How efficient is the human body at metabolizing food?

How efficient is the human body at metabolizing food?

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My friend and I were having a discussion over how "efficient" human digestion is. If a human ate a 1000 calorie hamburger, how many of those calories (how much energy) does the body process into usable energy (i.e. fat or other stores). I've heard things like "it's better to workout on an empty stomach because you will burn fat instead of energy from the food you just ate" - so from this I presume that storing energy as fat is less efficient than using the energy from digestion immediately (though in all honesty I'm not sure if that even makes sense.)

So, to summarize: how many calories of food does your body turn into usable calories via digestions? Percentages would be nice.

Bonus question: If the body does not process at 100%, does the FDA take this into account when labeling food products on their calorie content? If my Basal Metabolic Rate is 1800 calories a day, do I need to eat more than that since my body won't process all of the calories into usable energy?

I suspect that what you are actually looking for is the following:

- 1 gram of fat = 9 kcal - 1 gram of protein = 4 kcal - 1 gram of sugar = 4 kcal - 1 gram of alcohol = 7 kcal

Those are general and inexact values. They're just often used to give a rough idea of the amount of energy we get from different types of food.

A year and a half later, I stumbled upon this article by Ars Technica.

To summarize, research/experts acknowledge that the calorie system is a poor way to measure human metabolic performance. Gut fauna, calorie measuring device differences, and caloric differences produced by different levels of cooking all contribute to this system being unreliable.

To address the original question specifically: there isn't a clear answer or "rule of thumb", because it depends on far too many variables, many of which are directly attributable to the "calorie" being an inappropriate way to gauge food consumption and weight loss.

How Sweat Works: Why We Sweat When We're Hot, as Well as When We're Not

Did you know that there are only two mammals capable of long-distance running &mdash and that humans are one of them? (Horses are the other, since I'm sure you were curious).

While most other mammals can sprint faster than we can (a perk of having four legs), humans have basically evolved to be endurance runners. The theory is that this made us more efficient hunter-gatherers.

So what does this have to do with sweat? Well, if it weren't for how efficiently we produce and dissipate sweat, our ancestors wouldn't have been able to run long distances while hunting &mdash and we probably wouldn't be able to do things like run marathons.

But, as you already know, it's not just a long run that can make you work up a sweat. It's the little things, too, like being outside on a hot day or taking the stairs instead of the elevator. But, somewhat confusingly, you may also find yourself sweating at night, even though you're totally at rest, as well as when you're just scared, nervous or in pain.

Here's everything you ever wanted to know (or maybe didn't know you wanted to know) about how sweat works, including why we sweat when we're hot, as well as why we sweat even when we're not.

As Energy Sources

All chemical reactions in your body use adenosine triphosphate as an energy source. In order to be used as energy, proteins and fats must be converted to ATP. The conversion of protein to ATP is a costly process, requiring nearly as much ATP as it produces. This results in a gain in energy that is equivalent to as much as 4 and as little as 0 calories per gram of protein. In contrast, fats are easily broken down and converted to ATP. The result is an energy gain that is equivalent to approximately 9 calories per gram of fat.

Every one of us must undertake an apprenticeship with sorrow. We must learn the art and craft of grief, discover the profound way it ripens and deepens us. While grief is an intense emotion, it is also a skill we develop through a prolonged walk with loss. Facing grief is hard work … It takes outrageous courage to face outrageous loss. (Francis Weller) 1

Grief, experienced by every human, repeatedly and often compounded, must be metabolized. Involving every layer of our being, grief offers a powerful invitation to courageously embody and remember our most essential self and oneness with all of life.

Francis Weller, in his profound book, The Wild Edge of Sorrow, describes 5 gateways to grief 1 :

  1. Grief for what has been loved and lost. While this is a gate most recognized as we lose loved ones, animals, experiences, and dreams, this grief still often goes unmetabolized.
  1. Grief for the placeswithin ourselvesthat have not known love. Places that have not felt our own love are often shrouded in shame. Grief for these lost places arises and is metabolized when we no longer banish any part of ourselves.
  1. Grief for the sorrows of the world. Environmental demise, socioeconomic disparity, inequity, and systems that oppress bring incredible grief, often cumulative and overwhelming.
  1. Grief for what we expected and did not receive. In the diminished experience of who we truly are, there is sorrow and grief in forgetting that we are infinite – love embodied. Believing we are separate from Life essence/Pure consciousness is a great sorrow.
  1. Ancestral grief. There is so much in this gate to unpack as we appreciate the reckoning before us of generational trauma.

Grief, as a thread in the fabric of life, asks to be metabolized at the cellular level. Emotional digestion occurs in every moment, without our conscious involvement or pathologizing of the process. Happiness, delight, and joy are examples of emotions that metabolize instantaneously, nourishing our energy bodies without residue. The more difficult emotions invite awareness of our connection with all of life. They provide an opportunity to practice presence with all that is, bringing us closer to a state of oneness. They facilitate a profound spiritual practice, if we so choose. The digestion of grief is key to mental, emotional, and physical thriving.

The Neuroendocrine Dance

My walk with grief, and the stories and journeys of my patients, inspire this theory. Built on the integration of various schools of “systems thinking” and physiology, such as Endobiogenic medicine, Ayurvedic principles, Biotherapeutic Drainage, and more, this integrative theory for how grief is metabolized gives us insight into the mental and physical repercussions of undigested emotion and an invitation to witness emotional digestion as a foundational key to healing imbalance at all layers of being.

Endobiogenic medicine recognizes the neuroendocrine system as the manager of terrain. Through this lens, the physiologic dance between experience, thought, emotion, and physical expression is given structure. With any aggression – whether real, imagined, physical, mental, or emotional – norepinephrine (NE) is released within the central nervous system (CNS) and calibrates the intensity of the endocrine response to adapt appropriately to the aggression. NE signals the degree of intensity to the mind and body. Depending on individual genetics, physiology, past experiences, buffering capacity, and overall ability to manage adaptation efficiently, NE activity may be exaggerated and prolonged. NE solicits the hypothalamus, specifically corticotropin-releasing hormone (CRH) and thyrotropin-releasing hormone (TRH) (Figure 1).

Figure 1. NE-TRH Relationship

(Adapted from The Theory of Endobiogeny, by Hedayat KM et al) 2,3

CRH initiates corticotropic (adrenal) activity, managing adaptation through nutrient mobilization and calibration of subsequent endocrine axes. TRH and dopamine work centrally within the mind, and then in the body. TRH, as a neurohormone, increases the metabolic activity of neurons throughout the brain, supporting expansive thinking and creative solutions to novel stimuli. TRH, with histamine, synapses within the limbic system, giving emotional context to experiences. To supply the energy for increased central metabolic activity, TRH solicits peripheral activity to meet its own nutritional demands. TRH favors pancreatic release of glucagon, initiating glycogenolysis within the liver, which raises blood glucose for brain consumption. TRH also stimulates thyroid-stimulating hormone (TSH), the hormone of ideation. (Figure 2). Receptors for TSH on the exocrine pancreas favor increased digestive enzyme production and release to increase nutrient assimilation from the gut and support proteolysis after the adaptive demand has been met. TSH and increased peripheral thyroid activity favor increased cellular oxygen intake and utilization, increased calcium-driven enzymatic activity, and increased ATP production through mitochondrial oxidative phosphorylation.

Figure 2. Thyroid-Pancreas Relationship

(Adapted from The Theory of Endobiogeny, by Hedayat KM et al) 2,3

The waste materials produced are removed through circulation and emunctory function: CO2 exhalation via the lungs, proteinaceous and lipid soluble wastes via the liver, and water-soluble wastes via the kidney (Figure 3). The heart and cardiovascular system are key in circulating materials throughout the body. 2,3

Figure 3. Thyrotropic-Emunctories Relationship

(Adapted from The Theory of Endobiogeny, by Hedayat KM et al) 2,3

Clinical Consequences of Undigested Grief

Dysfunction in the normal physiology patterns can arise when there is: a persistent or exaggerated aggression, real or imagined the buffering capacity is overwhelmed toxicity accumulates and/or dysendocrinism develops in order to conserve resources and maintain the least number of elements of adaptation in active response. Undigested or overwhelming emotions initiate a prolonged sense of aggression. Clinically, the most common areas where I see disease form with undigested grief is in thyroid activity, both centrally (anxiety, depression, insomnia, rumination) and peripherally (hypo- and hyperthyroidism), and in the emunctories and supporting organs (liver, kidney, lungs, and pancreas). Increased cellular inflammation and necrosis occur, most often associated with the thyrotropic axis due to the neuroendocrine pathways described in Figure 3.

The skillful digestion of grief profoundly reminds us of our true essence and nourishes the expression of that essence with authenticity and grace. Knowledge of self and living in alignment with our true nature is key to healing and thriving. Undigested grief, stored in the body and the mind, feeds our false stories of separation, favoring a repeated sense of aggression and accumulated residue. There is a constriction of the pericardium, the protective energy around the heart, a guard against the hurt we don’t want to experience again. There is a congestion of the lungs, even fibrosis, as the stories of unprocessed grief continue to arise within the locus coeruleus, solicit NE and TRH, and stimulate inefficient cellular metabolism. There is increased CNS glucose demand, increased insulin production to resolve hyperglycemia, increased cellular oxidation, and free radical production beyond the protective cellular needs. Excess waste material due to increased metabolic activity places greater demand on the lungs, liver, and kidneys. There is greater demand on the pancreas for glycogen, insulin, and pancreatic enzyme production and release. Overthinking and worry, with excess TRH activity, favors pancreatic congestion. Congestion in any organ over time will reduce circulation, oxygenation, efficient removal of waste material, and efficiency of action, eventually resulting in dysfunction and disease. Disease may occur in the emunctory organ directly or in areas of the body associated with or affected by its lack of efficiency.

Humans must constantly adapt. Meeting the needs of the physical body, mental/emotional body, and energy body builds resilience and greater bandwidth for adaptation. Through the digestion and assimilation of nutrients, clean water, rest, and healthy movement, the needs of the physical body are met and strengthened. Through emotional digestion, the emotional and energy body are nourished. Building resiliency and clearing debris from each layer allows for a less encumbered expression of our essence through our entire being. When there is an unmet need or accumulated toxicity in any layer, there is an encumbrance of flow and an acute and palpable affect on surrounding layers.

Routes to Healing

Fundamental healing is a remembrance of Essence, a remembrance of Oneness with all of life. And, being in human form gives us the opportunity through body, mind/emotion, and energy layers to tangibly meet our needs and connect with and express pure consciousness in our unique forms and with our unique gifts (Figure 4).

Figure 4. The Layered Self

(Adapted from Tantra Illuminated, by Wallis CD) 4

The throat is one of the channels for the individual expression of the universal essence and our emotional nuance. This is not the path of mind to mouth, one often fraught with imagination and stories. It is the path of energy and emotion imbued with our essence. Emotions are energy in motion. The movement of sound, song, sigh, and cry, that comes from our cellular experience and processing of emotions, allows for their circulation and release from the heart and lungs through the throat – a key element of emotional digestion.

With an understanding of the physiology of emotional metabolism, a physical and metaphysical window opens to disease formation and resolution. A layered approach and an awareness of the neuroendocrine pathways managing adaptation, assimilation, and transmutation of emotional experiences are powerful tools for offering our patients and ourselves agency and pathways for healing. Giving voice to the grief, acknowledging its presence, and honoring the way it impacts and is processed through our whole being is the first step toward healing the light of loving attention is one of the most powerful healers.

Healing touch, energy, sound, light, color, music, breath work, and movement – as tools for bringing the enzymes of loving attention to the structure of grief – allow for its assimilation and transformation. Storytelling, painting, writing, community ritual, fire ceremonies, and group grief work are powerful tools for attention and digestion.

Botanical Helpers

Plants are powerful guides and support for each layer of our being, bringing healing and harmony and pointing us toward pure consciousness. A few of the plant helpers I have had the privilege to get to know on my own journey include the following:

Passiflora incarnata: Passionflower lowers alpha sympathetic (NE) activity, pacifying an exaggerated perception of aggressions and the transmission of that exaggerated signal to the adrenals and TRH. Passionflower invites heart-open presence with all that is. It is beauty embodied, reminding us to stay open to life and to hold the space for emotional digestion and radiant expression.

Cornus sanguinea bud: In its phytoembryotherapy preparation, dogwood helps soften the stories recycled by the locus coeruleus – stories of trauma and fear that were often stored away because there was insufficient capacity to digest them when first experienced. With less intensity coloring the story, there is less NE overstimulation, less perception of an emergency. Dogwood also has a tropism for the heart and circulatory system, improving blood fluidity, offering solace in heartbreak, acute infarction, and angina. Dogwood lowers the excess TSH activity (often evident with a quickly suppressed and low serum TSH) that favors necrosis, free radical formation, and inflammation. Dogwood supports the mental/emotional processing of trauma while also reducing the downstream physical harm.

Leonurus cardiaca: Motherwort dances where TRH meets the heart. The domain of motherwort is where the mind continues to mull and spin, anxious, busy without clear answer or action, and the heart rhythm becomes chaotic.

Melissa officinalis: Lemon balm invites a clear and calm mind while also supporting circulation to the digestive tract, reducing nausea, and calming excess stimulation of the adrenals and the thyroid. When the CNS is in a particular state of calm or resonance, memories are more easily accessed. Accessing these memories, in a safe space, is the first step of awareness and then metabolization of the grief.

Viburnum lantana bud: Also in phytoembryotherapy preparation, Lithy tree bud is supportive when grief and excess TRH, TSH, and peripheral thyroid activity have congested the lungs. Viburnum allows for the inspiration of life and the release of all that is no longer needed – a long, deep sigh.

Flower and gem essences are also powerful allies in our grief work. Many texts provide descriptions of essences and their effects. Also, exploring ways to communicate with the essences and listen for which ones “want” to work with a particular patient at a particular time can be a beautiful intuitive practice.

Each of our tool boxes for grief support will be individual, hopefully given nuance and precision by a physiologic model and voice to the metabolization of grief. Involving our patients in the awareness, physiological understanding, and choice of tools is also powerful medicine. Working with grief does take courage. And yet, the rewards are great!

Grief dares us to love once more. (Terry Tempest Williams) 5

Immune System

The immune system is our body's defense system against infections and diseases. Organs, tissues, cells, and cell products work together to respond to dangerous organisms (like viruses or bacteria) and substances that may enter the body from the environment. There are three types of response systems in the immune system: the anatomic response, the inflammatory response, and the immune response.

  • The anatomic response physically prevents threatening substances from entering your body. Examples of the anatomic system include the mucous membranes and the skin. If substances do get by, the inflammatory response goes on attack.
  • The inflammatory system works by excreting the invaders from your body. Sneezing, runny noses, and fever are examples of the inflammatory system at work. Sometimes, even though you don't feel well while it's happening, your body is fighting illness.
  • When the inflammatory response fails, the immune response goes to work. This is the central part of the immune system and is made up of white blood cells, which fight infection by gobbling up antigens. About a quarter of white blood cells, called the lymphocytes, migrate to the lymph nodes and produce antibodies, which fight disease.

Below we have given a short essay on Human Body is for Classes 1, 2, 3, 4, 5 and 6. This short essay on the topic is suitable for students of class 6 and below.

To prevent it from diseases and illnesses, a thorough knowledge of the human body is necessary. Medical science has unravelled many mysteries of the functions of our body. And, the more we find out, the more fascinating the human body appears to be. But there is still a lot that we don’t know or can’t explain.

The human skeleton is like a cage. It provides the necessary support to the body. It also helps in protecting our vital organs. There are 206 bones in an adult human body. These bones are made up of calcium and phosphorus. The box-like skull structure protects our brain.

The muscles constitute the flesh. There are over 600 muscles in our body. All our movements are the direct result of the contraction and expansion of these muscles.

A cell is the basic unit of the body and there are millions of cells in each human body. These cells get nourishment through food, drink and oxygen. The cell suffer wear and tear during work. But through adequate rest and food the damage to the cell is repaired.

Then, there are the circulatory, respiratory, disgestive and nervous systems in our body. They are all highly complex systems but each is wonderful in its own way. Human heart and brain must be two of the most wonderful creations ever. They are extremely complicated but also very efficient parts of our body.

For us to live and remain healthy, it is important for all these parts and systems to work well together, in harmony with each other.

How Dieting Works

Drinking water and eating fruits and vegetables are part of a healthy diet. See more weight loss tips pictures.

Dieting is one of those things that is completely integrated into American culture. On any given day, a huge portion of the U.S. population is "on a diet" and "counting calories" in one way or another. And look at how many of the diet names in the following list you recognize:

You probably recognize many of these names because you hear them all the time!

In this article, we will look first at weight gain and why gaining weight is so easy. Then we will look at what you can do about weight gain -- in the form of diet and exercise -- to maintain a consistent weight.

Have you ever wondered why, for so many people (and especially for anyone older than 30 years old), weight gain seems to be a fact of life? It's because the human body is way too efficient! It just does not take that much energy to maintain the human body at rest and when exercising, the human body is amazingly frugal when it comes to turning food into motion.

At rest (for example, while sitting and watching television), the human body burns only about 12 calories per pound of body weight per day (26 calories per kilogram). That means that if you weigh 150 pounds (68 kg), your body uses only about:

150 X 12 = 1,800 calories per day

Twelve calories per pound per day is a rough estimate -- see How Calories Work for details.

Those 1,800 calories are used to do everything you need to stay alive:

  • They keep your heart beating and lungs breathing.
  • They keep your internal organs operating properly.
  • They keep your brain running.
  • They keep your body warm.

In motion, the human body also uses energy very efficiently. For example, a person running a marathon (26 miles or 42 km) burns only about 2,600 calories. In other words, you burn only about 100 calories per mile (about 62 calories per km) when you are running.

You can see just how efficient the human body is if you compare your body to a car. A typical car in the United States gets between 15 and 30 miles per gallon of gasoline (6 to 12 km/L). A gallon of gas contains about 31,000 calories. That means that if a human being could drink gasoline instead of eating hamburgers to take in calories, a human being could run 26 miles on about one-twelfth of a gallon of gas (0.3 L). In other words, a human being gets more than 300 miles per gallon (120 km/L)! If you put a human being on a bicycle to increase the efficiency, a human being can get well over 1,000 miles per gallon (more than 500 km/L)!

That level of efficiency is the main reason why it is so easy to gain weight, as we will see in the next section.

A meal at McDonald's can add up to almost a whole day's worth of calories.

The 1,800 calories that a typical person at rest needs per day is just not that many. For example, if you go to your neighborhood McDonald's restaurant and order the Big Xtra meal, you will get a sandwich, a large order of french fries and a large Coke®. This meal contains:

  • 710 calories in the sandwich*
  • 540 calories in the french fries*
  • 310 calories in the drink*

In other words, just this one meal provides 1,560 calories you need during a day. If you get an M&M® McFlurry&trade with it for dessert, you'll get 630 more calories, so you are already consuming almost 2,200 calories just at this one meal!

Similarly, if you go to Pizza Hut and get a Meat Lover's Pan Pizza®, each slice contains 360 calories.* If you eat three slices and get a large drink to go with it, that's 1,390 calories -- just 410 calories shy of a full day's worth of calories. (*See the Pizza Hut Nutrition Guide for details.)

Similarly, if you eat 12 SnackWell's Crème Sandwich Cookies -- which, if you think about it, really is not that hard to do -- you've taken in 660 calories. That's more than one-third of the daily caloric intake.

The point here is not to slam these products or make them look bad. For example, I've got two kids and I go to McDonald's at least once a week. The point is that, in America and most other developed countries, it is incredibly easy to find and consume calories. Let's take a look at what someone might consume in a typical day.

Face it, many of us are over-worked, over-booked and totally over-extended. So, convenient food often takes the lead in our daily diets. In a typical day someone might consume something like this:

  • You might have two Pop-Tarts® for breakfast,
  • then hit Pizza Hut for lunch,
  • grab some SnackWell's and a cola for a snack,
  • head for McDonald's for dinner
  • and top it off with some potato chips while watching TV.

You can see how the number of calories coming in can easily reach 3,000, 4,000 or 5,000 per day without any effort at all. That's the problem.

Your body, it turns out, is extremely efficient at capturing and storing excess calories. Whenever your body finds that it has excess calories on hand, it converts them to fat and saves them for a rainy day. (See How Calories Work and How Fat Cells Work for details). It only takes 3,500 excess calories to create 1 pound of new fat on your body. If you are taking in just 500 extra calories per day, then you are gaining a pound of fat per week (500 calories x 7 days in a week = 3,500 calories/week). Since it is easy to get 500 calories from just one ice cream cone or a few cookies, you can see that weight gain is completely effortless in today's society. Food is just too easy to find.

Let's imagine that you are overweight and you would like to lose several excess pounds. To lose 1 pound of fat, what you have to do is burn off 3,500 calories. That is, over a period of time, you have to consume 3,500 calories less than your body needs. There are several ways you can create that deficit. If you assume that you weigh 150 pounds and that your body at rest needs 1,800 calories per day (150 * 12 = 1,800) to live, here are several examples (some realistic, some not):

    You could lie in bed and starve yourself. Since you are lying in bed, you are consuming 1,800 calories per day. Since you are starving yourself, you are taking in no calories. That means that, every day, you create a deficit of 1,800 calories and, approximately every two days, you will lose 1 pound of body weight.

As you can see from these examples, the only way to lose fat is to consume fewer calories per day than your body needs. For every 3,500 calories that your body takes from its fat reserves, you lose 1 pound (0.45 kg) of body fat. You can create the deficit either by monitoring and restricting your intake of calories, or by exercising, or both.

The idea behind most diets -- everything from Weight Watchers to the grapefruit diet -- is simply to help you somehow lower the number of calories that you consume each day. That's all they do.

Why Diets Tend Not to Work

The reason why most diets tend not to work for very long is because they are not sustainable. A person gains weight because he or she consumes more calories per day than needed. The diet creates a temporary deficit. When the diet ends, the person goes back to normal eating and the weight comes back.

Let's look at an example. Say that you weigh 150 pounds. That means that you burn 1,800 calories per day in a resting state. Let's also imagine that in the course of a day you burn 200 more calories living your life -- walking up and down steps, carrying in the groceries and so on. Your calorie needs then are, on average, 2,000 calories per day. Now let's further imagine that, on average, you consume 2,050 calories per day. On a daily basis your body is taking in, and therefore storing, 50 calories more than it needs. So every 70 days (3,500 calories in a pound / 50 calories each day = 70 days) you gain 1 pound (0.45 kg). If that "50 extra calories per day" trend continues, then over the course of a year you would gain 5 pounds. This, by the way, is the pattern for a big portion of the U.S. population. If you over-consume by just a few calories per day, over time you will gain weight. Keep in mind that just one Oreo-type cookie contains 50 calories, so over-consuming is incredibly easy.

Now, you go on a diet -- the amazing "Get Slim Miracle Diet." On this diet, you consume nothing but 2 cups of brown rice and a can of Vienna sausages, along with all the onions you care to eat, every day. You start this diet and you are consuming only 1,000 calories per day. You also start jogging 2 miles a day. That means that, on a typical day, you are consuming 1,200 calories less than you need. Over the course of three days (3,500 calories in a pound / 1,200 calories each day = approximately 3 days), you will lose 1 pound of weight. You keep on this diet for two months and lose 20 pounds.

The day you go off this diet, what is going to happen? First, you are probably going to eat a lot more than normal because you have been eating nothing but rice and Vienna sausages for two months! Then you will settle into your "normal eating pattern" that you had before the diet. And eventually all of the weight comes back.

This is why diets don't work for most people. You do lose weight, but then go off the diet and gain it back. What is needed instead is a sustainable diet -- a food consumption and exercise plan -- that lets you live a normal life and eat normal foods in a normal way.

Building a Sustainable Diet

Building a sustainable diet and exercise plan is the key to maintaining a consistent weight. This is not easy for many people. As described in the previous sections, the landscape is literally covered with calories, and exercise takes time and energy.

The first step to building a sustainable diet is to start counting the calories that you consume in a day so that you become conscious of two things:

    You need to understand exactly how many calories you are eating on a "normal" day.

In the United States, any food that you buy in the grocery store is required by the U.S. Food & Drug Administration to have a nutritional label with that food's calorie content. You can also look at a chart like this one to find out the number of calories in different foods. Any chain restaurant will supply you with nutrition information both at the store and on the Web (or you can see a Web site like this).

The second step is to figure out how many calories you need in a day. You can use the "12 calories per pound" rule, or you can get more precise by looking at the formulas in How Calories Work.

Pick your "ideal weight" -- the weight that you would like to maintain. Then calculate how many calories a day you can consume to maintain that weight.

The third step is to compare the two numbers -- You may be startled by the difference between the "number of calories you need" and "the number of calories that you take in" in a day. That is where the extra pounds are coming from.

The fourth step is to figure out how to bring the two numbers in line. What you will soon realize is that 1,600 or 1,800 or 2,000 calories per day just isn't that many. You have to watch and count everything you eat and drink every day and stick to your daily limit.

The fifth step might be to add exercise to the mix so that you can raise the number of calories you can consume per day. Online resources like this exercise calculator will show you how many calories different forms of exercise can burn. Burning 250 or 500 calories per day through exercise can make a big difference.

In an effort to reduce the number calories you take in per day, here are several strategies that you might find effective:

    Be conscious of every calorie you consume, and keep a daily journal. Stick an index card in your pocket each day and write down everything you eat and drink.

  • You take in fewer calories.
  • You get to chew the orange, which has a psychological effect.
  • It fills your stomach, which curbs your appetite. An orange actually gives you a feeling of fullness, while orange juice does not.

The same holds true of any beverage that contains calories -- the calories come in but your appetite remains the same.

By simply refusing all foods that contain lots of sugar, you make it easy to eliminate a big class of snack foods.

  • potato chips
  • cheese crisps
  • french fries
  • onion rings
  • donuts
  • fritters
  • fried chicken

Fat from deep frying gives these foods lots of calories for their size. Eliminating fried foods and sugar together pretty much eliminates all high-calorie snacks. Entire aisles in the grocery store become irrelevant to you.

Apples contain few calories for their size.

Here's a good mental exercise that helps you understand the point: Most people would not find it hard to eat a dozen Oreo-type cookies. Or a dozen SnackWell's cookies. That's 600 calories. Now imagine trying to eat six bananas at one sitting -- you would explode! But it's the same number of calories. Look for low-density foods like bananas that fill you up without giving you that many calories. Foods that are low-density include:

Things that are high-density include any food or beverage high in sugar and/or fat. Nuts, most meats, candy, cookies, crackers, potato chips, fried anything, cola, beer, and so on are all high-density and should be avoided.

If you follow these guidelines and, through diet and exercise, keep the number of calories you consume below the number of calories needed, you will lose fat and maintain a consistent weight.

Metabolism of Carbohydrates: 10 Cycles (With Diagram)

This article throws light upon the ten major pathways/cycles of carbohydrate metabolism. The ten pathways/cycles of carbohydrate metabolism are:

(1) Glycolysis (2) Conversion of Pyruvate to Acetyl COA (3) Citric Acid Cycle (4) Gluconeogenesis (5) Glycogen Metabolism (6) Glycogenesis (7) Glycogenolysis (8) Hexose Monophosphate Shunt (9) Glyoxylate Cycle and (10) Photosynthesis.

Carbohydrates are the major source of energy for the living cells. The monosaccharide glucose is the central molecule in carbohydrate metabolism since all the major pathways of carbohydrate metabolism are connected with it (Fig. 67.3).

Glucose is utilized as a source of energy, it is synthesized from non-carbohydrate precursors and stored as glycogen to release glucose as and when the need arises. The other monosaccharide’s important in carbohydrate metabolism are fructose, galactose and mannose.

The fasting blood glucose level in normal humans is 60-100 mg/dl (4.5-5.5 mmol/l) and it is very efficiently maintained at this level.

The outlines of major pathways/cycles of carbohydrate metabolism are described:

Cycle # 1. Glycolysis:

Glycolysis is derived from the Greek words (glycose—sweet or sugar lysis—dissolution). It is a universal pathway in the living cells. Glycolysis is defined as the sequence of reactions converting glucose (or glycogen) to pyruvate or lactate, with the production of ATP (Fig. 67.4).

Salient features:

1. Glycolysis (also known as Embden-Meyerhof pathway) takes place in all cells of the body. The enzymes of this pathway are present in the cytosomal fraction of the cell.

2. Glycolysis occurs in the absence of oxygen (anaerobic) or in the presence of oxygen (aerobic). Lactate is the end product under anaerobic condition. In the aerobic condition, pyruvate is formed, which is then oxidized to CO2 and H2O.

3. Glycolysis is a major pathway for ATP synthesis in tissues lacking mitochondria, e.g. erythrocytes, cornea, lens etc.

4. Glycolysis is very essential for brain which is dependent on glucose for energy. The glucose in brain has to undergo glycolysis before it is oxidized to CO2 and H2O.

5. Glycolysis (anaerobic) may be summarized by the net reaction

Glucose + 2ADP + 2Pi → 2 Lactate + 2ATP

6. Reversal of glycolysis along with the alternate arrangements made at the irreversible steps will result in the synthesis of glucose (gluconeogenesis).

Cycle # 2. Conversion of Pyruvate to Acetyl COA:

Pyruvate is converted to acetyl CoA by oxidative decarboxylation. This is an irreversible reaction, catalysed by a multi-enzyme complex, known as pyruvate dehydrogenase complex (PDH), which is found only in the mitochondria. High concentrations of PDH are found in cardiac muscle and kidney. The enzyme PDH requires five cofactors (coenzymes), namely — TPP, lipoamide, FAD, coenzyme A and NAD + (lipoamide contains lipoic acid linked to ɛ-amino group of lysine).

The overall reaction of PDH is:

Cycle # 3. Citric Acid Cycle:

The citric acid cycle (Krebs cycle or tricarboxylic acid—TCA cycle) is the most important metabolic pathway for the energy supply to the body. About 65-70% of the ATP is synthesized in Krebs cycle. Citric acid cycle essentially involves the oxidation of acetyl CoA to CO2 and H2O.

The citric acid cycle is the final common oxidative pathway for carbohydrates, fats and amino acids. This cycle not only supplies energy but also provides many intermediates required for the synthesis of amino acids, glucose, heme etc. Krebs cycle is the most important central pathway connecting almost all the individual metabolic pathways (either directly or indirectly). The enzymes of TCA cycle are located in mitochondrial matrix, in close proximity to the electron transport chain.

Krebs cycle basically involves the combination of a two carbon acetyl CoA with a four carbon oxaloacetate to produce a six carbon tricarboxylic acid, citrate. In the reactions that follow, the two carbons are oxidized to CO, and oxaloacetate is regenerated and recycled. Oxaloacetate is considered to play a catalytic role in citric acid cycle. The reactions of Krebs cycle are depicted in Fig. 67.5.

Cycle # 4. Gluconeogenesis:

The synthesis of glucose or glycogen from non-carbohydrate compounds is known as gluconeogenesis. The major substrates/precursors for gluconeogenesis are lactate, pyruvate, glucogenic amino acids, propionate and glycerol.

Location of gluconeogenesis:

Gluconeogenesis occurs mainly in the cytosol, although some precursors are produced in the mitochondria. Gluconeogenesis mostly takes place in liver and, to some extent, in kidney matrix (about one-tenth of liver capacity).

Reactions of gluconeogenesis:

Gluconeogenesis closely resembles the reversed pathway of glycolysis, although it is not the complete reversal of glycolysis. Essentially, 3 (out of 10) reactions of glycolysis are irreversible. The seven reactions are common for both glycolysis and gluconeogenesis. The three irreversible steps of glycolysis are catalysed by the enzymes, namely hexokinase, phosphofructokinase and pyruvate kinase.

Cycle # 5. Glycogen Metabolism:

Glycogen is the storage form of glucose in animals, as is starch in plants. It is stored mostly in liver (6-8%) and muscle (1-2%). Due to more muscle mass, the quantity of glycogen in muscle (250 g) is about three times higher than that in the liver (75 g).

Functions of glycogen:

The prime function of liver glycogen is to maintain the blood glucose levels, particularly between meals. Liver glycogen stores increase in a well-fed state which are depleted during fasting. Muscle glycogen serves as a fuel reserve for the supply of ATP during muscle contraction.

Cycle # 6. Glycogenesis:

The synthesis of glycogen from glucose is glycogenesis. Glycogenesis takes place in the cytosol and requires ATP and UTP, besides glucose.

Cycle # 7. Glycogenolysis:

The degradation of stored glycogen in liver and muscle constitutes glycogenolysis. The pathway for the synthesis and degradation of glycogen are not reversible. An independent set of enzymes present in the cytosol carry out glycogenolysis. Glycogen is degraded by breaking α-1, 4- and α-1, 6-glycosidic bonds.

Cycle # 8. Hexose Monophosphate Shunt:

Hexose monophosphate pathway or HMP shunt is also called pentose phosphate pathway or phosphogluconate pathway. This is an alternative pathway to glycolysis and TCA cycle for the oxidation of glucose. However, HMP shunt is more anabolic in nature, since it is concerned with the biosynthesis of NADPH and pentose’s.

Location of the pathway:

The enzymes of HMP shunt are located in the cytosol. The tissues such as liver, adipose tissue, adrenal gland, erythrocytes, testes and lactating mammary gland, are highly active in HMP shunt. Most of these tissues are involved in the biosynthesis of fatty acids and steroids which are dependent on the supply of NADPH.

Reactions of HMP shunt:

The sequence of reactions of HMP shunt is depicted in Fig. 67.6.

Significance of HMP shunt:

HMP shunt is unique in generating two impor­tant products—pentose’s and NADPH—needed for the biosynthetic reactions and other functions.

A. Importance of pentose’s:

In the HMP shunt, hexoses are converted into pentose’s, the most important being ribose 5-phosphate. This pentose or its derivatives are useful for the synthesis of nucleic acids (RNA and DNA) and many nucleotides such as ATP, NAD + , FAD and CoA.

B. Importance of NADPH:

1. NADPH is required for the reductive biosynthesis of fatty acids and steroids, hence HMP shunt is more active in the tissues concerned with lipogenesis, e.g. adipose tissue, liver etc.

2. NADPH is used in the synthesis of certain amino acids involving the enzyme glutamate dehydrogenase.

3. There is a continuous production of H2O2 in the living cells which can chemically damage unsaturated lipids, proteins and DNA. This is, however, prevented to a large extent through antioxidant reactions involving NADPH. Gluta­thione mediated reduction of H2O2 is given hereunder

Glutathione (reduced, GSH) detoxifies H2O2, peroxidase catalyses this reaction. NADPH is responsible for the regeneration of reduced glutathione from the oxidized one.

Cycle # 9. Glyoxylate Cycle:

The animals, including man, cannot carry out the net synthesis of carbohydrate from fat. However, the plants and many microorganisms are equipped with the metabolic machinery—namely the glyoxylate cycle—to convert fat into carbohydrates. This pathway is very significant in the germinating seeds where the stored triacylglycerol (fat) is converted to sugars to meet the energy needs.

The glyoxylate cycle is regarded as an anabolic variant of citric acid cycle and is depicted in Fig. 67.7.

Cycle # 10. Photosynthesis:

The synthesis of carbohydrates in green plants photosynthesis. It is now recognized that photosynthesis primarily involves the process of energy transduction in which light energy is converted into chemical energy (in the form of oxidizable carbon compounds).

It is an established fact that all the energy consumed by the biological systems arises from the solar energy that is trapped in the photosynthesis. The basic equation of photosynthesis is given below.

In the above equation, (CH2O) represents carbohydrate. Photosynthesis in the green plants occurs in the chloroplasts, a specialized organelles. The mechanism of photosynthesis is complex, involving many stages, and participation of various macromolecules and macromolecules.

The role of photosystems:

The initial step in the photosynthesis is the by assimilation of carbon dioxide is referred to as absorption of light by chlorophyll molecules in the chloroplasts. This results in the production of excitation energy which is transferred from one chlorophyll molecule to another, until it is trapped by a reaction center. The light-activated transfer of an electron to an acceptor (photosystems) occurs at the reaction center.

Photosynthesis primarily requires the interactions of two distinct photosystems (I and II). Photosystem I generates a strong reductant that results in the formation of NADPH. Photosystem II produces a strong oxidant that forms O2 from H2O. Further, the generation of ATP occurs as electrons flow from photosystem II to photosystem I (Fig. 67.8). Thus, light is responsible for the flow of electrons from H2O to NADPH with a concomitant generation of ATP.

The Calvin cycle:

The dark phase of photosynthesis is referred to as Calvin cycle. In this cycle, the ATP and NADPH produced in the light reaction (described above) are utilized to convert CO2 to hexoses and other organic compounds (Fig. 67.9). The Calvin cycle starts with a reaction of CO2 and ribulose 1, 5-bisphosphate to form two molecules 3-phosphoglycerate. This 3-phosphoglycerate can be converted to fructose 6-phosphate, glucose 6-phosphate and other carbon compounds.

The Body’s Ecosystem

The Scientist Staff
Aug 1, 2014


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T he human body is teeming with microbes&mdashtrillions of them. The commensal bacteria and fungi that live on and inside us outnumber our own cells 10-to-1, and the viruses that teem inside those cells and ours may add another order of magnitude. Genetic analyses of samples from different body regions have revealed the diverse and dynamic communities of microbes that inhabit not just the gut and areas directly exposed to the outside world, but also parts of the body that were long assumed to be microbe-free, such as the placenta, which turns out to harbor bacteria most closely akin to those in the mouth. The mouth microbiome is also suspected of influencing bacterial communities in the lungs. Researchers are also examining the basic biology of the microbiomes of the penis, the vagina, and the skin.

THE BODY'S MICROBIOMES: Genomic surveys of the body&rsquos bacterial.

Altogether, the members of the human body’s microbial ecosystem make up anywhere from two to six pounds of a 200-pound adult’s total body weight, according to estimates from the Human Microbiome Project, launched in 2007 by the National Institutes of Health (NIH). The gastrointestinal tract is home to an overwhelming majority of these microbes, and, correspondingly, has attracted the most interest from the research community. But scientists are learning ever more about the microbiomes that inhabit parts of the body outside the gut, and they’re finding that these communities are likely just as important. Strong patterns, along with high diversity and variation across and within individuals, are recurring themes in microbiome research. While surveys of the body’s microbial communities continue, the field is also entering a second stage of inquiry: a quest to understand how the human microbiome promotes health or permits disease.

“None of us in the field—and this is true for the gut, this is true for the skin—none of us can actually tell how our experimental observations really relate to human disease, but we’re getting closer to mechanistic insights,” says immunologist Yasmine Belkaid, chief of mucosal immunology at the National Institute of Allergy and Infectious Disease.

Mouth Microbes

© CAROL DEL ANGEL/IKON/GETTY IMAGES The late zoologist Charles Atwood Kofoid couldn’t possibly have known that he and his University of California, Berkeley, colleagues had begun to chip away at the human oral microbiome when, in 1929, they described in the Journal of the American Dental Association “animal parasites of the mouth and their relation to dental disease.” Scientists studying periodontal diseases have for decades realized that certain pathogenic bacteria contribute to inflammation and the eventual destruction of tissues within the oral cavity. But it’s now recognized that the mouth is populated with commensal microbes, too, and that these typically benign bacteria can contribute to a person’s health beyond their gums, tongue, and teeth.

Exploring the composition of the mouth microbiome is not without its challenges, however. “The whole world passes through the oral cavity,” says Purnima Kumar, an assistant professor of periodontology at Ohio State University College of Dentistry. “When we collect a sample, we don’t know if it’s just something that’s passing by, or if it’s truly a member of the community,” she says.

Some microbes that dwell in the mouth readily move on from the oral cavity, passing with saliva and food farther through the gastrointestinal tract, for example, or becoming aerosolized and spreading into the lungs. (See “A Lungful of Microbes,” below.) A recent study showed that the placental microbiome more closely resembles that of the mouth than of any other body site, suggesting the oral cavity, by way of the maternal bloodstream, might also supply the organ with commensal microbes. (See “The Maternal Microbiome,” below.)

The mouth may also pass along not-so-friendly bacteria to other body sites. “There’s a lot of evidence linking oral bacteria to distal infections,” says Kumar. To date, oral bacteria have been implicated in cardiovascular disease, pancreatic cancer, colorectal cancer, rheumatoid arthritis, and preterm birth, among other things.

The first step in understanding how mouth microbes affect human health and disease is to determine which species inhabit the human oral cavity. In 2010, microbiologist Floyd Dewhirst from the Forsyth Institute in Cambridge, Massachusetts, and his colleagues published the first comprehensive examination of mouth-dwelling microbes, finding distinct communities on the tongue, on the roof of the mouth, within the biofilms that coat the teeth and gums, and elsewhere in the oral cavity (J Bacteriol, 192:5002-17, 2010). Researchers have identified some 700 microbial species that inhabit the human mouth. “We’re doing really well in terms of who’s there,” Dewhirst says.

Scientists are also starting to get a better understanding of how microbes are organized within the oral cavity. Accumulating evidence suggests that the structure of this microbiome “is not haphazard or random,” says Boston University’s Salomon Amar. “We don’t have the full picture yet, but we understand that there is [a] first layer of microorganisms that allows for the attachment of the second-comers, the third-comers, the fourth, and so on, in a very hierarchical type of organization.”

Such a diversity of species makes for varied cellular interactions. At any one time, “there might be 200 or 300 species interacting with one another and the host,” says William Wade, a professor of oral microbiology at Barts and The London School of Medicine and Dentistry’s Blizard Institute. “Trying to model these interactions is extremely difficult.” By better understanding the dynamics of how these communities promote health or thwart pathogenesis, however, researchers may one day be able to disrupt the oral microbiome in targeted ways to prevent harmful growth.—Tracy Vence

A Lungful of Microbes

© ROMAN YA/SHUTTERSTOCK.COM If the human digestive tract were a river extending from the mouth through the stomach and intestines, the lungs would be adjacent pools that are fed by the current, according to Gary Huffnagle of the University of Michigan who began studying the bacterial communities that inhabit these organs nearly a decade ago. “There’s a constant flow into [the] lungs of aspirated bacteria from the mouth,” he says. But through the action of cilia, the cough reflex, and other cleansing responses, there’s also an outward flow of microbes, making the lung microbiome a dynamic community.

Like many other body sites now known to harbor commensal bacteria, the disease-free lung was long thought by researchers and clinicians to be largely sterile. Over the last 10 years, however, evidence has been building that the lungs are also populated by a persistent community of microbial residents—albeit a small one. The lung microbiome is about 1,000 times less dense than the oral microbiome, and about 1 million to 1 billion times sparser than the microbial community of the gut, says Huffnagle. That is in part because the lung lacks the microbe-friendly mucosal lining found in the mouth and gastrointestinal tract, instead harboring a thin layer of much-less-inviting surfactant to keep the respiratory organs from drying out, as well as ciliated cells that beat rhythmically to move debris and invading microbes.

In a review article published this March (The Lancet Respiratory Medicine, 2:238-46, 2014), Huffnagle and his colleagues argued that the lungs are like the South Pacific, with small islands of clustered bacteria and wide stretches of unpopulated regions between them. It appears that the lung microbiome is populated from the oral microbiome and the air, and among this population exists a small subset of bacteria that can survive the unique environment of these organs. The most common bacteria found in healthy lungs are Streptococcus, Prevotella, and Veillonella species.

The lung microbiome is about 1,000 times less dense than the oral microbiome, and about 1 million to 1 billion times sparser than the microbial community of the gut.

Recent studies have linked shifts in the lung microbiome to chronic diseases, such as cystic fibrosis (CF) or chronic obstructive pulmonary disease (COPD). In a 2012 study led by epidemiologist John LiPuma of the University of Michigan, the researchers collected specimens from the lungs of CF patients for more than a decade and found that, as the disease progressed, the lung microbiome became less diverse, although overall microbe density stayed the same (PNAS, 10.1073/pnas.1120577109, 2012). They ascribed this shift in the microbiome to the use of antibiotics, which are typically administered to those with CF. “Are antibiotics bad? We’re not saying that at all,” LiPuma says. “The question this paper raises is: Is there a tipping point where antibiotics start to turn against us in CF?”

Leopoldo Segal of the New York University Langone Medical Center who studies small-airway disorders with an eye toward early detection of COPD, has found in a series of studies that inflammation of the lungs is often accompanied by a shift in their bacterial makeup. But the mechanisms behind these changes, and their consequences, are still not well understood. Other studies have uncovered associations between shifts in the lung microbiome and HIV or asthma, but again, causality has been difficult to show.

According to Yvonne Huang of the University of California, San Francisco, Medical Center, characterization of lung microbiomes in relation to health and disease progression is just starting to yield meaningful results. “This field is where studies of gut microbiome were 10 to 15 years ago.” —Rina Shaikh-Lesko

Microbes of the Penis and Vagina

© UIG GETTY IMAGES Microbiologist David Nelson of Indiana University in Bloomington was investigating Chlamydia infections when he and his colleagues found evidence to suggest that the sexually transmitted pathogens in the male urogenital tract were mingling with other microbes (PLOS ONE, 5:e14116, 2010). Specifically, Nelson learned that the Chlamydia strains of the urogenital tract encode an enzyme that allows them to make tryptophan from an organic compound called indole, which is produced by other bacteria inhabiting the penis. “There was a signature in the chlamydial genome that suggested this organism might be interacting with other microorganisms,” says Nelson. “That’s what initially piqued our interest. And when we went in and started to look, we found that there were a lot more [microbes] than we would have anticipated being there.”

Nelson and his team were among those discovering that the penis harbors its own unique microbiomes, inside and out. Some men pass urine containing a variety of lactobacilli and streptococci species, likely washed from the urethra, whereas others’ urine has more anaerobes, such as Prevotella and Fusobacterium. In terms of overall composition, “we see a lot of parallels to the gut,” says Nelson, noting that there doesn’t seem to be a standout formula for a “healthy” urogenital tract. Commensal microbes within the urethra could make a man more susceptible to infection by supporting colonization by pathogens such as Chlamydia, whereas bacteria that consume the environment’s nutrients could help prevent infection. “We just don’t know at this point,” says Nelson.

On the outside of the penis, circumcision has the largest known influence on the composition of the microbiome. “Men who are uncircumcised have significantly more bacteria on their penis, and the types of bacteria are also very different,” explained Cindy Liu, now a research pathologist at Johns Hopkins Medicine in Baltimore.

In 2010, Lance Price of the Flagstaff, Arizona, office of Translational Genomics Research Institute and his colleagues, including Liu, showed that the base of the penis’s head, or glans, harbored fewer anaerobic bacteria within six months after the men in a study were circumcised (PLOS ONE, 10.1371/journal.pone.0008422, 2010). Last year, the team confirmed its finding in a larger cohort (mBio, 4:e00076-13, 2013). “It really appears that [the penis microbiome] depends on whether you’re circumcised or uncircumcised—different organisms dominate,” says Price.

Some of the anaerobes commonly found on the uncircumcised penis and on occasion inside the male urogenital tract are the same species associated with bacterial vaginosis (BV) in women, says Liu. Deborah Anderson, an OB-GYN and microbiologist at Boston University School of Medicine, and her colleagues have found similar results. “One hypothesis is that the male microbiome might reflect or be related to [his] partner’s microbiome,” says Anderson.

Researchers studying the vagina have for years characterized its microbial community as being dominated by Lactobacillus bacteria, which ferment carbohydrates to lactic acid, yielding a low pH that is toxic to many pathogenic microbes. When levels of Lactobacillus drop, the pH becomes more neutral, and the risk of infections such as BV rises. But with research revealing notable variation among women’s vaginal microbiomes, as well as some interesting dynamics of the microbial communities within a single organ, “that dogma is changing a little bit,” says Gregory Buck of the Vaginal Microbiome Consortium at Virginia Commonwealth University (VCU).

A few years ago, Larry Forney of the University of Idaho, Jacques Ravel of the University of Maryland School of Medicine, and their collaborators published a survey of the vaginal microbiomes of nearly 400 women and found that the majority harbored bacterial communities dominated by one of four Lactobacillus strains (PNAS, 108:4680-87, 2011). More than a quarter of the women studied, however, did not follow this pattern. Instead, their vaginas had fewer lactobacilli and greater numbers of other anaerobic bacteria, although the bacterial communities always included members of genera known to produce lactic acid.

The researchers also found that the composition of a woman’s vaginal microbiome was linked to her ethnicity. Eighty percent of Asian women and nearly 90 percent of white women harbored vaginal microbiomes that were dominated by Lactobacillus, while only about 60 percent of Hispanic and black women did. Vaginal pH varied with ethnicity as well, with Hispanic and black women averaging 5.0 and 4.7, respectively, and Asian and white women averaging 4.4 and 4.2. “There is a racial difference in the vaginal environment and the microbial [community] in parallel,” says Buck.

To complicate matters even further, it is now recognized that the vaginal microbiome is not stable. After menopause, the vagina harbors fewer lactobacilli than during the reproductive stage of women’s lives, with the notable exception of individuals on hormone-replacement therapies. More recent research from Forney, Ravel, and their colleagues has also revealed that the composition of the vaginal microbiome can change in as little as 24 hours. And once again, there are differences among individuals, with some women’s microbiomes appearing to be more stable than others’ (Sci Transl Med, 4:132ra52, 2012).

“In the past we’ve made some generalizations about what kinds of bacteria are found in the vagina, what kinds of bacteria are good or healthy or protective,” says Forney. “What the research is showing is there are tremendous differences between women in terms of the kinds of bacteria that are present and the changes in the communities that occur over time.”—Tracy Vence and Jef Akst

The Maternal Microbiome

© MAN_HALF-TUBE/ISTOCKPHOTO.COM Throughout her training in obstetrics, Kjersti Aagaard was taught that the womb is a sterile sanctuary for baby to develop, and “the only time it’s not is when we have a pathogenic infection,” says Aagaard, who studies the in utero environment of humans and animal models at Baylor College of Medicine and Texas Children’s Hospital. But recent evidence doesn’t seem to support such an idea.

In 2012, Aagaard and her colleagues found that while the vaginal microbiome did change during pregnancy, it didn’t resemble the microbial makeup of newborn babies: the vagina harbored bacterial communities of about 80 percent Lactobacillus, while newborn humans have a relatively greater abundance of other taxa, such as Actinobacteria, Proteobacteria, and Bacteroides (PLOS ONE, 7:e36466, 2012). This suggested that babies aren’t merely painted with vaginal microbes during childbirth, but that bacterial exposure might happen sooner.

A few years earlier, Juan Miguel Rodríguez’s group at the Complutense University of Madrid in Spain had inoculated pregnant mice with labeled bacteria and identified the strain in the meconium (the feces that develop in a fetus) of pups delivered by C-section, similarly implying that an infant’s first meeting with microbes is not at birth (Res Microbiol, 159:187-93, 2008). And in her latest study, Aagaard and colleagues collected placental tissue from 320 mothers immediately after they gave birth and documented a diverse community of microbes that resembled the mother’s oral microbial community more than any other site on the body (Sci Transl Med, 6:237ra65, 2014). “Based on the sum of evidence, it is time to overturn the sterile-womb paradigm and recognize the unborn child is first colonized in the womb,” says Seth Bordenstein of Vanderbilt University.

And then there’s breast milk, which for many decades was also considered sterile, but which is, in fact, a creamy bacterial soup.

The birthing process, then, would be the second stop on a tour of the maternal microbiome. Once on the outside, a baby’s first embrace with his mother is really a group hug with her skin microbiome. And then there’s breast milk, which for many decades was also considered sterile, but which is, in fact, a creamy bacterial soup.

When Rodríguez first began examining breast milk in the 1990s and found evidence that it served as a potential source of microbes in infant feces, many people didn’t believe him. They assumed that his samples were contaminated, “maybe from the mother’s skin or maybe the mouth of baby,” he says, but the bacterial strains he found in breast milk didn’t exist in the mouth or on the skin. And later, his group confirmed that these breast-milk bacteria were finding their way into the infant gut.

In 2011, Mark McGuire of the University of Idaho and his colleagues characterized the microbiome of human breast milk from 16 women and found a diverse community of microbes (PLOS ONE, 6:e21313, 2011). The most abundant bacteria were Streptococcus, Staphylococcus, Serratia, and Corynebacteria, although each woman’s sample was different. “It was very personalized,” says McGuire. “Part of that personalization means she’s sampling her environment and providing that environment to her offspring, and maybe that’s a way to train the immune system and help the infant expand what it’s going to be exposed to early in life.”

In addition to introducing microbes to populate her infant’s gut, a mother’s microbiome during pregnancy and lactation appears to affect her own health. Changes in the gut microbiome during pregnancy correlate with gains in body fat and dips in insulin sensitivity in mice, for example (Cell, 150:470-80, 2012). And several years ago, Rodríguez discovered that the breast-milk microbiomes of women with mastitis, a painful infection of the breast tissue, are characterized by what he calls a “huge dysbiosis”: a single strain of pathogenic bacteria dominating the sample. Providing oral supplements of the missing bacteria helped the women clear the infection. “For the first time we said, ‘Maybe this is important for the treatment of mastitis or painful breast-feeding,’” says Rodríguez, whose team is now wrapping up subsequent trials testing the ability of therapeutic bacteria, rather than antibiotics, to treat mastitis during breastfeeding.—Kerry Grens

Microbial Suit of Armor

© KOWALSKA-ART/ISTOCKPHOTO.COM The skin is characterized by a multiplicity of habitats, including invaginations, appendages, glands, and follicles. Such environmental heterogeneity not surprisingly breeds diversity at the level of the microbiome. The skin is in constant contact with the outside world, making the bacterial communities that populate the skin some of the most varied of human microbiomes. “Between humidity and hygiene approaches and clothing and everything else, [the environment that skin microbes are exposed to] has infinitely more variation,” says Richard Gallo, chief of dermatology at the University of California, San Diego, School of Medicine.

Nevertheless, the skin is not simply covered with a random suite of bacterial species from the environment. Surveys of the bacterial communities that live in and on the skin of healthy adults have revealed three distinct skin microbiomes, each with fairly consistent patterns of microbial composition. The oily, or sebaceous, glands of the head, neck, and trunk—which secrete a mixture of lipids called sebum—are dominated by Propionibacterium species, including P. acnes, which is associated with blemishes. Moist sites, such as the crease of the elbow, below a woman’s breasts, or between the toes, are frequented by genus Corynebacterium. And the dry surfaces of the body, the uncreased expanses of skin such as the forearm or leg, are home to Staphylococcus species, in particular S. epidermidis.

While causal links between the skin’s commensal microbes and health or disease remain to be demonstrated, the evidence that has accumulated in the past few years paints a suggestive picture. Recent research has begun to document how skin commensals interact with one another, with pathogenic microbes, and with human cells. Staphylococcus epidermidis secretes antimicrobial substances that help fight pathogenic invaders, and P. acnes uses the skin’s lipids to generate short-chain fatty acids that can similarly ward off microbial threats. Meanwhile, these and other skin microbes can impact the local molecular environment, and may be able to alter the behavior of human immune cells.

“The field is exploding in terms of the types of observations that have been made,” says Gallo, “and they’re reaching into every aspect of immunology.”

Recently, molecular microbiologist Gitte Julie Christensen of Aarhus University in Denmark and her colleagues found that the P. acnes strains associated with healthy skin carry genes for thiopeptides, antimicrobial compounds that inhibit the growth of gram-positive species. Strains associated with acne, on the other hand, don’t appear to produce such compounds. In culture, Christensen says, “we can see that these health-associated strains are much better at killing other bacteria than the other strains.”

S. epidermidis itself plays a notable role in host immunity. In 2009, Gallo and colleagues showed that the species secreted lipoteichoic acid, which prevents inflammatory cytokine release from keratinocytes of human skin (Nat Med, 15:1377-82, 2009), and in 2012, Yasmine Belkaid of the National Institute of Allergy and Infectious Disease and her colleagues demonstrated that the addition of S. epidermidis to the skin of germ-free mice altered T-cell function to boost host immunity (Science, 337:1115-19, 2012). “We were able to show that these microbes were sufficient to make the immune system of the skin capable to control infection,” says Belkaid.

Another hint of the skin microbiome’s involvement in immunity came last October, when National Cancer Institute dermatologist Heidi Kong and her colleagues found that immunodeficient patients tended to have more “permissive” skin (Genome Res, doi: 10.1101/gr.159467.113, 2013). That is, people with primary immunodeficiencies harbored more bacterial and fungal communities, including species not normally found on healthy adults. “It’s possible that the focal defects in the immune system allow or permit these otherwise uncommon bacteria and fungi to be present on these patients,” says Kong.

As the importance of the skin microbiome in health and disease is further investigated, researchers are also looking into the possibility of manipulating it. With all our modern changes in lifestyle, diet, hygiene practices, and more, “we have dramatically altered our skin microbiota,” says Belkaid. “I think anything we can do to restore more balance or more appropriate microbe composition in the skin, as in all the different tissues, is extremely important.” —Jef Akst

Oxygen Required for Metabolism

Metabolism is a generic term for all the chemical reactions that break down or "burn" food to provide energy for the operation of an organism. The word "burn" is used advisedly, because the energy yield from a food in the human metabolic process is comparable to the energy obtained by actual combustion. The energy available from a food is commonly stated in dietary Calories, and the Calorie rating of a food may actually be obtained by burning it in a pure oxygen atmosphere in a calorimeter to measure the energy yield from this combustion.

Like ordinary combustion, the metabolism of food requires a supply of oxygen and produces carbon dioxide as a combustion product. For various foods one can state a representative energy yield, an amount of oxygen required, and an expected amount of carbon dioxide released. Here are some values from Nelson.

Food Energy released
Oxygen required
liters O2/g
Carbohydrate 4.1 0.81 0.81
Fat 9.3 1.96 1.39
Protein 4.0 0.94 0.75
Alcohol 7.1 1.46 0.97

Note that the amount of energy produced for the four types of food is roughly proportional to the amount of oxygen use, so that the metabolic rate can be measured by measuring the rate of oxygen consumption. However, the amount of carbon dioxide produced by the four types of food is different, so the ratio CO2/O2 provides some information about the type of food being utilized.

The average for the three types of food in the above table is 4.7 kcal of energy release for each liter of oxygen consumed. On the average, an adult at rest consumes about 16 liters of oxygen per hour. This gives a nominal basal metabolic rate of 75 kcal/hr which translates to 87 watts.

Crucial to the metabolic process is the molecule adenosine triphosphate (ATP), considered by biologists to be the energy currency of life. Almost every process in the body that uses energy gets it from ATP, and in the process converts it to ADP. The energy from the oxidation of food (metabolism) is used to convert the ADP back to ATP, making the energy available for body processes. One of the main pathways for doing so is in the oxidation of glucose.