We just learned that our body is composed of billions of cells. To function, these cells need essential nutrients—carbohydrates, proteins, fats, vitamins, and minerals—which we obtain from foods. However, before our cells can access these nutrients, foods need to be broken down or digested into their simplest units, so that the nutrients can be absorbed and enter the bloodstream. Digestion is a complex process that involves many organs and chemicals, as we’ll explore on this page.
An Overview of the Organs Involved in Digestion
The function of the digestive system is to break down the foods you eat, release their nutrients, and absorb those nutrients into the body. Although the small intestine is the workhorse of the system where the majority of digestion and absorption occurs, each of the digestive system organs makes a vital contribution to this process.
The easiest way to understand the digestive system is to divide its organs into two main categories: the gastrointestinal tract (GI tract) and the accessory organs.
- The GI tract is a one-way tube about 25 feet in length, beginning at the mouth and ending at the anus. Between these two points, the GI tract also contains the pharynx, esophagus, stomach, small and large intestines, and the rectum. The small intestine is comprised of three parts: the duodenum, the jejunum, and the ileum. The large intestine, also called the colon, is similarly divided into three sections: the ascending colon, transverse colon, and descending colon. Both the mouth and anus are open to the external environment; thus, food and wastes within the GI tract are technically considered to be outside the body. Only through the process of absorption do the nutrients in food enter into and nourish the body’s “inner space.”
- Accessory organs, despite their name, are critical to the function of the digestive system. They are considered accessory organs since they are not actually part of the intestinal tract itself, but have ducts that deliver digestive juices into the tract to help aid in digestion. There are four accessory organs: the salivary glands, liver, gallbladder, and pancreas. All of these organs secrete fluids containing a variety of chemicals such as enzymes and acids that aid in digestion.
Figure 3.9. An overview of the organs involved in digestion. The parts of the GI tract are highlighted in blue, and the accessory organs are highlighted in yellow.
An Overview of the Digestive Process
The process of digestion includes five main activities: ingestion, mechanical digestion, chemical digestion, absorption, and excretion.
The first of these processes, ingestion, refers to the entry of food into the GI tract through the mouth. There, the food is chewed and mixed with saliva, which contains enzymes that begin breaking down the carbohydrates and lipids in food. Mastication (chewing) increases the surface area of the food and allows for food to be broken into small enough pieces to be swallowed safely.
Food (now called a bolus since it has been chewed and moistened) leaves the mouth when the tongue and pharyngeal muscles propel the bolus into the esophagus. The bolus will travel down the esophagus through an involuntary process called peristalsis. Peristalsis consists of sequential, alternating waves of contraction and relaxation of the smooth muscles in the GI tract, which act to propel food along (Figure 3.10). These waves also play a role in mixing food with digestive juices. Peristalsis is so powerful that foods and liquids you swallow enter your stomach even if you are standing on your head.
Figure 3.10. Peristalsis moves food through the digestive tract with alternating waves of muscle contraction and relaxation.
Digestion includes both mechanical and chemical processes. Mechanical digestion is a purely physical process of making food particles smaller to increase both surface area and mobility. Mechanical digestion does not change the chemical nature of the food. It includes mastication, tongue movements that help break food into smaller bits and mix it with saliva, mixing and churning of the stomach to further break food apart and expose more of its surface area to digestive juices, and peristalsis to help move food along the intestinal tract. Segmentation is also an example of mechanical digestion. Segmentation, which occurs mainly in the small intestine, consists of localized contractions of circular muscle of the GI tract. These contractions isolate small sections of the intestine, moving their contents back and forth while continuously subdividing, breaking up, and mixing the contents. By moving food back and forth in the intestinal tract, segmentation mixes food with digestive juices and facilitates absorption.
Figure 3.11. Segmentation separates chyme and then pushes it back together, mixing it and providing time for digestion and absorption.
In chemical digestion, digestive secretions that contain enzymes start to break down the macronutrients into their chemical building blocks (for example, starch into glucose). Enzymes are chemicals that help speed up or facilitate chemical reactions in the body. They bring together two compounds to react, without undergoing any changes themselves. For example, the main chemical reaction in digestion is hydrolysis.Hydrolysis is the splitting of one molecule into two with the addition of water. For example, the sugar sucrose (a double sugar) needs to be broken down to its building blocks, glucose and fructose (both single sugars), before it can be absorbed. This breakdown happens through hydrolysis, and the enzyme, sucrase, brings together the sucrose molecule and the water molecule to react. This process is illustrated in the following animation.
Video: Enzyme Action and the Hydrolysis of Sucrose by McGraw-Hill Animations, YouTube (June 3, 2017). 1:46 minutes.
Nutrients are of little to no value to the body unless they enter the bloodstream. This occurs through the process of absorption, which takes place primarily within the small intestine. There, most nutrients are absorbed from the lumen (or inside space) of the GI tract into the bloodstream. Larger lipids are absorbed into lymph but eventually enter the bloodstream as well.
In excretion, the final step of digestion, undigested materials are removed from the body as feces. The feces is stored in the rectum until it leaves the body through the anus.
Functions of the Digestive Organs
Now that you have an overview of the digestive organs and the digestive process, let’s discuss in more detail what types of mechanical and chemical digestion take place in each of the organs of the GI tract. Let’s imagine eating a peanut butter and jelly sandwich that contains carbohydrates, proteins, fats, vitamins, and minerals. How does each organ participate in breaking this sandwich down into units that can be absorbed and utilized by cells throughout the body?
Mouth
Ingestion of the peanut butter and jelly sandwich happens in the mouth or oral cavity. This is where mechanical and chemical digestion also begin. Teeth physically crush and grind the sandwich into smaller particles and mix the food particles with saliva. Salivary amylase (a digestive enzyme) is secreted by salivary glands (salivary glands produce saliva which is a mixture of water, enzymes, and other chemicals) and begins the chemical breakdown of carbohydrates in the bread, while lingual lipase (another digestive enzyme) starts the chemical breakdown of triglycerides (the main form of fat in food) in the peanut butter.
Esophagus
The esophagus is a muscular tube that transports food from the mouth to the stomach. No chemical digestion occurs while the bolus is mechanically propelled through this tube by peristalsis.
Stomach
The stomach is an expansion of the GI tract that links the esophagus to the first part of the small intestine (the duodenum). The empty stomach is only about the size of your fist but can stretch to hold as much as 4 liters of food and fluid—more than 75 times its empty volume—and then return to its resting size when empty. An important function of the stomach is to serve as a temporary holding chamber. You can ingest a meal far more quickly than it can be digested and absorbed by the small intestine. Thus, the stomach holds food and secretes only small amounts into the small intestine at a time. (The length of time food spends in the stomach varies by the macronutrient composition of the meal. A high-fat or high-protein meal takes longer to break down than one rich in carbohydrates. It usually takes a few hours after a meal to empty the stomach contents completely into the small intestine.)
When the peanut butter and jelly sandwich enters the stomach, a highly muscular organ, powerful peristaltic contractions help mash, pulverize, and churn it into chyme. Chyme is a semiliquid mass of partially digested food along with gastric juices secreted by cells in the stomach. These gastric juices contain hydrochloric acid, which lowers the pH of the chyme in the stomach. This acidic environment kills many bacteria or other germs that may have been present in the food, and it causes the three-dimensional structure of dietary proteins to unfold. Gastric juices also contain the enzyme pepsin, which begins the chemical breakdown of proteins in the peanut butter and bread. Gastric lipase continues the breakdown of fat from the peanut butter.
Small Intestine
Chyme released from the stomach enters the small intestine, where most digestion and absorption occurs. The small intestine is divided into three parts, all part of one continuous tube: the duodenum, the jejunum, and the ileum.
Once the chyme enters the duodenum (the first segment of the small intestine), the pancreas and gallbladder are stimulated to release juices that aid in digestion. The pancreas (located behind the stomach) produces and secretes pancreatic juices which consist mostly of water, but also contain bicarbonate that neutralizes the acidity of the stomach-derived chyme and enzymes that further break down proteins, carbohydrates, and lipids. The small intestine’s absorptive cells also synthesize digestive enzymes that aid in the breakdown of sugars and proteins.
The gallbladder (a small sac located behind the liver) stores, concentrates, and secretes a fluid called bile that helps to digest fats. Bile is made in the liver and stored in the gallbladder. Bile is an emulsifier; it acts similar to a detergent (that would remove grease from a frying pan) by breaking large fat droplets into smaller fat droplets so they can mix with the watery digestive juices.
Peristalsis and segmentation control the movement and mixing of chyme through the small intestine. As in the esophagus and stomach, peristalsis consists of circular waves of smooth muscle contractions that propel food forward. Segmentation helps to mix food with digestive juices and facilitates absorption.
Nutrient absorption takes place mainly in the latter part of the small intestine, the ileum. The small intestine is perfectly structured for maximizing nutrient absorption. Its surface area is greater than 200 square meters—about the size of a tennis court! The large surface area is due to the multiple levels of folding, villi, and microvilli that cover the internal tissue of the small intestine. Villi are tiny finger-like projections that are covered with enterocytes or absorptive cells. The absorptive cell membrane is made of even smaller projections, called microvilli (Figure 3.12). These microvilli are referred to collectively as the brush border since their appearance resembles the bristles on a brush.
Figure 3.12. Histology of the small intestine. (a) The absorptive surface of the small intestine is vastly enlarged by the presence of circular folds, villi, and microvilli. (b) Micrograph of the circular folds. (c) Micrograph of the villi. (d) Electron micrograph of the microvilli.
Digested nutrients are absorbed into either capillaries or lymphatic vessels contained within each villus. Amino acids (from protein digestion), small fatty acids (from triglyceride digestion), sugars (from carbohydrate digestion), water-soluble vitamins, and minerals are transported from the intestinal cells into the bloodstream through capillaries. The larger fatty acids, fat-soluble vitamins, and other lipids (that are packaged in lipid transport particles) are transported first through lymphatic vessels and then eventually meet up with the blood. Water-soluble nutrients that enter the bloodstream are transported directly to the liver where the liver processes, stores, or releases these nutrients to other body cells.
Figure 3.13. The digestion and absorption of nutrients in the small intestine.
Large Intestine
Most of the nutrients from the peanut butter and jelly sandwich have now been digested and absorbed. Any components that still remain (usually less than ten percent of food consumed) and the indigestible fiber move from the small intestine to the large intestine (colon). A main task of the large intestine is to absorb much of the remaining water. Water is present not only from the solid foods and beverages consumed, but also the digestive juices released by the stomach and pancreas. As water is reabsorbed, liquid chyme becomes a semisolid, referred to as feces. Feces is composed of undigested food residues, unabsorbed digested substances, millions of bacteria, old cells from the lining of the GI tract, inorganic salts, and enough water to let it pass smoothly out of the body.
Feces is stored in the rectum (a temporary holding area) until it is expelled through the anus via defecation. No further chemical breakdown of food takes place in the large intestine except that accomplished by the bacteria that inhabit this portion of the GI tract. There are trillions of bacteria residing in the large intestine (referred to as the bacterial flora), exceeding the total number of cells in the human body. This may seem rather unpleasant, but the great majority of bacteria in the large intestine are harmless and many are even beneficial—facilitating chemical digestion and absorption, improving immune function, and synthesizing vitamins such as biotin, pantothenic acid, and vitamin K.
The figure below summarizes the functions of the digestive organs.
Figure 3.14. Summary of digestion and absorption. Digestion begins in the mouth and continues as food travels through the small intestine. Most absorption occurs in the small intestine.
Video: “The Digestive System” by National Geographic, YouTube (November 26, 2012), 5:07 minutes.
Self-Check:
Attributions:
- “The Digestive System,” unit 23 from J. Gordon Betts, Kelly A. Young, James A. Wise, Eddie Johnson, Brandon Poe, Dean H. Kruse, Oksana Korol, Jody E. Johnson, Mark Womble, Peter DeSaix, Anatomy and Physiology, CC BY 4.0
- University of Hawai‘i at Mānoa Food Science and Human Nutrition Program, “The Digestive System,” CC BY-NC 4.0
Images:
- Figure 3.9. “GI tract and accessory organs” by Alice Callahan is licensed under CC BY 4.0; edited from “Digestive system diagram edit” by Mariana Ruiz, edited by Joaquim Alves Gaspar, Jmarchn is in the Public Domain
- Figure 3.10. “Peristalsis” by J. Gordon Betts, Kelly A. Young, James A. Wise, Eddie Johnson, Brandon Poe, Dean H. Kruse, Oksana Korol, Jody E. Johnson, Mark Womble, Peter DeSaix, Anatomy and Physiology, OpenStax, licensed under CC BY 4.0
- Figure 3.11. “Segmentation” by OpenStax College is licensed under CC BY 3.0
- Figure 3.12. “Histology Small Intestines” by OpenStax College is licensed under CC BY 3.0
- Figure 3.13. “Absorption of Nutrients” by Tamberly Powell is licensed under CC BY 4.0; edited from University of Hawai‘i at Mānoa Food Science and Human Nutrition Program, “The Digestive System,” CC BY-NC 4.0
- Figure 3.14. “Functions of the Digestive Organs” by Tamberly Powell is licensed under CC BY 4.0; edited from “Figure 23.28 Digestion and Absorption” by J. Gordon Betts, Kelly A. Young, James A. Wise, Eddie Johnson, Brandon Poe, Dean H. Kruse, Oksana Korol, Jody E. Johnson, Mark Womble, Peter DeSaix , Anatomy and Physiology, OpenStax, licensed under CC BY 4.0
The concept of energy balance seems simple: Balance the calories you consume with the calories you expend. But many factors play a role in energy intake and energy expenditure. Some of these factors are under our control and others are not. In this section, we define energy balance, look at the different components of energy expenditure, and discuss the variables that influence energy expenditure. We’ll also consider some of the factors that can affect energy intake and consider why energy balance is more complex than it seems.
Energy Balance
Our body weight is influenced by our energy intake (calories we consume) and our energy output (energy we expend during rest and physical activity). This relationship is defined by the energy balance equation:
Energy Balance = energy intake - energy expenditure
When an individual is in energy balance, energy intake equals energy expenditure, and weight should remain stable.
Positive energy balance occurs when energy intake is greater than energy expenditure, usually resulting in weight gain.
Negative energy balance is when energy intake is less than energy expenditure, usually resulting in weight loss.
Energy intake is made up of the calories we consume from food and beverages. These calories come from the macronutrients (carbohydrates, proteins, and fats) and alcohol. Remember that when the body has a surplus of energy, this energy can be stored as fat. When the body has an inadequate supply of food calories to match energy expenditure, it will turn to stored energy (i.e., adipose tissue, glycogen, and some muscle protein) to meet energy demands, resulting in weight loss.
You may see simple rules about energy balance, such as the idea that a pound of fat contains 3,500 calories, so cutting 3,500 calories from your diet will result in a pound of weight loss. This is sometimes extrapolated to say that cutting your caloric intake by 500 calories per day will lead to one pound of weight loss per week, or 52 pounds of weight loss in one year. However, this is a myth, because the body responds to caloric restriction by making adjustments to energy expenditure and resisting changes in body weight, especially over long periods of time.1
Energy balance is complex, dynamic, and variable between individuals—something we’ll explore a bit more later on this page—but it is still a vital concept in understanding body weight. Next, let’s look at the energy expenditure side of the energy balance equation, to see the components that make up energy expenditure and the factors that influence them.
Components of Energy Expenditure
The sum of caloric expenditure is referred to as total energy expenditure (TEE). There are three main components of TEE:
- Basal metabolic rate (BMR)
- Thermic effect of food (TEF)
- Physical activity
Figure 7.3. Components of total energy expenditure include basal metabolism, the thermic effect of food, and physical activity.
1. Basal Metabolic Rate (BMR)
BMR is the energy expended by the body when at rest. These are the behind-the-scenes activities that are required to sustain life. Examples include:
- respiration
- circulation
- nervous system activity
- protein synthesis
- temperature regulation
Basal metabolic rate does not include the energy required for digestion or physical activity.
BMR is usually the largest component of energy expenditure, making up about 60 to 75 percent of total energy output. For example, a sedentary person might need about 1800 calories in a day, with about 1200 of them being for BMR.
Figure 7.4. Components of energy expenditure and their percent contribution to the total in sedentary to moderately active people.
BMR can vary widely among individuals. An individual’s lean body mass—made up of organs, bone, and muscle—is the biggest determinant of BMR, because lean body tissue is more metabolically active than fat tissue. This means that a muscular person expends more energy than a person of similar weight with more fat. Likewise, increasing your muscle mass can cause an increase in your BMR. However, skeletal muscle at rest only accounts for about 18 percent of the total energy expended by lean mass. Most is used to meet the energy needs of vital organs. The liver and brain, for example, together account for nearly half of the energy expenditure by lean mass.
Figure 7.5. Energy expenditure of organs.
BMR depends not only on body composition but also on body size, sex, age, nutritional status, genetics, body temperature, and hormones (Table 7.1). People with a larger frame size have a higher BMR simply because they have more mass. On average, women have a lower BMR than men, because they typically have a smaller frame size and less muscle mass. As we get older, muscle mass declines, and therefore BMR declines as well.
Nutritional status also affects basal metabolism. If someone is fasting or starving, or even just cutting their caloric intake for a diet, their BMR will decrease. This is because the body attempts to maintain homeostasis and adapts by slowing down its basic functions (BMR) to help preserve energy and balance the decrease in energy intake. This is a protective mechanism during times of food shortages, but it also makes intentional weight loss more difficult.
|
Factors That Increase BMR |
Factors That Decrease BMR |
|
Higher lean body mass |
Lower lean body mass |
|
Larger frame size |
Smaller frame size |
|
Younger age |
Older age |
|
Male sex |
Female sex |
|
Stress, fever, illness |
Starvation or fasting |
|
Elevated levels of thyroid hormone |
Lower levels of thyroid hormone |
|
Pregnancy or lactation |
|
|
Stimulants such as caffeine and tobacco |
|
Table 7.1. Factors that impact BMR.
2. Thermic Effect of Food (TEF)
This is the energy needed to digest, absorb, and metabolize the nutrients in foods. It accounts for 5 to 10 percent of total energy expenditure and does not vary greatly amongst individuals.
3. Physical activity
Physical activity is another important way the body expends energy. Physical activity usually contributes anywhere from 15 to 30 percent of energy expenditure and can be further divided into two parts:
- exercise-related activity thermogenesis (EAT)
- non-exercise activity thermogenesis (NEAT)
EAT is planned, structured, and repetitive physical activity with the objective of improving health (participating in a sport like soccer or strength training at the gym, for example).
NEAT is the energy expenditure for unstructured and unplanned activities. This includes daily-living activities like cleaning the house, yard work, shopping, and occupational activities. NEAT also includes the energy required to maintain posture and spontaneous movements such as fidgeting and pacing.2
NEAT can vary by up to 2,000 calories a day for two people of similar size, according to Dr. James Levine, the Mayo Clinic researcher who first coined the term. NEAT may be an important component of obesity prevention and is currently an area of research.
Factors Affecting Energy Intake
Given the importance of energy’s role in sustaining life, it’s not surprising that energy balance is tightly regulated by complex physiological processes. A region in the brain called the hypothalamus is the main control center for hunger and satiety. There is a constant dialogue between our brains and gastrointestinal tracts through hormonal and neural signals, which determine if we feel hungry or full. The hypothalamus can sense nutrient levels in the blood, and when nutrient levels are low, the hunger center is stimulated. Conversely, when nutrient levels are high, the satiety center is stimulated.
Figure 7.6. The hypothalamus, shown in blue, is about the size of an almond and serves as the hunger center of the brain, receiving signals from the gastrointestinal tract, adipose tissue, and blood and signaling hunger and satiety.
Hunger is the physiological need to eat. When the stomach is empty, it contracts and starts to grumble and growl. The stomach’s mechanical movements relay neural signals to the hypothalamus. (Of course, the stomach also contracts when it’s full and hard at work digesting food, but we can’t hear these movements as well because the stomach’s contents muffle the noise.) The stomach is also the main organ that produces and secretes the “hunger hormone,” ghrelin, the only gut hormone found to increase hunger. Ghrelin levels are high before a meal and fall quickly once nutrients are absorbed.3
Appetite is the psychological desire to eat. Satiety is the sensation of feeling full. After you eat a meal, the stomach stretches and sends a neural signal to the brain stimulating the sensation of satiety and relaying the message to stop eating. There are many hormones that are associated with satiety, and various organs secrete these hormones, including the gastrointestinal tract, pancreas, and adipose tissue. Cholecystokinin (CCK) is an example of one of these satiety hormones and is secreted in response to nutrients in the gut, especially fat and protein. In addition to inhibiting food intake, CCK stimulates pancreatic secretions, gall bladder contractions, and intestinal motility—all of which aide in the digestion of nutrients.3
Adipose tissue also plays a role in regulating food intake. Adipose is the primary organ that produces the hormone leptin, and as fat stores increase, more leptin is produced. Higher levels of leptin communicate to the satiety center in the hypothalamus that the body is in positive energy balance. Leptin acts on the brain to suppress hunger and increase energy expenditure. The discovery of leptin’s functions sparked excitement in the research world and the weight loss industry, as it was hypothesized that leptin might be used as a weight loss drug to decrease food intake. In several clinical trials, it was found that people who are overweight or obese are actually resistant to the hormone, meaning their brain does not respond as well to it. Therefore, when you administer leptin to an overweight or obese person, there is generally no sustained effect on food intake.4
Figure 7.7. The structure of the hormone leptin (left), which is primarily produced by adipose tissue. The obese mouse in the photo has a gene mutation that makes it unable to produce leptin, resulting in constant hunger, lethargy, and severe obesity. For comparison, a mouse with normal leptin production is also shown. Such gene mutations are rare, but they serve as a dramatic illustration of the importance of the hormone in signaling energy balance.
The Complexity of Energy Balance
Energy balance seems like it should be a simple math problem, and in fact, it is based on a fundamental truth in physics—the first law of thermodynamics. This law states that energy can’t be created or destroyed; it can only change form. That is, calories that are consumed must go somewhere, and if they aren’t metabolized (which converts caloric energy to heat and work energy), they’ll have to be stored, usually in the form of adipose tissue. What makes energy balance challenging is the reality that both energy intake and energy expenditure are dynamic variables that are constantly changing, in response to each other and overall energy balance.5,6
Let’s first look at the energy intake side. As we’ve already discussed, how much food we eat each day is not just a matter of willpower or self-control. It’s the result of powerful physiological and psychological forces that tell us if we need to eat, or if we’ve had enough. Our brains are hard-wired to seek food if we’re in negative energy balance, an instinct required for survival. This means that if you start to exercise more—increasing your energy expenditure—you will also feel hungrier, because your body needs more fuel to support the increase in physical activity. If you eat fewer calories, perhaps in an effort to lose weight, your stomach will produce more ghrelin, and your adipose tissue will produce less leptin. These and other shifting hormone levels work together to increase hunger and make you focus on obtaining more calories. People who try to gain weight run into the opposite problem. Their leptin levels increase, suppressing hunger. It’s also uncomfortable to eat beyond satiety, and food doesn’t taste as good once you're full.
Even measuring how much energy is consumed is not as simple as you might think. We can measure the caloric content of food from a chemical standpoint, but we can only estimate how much energy a person will absorb from that food. This will depend on how well the food is digested and how well the macronutrients are absorbed—factors which vary depending on the food itself, the digestion efficiency of the person eating it, and even the microbes living in their gut. Two people may eat the exact same meal, yet not absorb the same number of calories.
Energy expenditure is also dynamic and changes under different conditions, including increased or decreased caloric intake. Decreased caloric intake and negative energy balance cause a drop in BMR to conserve energy. Muscles also become more efficient, requiring less energy to work, and without realizing it, people in negative energy balance often decrease their NEAT activity level. These adaptations help to conserve body weight and make it more difficult to stay in negative energy balance. People may still be able to lose weight despite their bodies working to prevent it, but maintaining a new, lower weight requires constant vigilance, and weight regain is common.
Research has also shown that people respond differently to positive energy balance. When a group of people are overfed, the amount of weight gained amongst study participants varies widely. In a study of identical twins who were given an extra 1,000 calories per day for 100 days, weight gain varied between 10 and 30 pounds among participants. Weight gain between twins was more similar (though not exactly the same), which may be attributed to genetic factors.7 People gain and lose weight differently; we don't necessarily follow formulas.
When people say that the answer to weight gain is to eat less and move more, they may be partially correct. But this is also an oversimplified answer, because of all the complexities underlying energy intake and energy expenditure.
Self-check:
References:
- “Balancing Energy Input with Energy Output”, section 11.2 from the book An Introduction to Nutrition (v. 1.0), CC BY-NC-SA 3.0
- 1Webb, D. (2014, November). Farewell to the 3,500-Calorie Rule. Today’s Dietitian, 26(11), 36. https://www.todaysdietitian.com/newarchives/111114p36.shtml
- 2Chun, N., Park, M., Kim, J., Park, H., Hwang, H., Lee, C., Han, J., So, J., Park, J., & Lim, K. (2018). Non-exercise activity thermogenesis (NEAT): a component of total daily expenditure. J Exerc Nutrition Biochem, 22(2), 23–30. doi: 10.20463/jenb.2018.0013
- 3Austin J., & Marks, D. (2008). Hormonal Regulators of Appetite. Int J Pediatr Endocrinol. 2009. doi: 10.1155/2009/141753
- 4Dardeno, T. A. et al. (2010). Leptin in Human Physiology and Therapeutics. Front Neuroendocrinol, 31 (3), 377–93. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2916735/?tool=pubmed
- 5Hall, K. D., & Guo, J. (2017). Obesity Energetics: Body Weight Regulation and the Effects of Diet Composition. Gastroenterology, 152(7), 1718-1727.e3. https://doi.org/10.1053/j.gastro.2017.01.052
- 6Hall, K. D., Heymsfield, S. B., Kemnitz, J. W., Klein, S., Schoeller, D. A., & Speakman, J. R. (2012). Energy balance and its components: Implications for body weight regulation123. The American Journal of Clinical Nutrition, 95(4), 989–994. https://doi.org/10.3945/ajcn.112.036350
- 7Bouchard, C., Tremblan, A.,...Fournier, G. (1990). The response to long-term overfeeding in identical twins. N. Engl. J. Med. 322, 1477-1482.
Images:
- “Energy balance” by Tamberly Powell is licensed under CC BY-NC-SA 2.0 with images used from "Yoga" by Matt Mad is licensed under CC BY-NC-ND 2.0, "Salmon (sustainable fishing), whole grain wild rice, sesame-spinach, avocado, edamame, home-made teriyaki sauce" by Marco Verch is licensed under CC BY 2.0, "Bitten Apple" by DLG Images is licensed under CC BY 2.0, "Exercise" by Andy Cross is licensed under CC BY-NC 2.0, "The Habit: Bacon Cheeseburger" by Person-with-No Name is licensed under CC BY 2.0, and "Watching Gabiera" by Carlos Ebert is licensed under CC BY 2.0
- Fig 7.3. “Components of total energy expenditure” from “Balancing Energy Input with Energy Output,” section 11.2 from the book An Introduction to Nutrition (v. 1.0), CC BY-NC-SA 3.0
- Fig 7.4. “Components of energy expenditure and the percentage they contribute” by Tamberly Powell is licensed under CC BY-NC-SA 2.0
- Fig 7.5. “Energy Expenditure of Organs” by Tamberly Powell is licensed under CC BY-NC-SA 2.0
- Table 7.1. Factors that Impact BMR by Tamberly Powell is licensed under CC BY-NC-SA 2.0
- "Raking Alternative" by Jack Zalium is licensed under CC BY-ND 2.0
- Fig 7.6."The Hypothalamus-Pituitary Complex" by OpenStax College is licensed under CC BY-SA 3.0
- Fig 7.7. "Leptin" by Vossman is licensed under CC BY-SA 3.0; "Fatmouse" by Human Genome wall for SC99 is in the Public Domain
