Proteins are the “workhorses” of the body and participate in many bodily functions. As we’ve already discussed, proteins come in all sizes and shapes, and each is specifically structured for its particular function. This page describes some of the important functions of proteins. As you read through them, keep in mind that synthesis of all of these different proteins requires adequate amounts of amino acids. As you can imagine, consuming a diet that is deficient in protein and essential amino acids can impair many of the body’s functions. (More on that later in the unit.)
Figure 6.9. Examples of proteins with different functions, sizes, and shapes.
Major types and functions of proteins are summarized in the table below, and the subsequent sections of this page give more detail on each of them.
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Protein Types and Functions |
||
|
Type |
Examples |
Functions |
|
Structure |
Actin, myosin, collagen, elastin, keratin |
Give tissues (bone, tendons, ligaments, cartilage, skin, muscles) strength and structure |
|
Enzymes |
Amylase, lipase, pepsin, lactase |
Digest macronutrients into smaller monomers that can be absorbed; performs steps in metabolic pathways to allow for nutrient utilization |
|
Hormones |
Insulin, glucagon, thyroxine |
Chemical messengers that travel in blood and coordinate processes around the body |
|
Fluid and acid-base balance |
Albumin, hemoglobin |
Maintains appropriate balance of fluids and pH in different body compartments |
|
Transport |
Hemoglobin, albumin, protein channels, carrier proteins |
Carry substances around the body in the blood or lymph; help molecules cross cell membranes |
|
Defense |
Collagen, lysozyme, antibodies |
Protect the body from foreign pathogens |
Table 6.2. Protein types and functions
Structure
More than one hundred different structural proteins have been discovered in the human body, but the most abundant by far is collagen, which makes up about 6 percent of total body weight. Collagen makes up 30 percent of bone tissue and comprises large amounts of tendons, ligaments, cartilage, skin, and muscle. Collagen is a strong, fibrous protein made up of mostly glycine and proline amino acids. Within its quaternary structure, three protein strands twist around each other like a rope and then these collagen ropes overlap with others.
Figure 6.10. Triple-helix structure of collagen
This highly ordered structure is even stronger than steel fibers of the same size. Collagen makes bones strong but flexible. Collagen fibers in the skin’s dermis provide it with structure, and the accompanying elastin protein fibrils make it flexible. Pinch the skin on your hand and then let go; the collagen and elastin proteins in skin allow it to go back to its original shape. Smooth-muscle cells that secrete collagen and elastin proteins surround blood vessels, providing the vessels with structure and the ability to stretch back after blood is pumped through them. Another strong, fibrous protein is keratin, an important component of skin, hair, and nails.
Enzymes
Enzymes are proteins that conduct specific chemical reactions. An enzyme’s job is to provide a site for a chemical reaction and to lower the amount of energy and time it takes for that chemical reaction to happen (this is known as “catalysis”). On average, more than 100 chemical reactions occur in cells every single second, and most of them require enzymes. The liver alone contains over 1,000 enzyme systems. Enzymes are specific and will use only particular substrates that fit into their active site, similar to the way a lock can be opened only with a specific key. Fortunately, an enzyme can fulfill its role as a catalyst over and over again, although eventually it is destroyed and rebuilt. All bodily functions, including the breakdown of nutrients in the stomach and small intestine, the transformation of nutrients into molecules a cell can use, and building all macromolecules, including protein itself, involve enzymes.
Figure 6.11. Enzymes are proteins. An enzyme’s job is to provide a site for substances to chemically react and form a product, and decrease the amount of energy and time it takes for this to happen.
VIDEO: “Enzymes,” by Amoeba Sisters, YouTube (August 28, 2016), 5:46 minutes. This video demonstrates the action of enzymes.
Hormones
Proteins are responsible for hormone synthesis. Hormones are the chemical messengers produced by the endocrine glands. When an endocrine gland is stimulated, it releases a hormone. The hormone is then transported in the blood to its target cell, where it communicates a message to initiate a specific reaction or cellular process. For instance, after you eat a meal, your blood glucose levels rise. In response to the increased blood glucose, the pancreas releases the hormone insulin. Insulin tells the cells of the body that glucose is available and to take it up from the blood and store it or use it for making energy or building macromolecules. A major function of hormones is to turn enzymes on and off, so some proteins can even regulate the actions of other proteins. While not all hormones are made from proteins, many of them are.
Fluid and Acid-Base Balance
Adequate protein intake enables the basic biological processes of the body to maintain homeostasis (constant or stable conditions) in a changing environment. One aspect of this is fluid balance, keeping water distributed properly in the different compartments of the body. If too much water suddenly moves from the blood into a tissue, the results are swelling and, potentially, cell death. Water always flows from an area of high concentration to an area of low concentration. As a result, water moves toward areas that have higher concentrations of other solutes, such as proteins and glucose. To keep the water evenly distributed between blood and cells, proteins continuously circulate at high concentrations in the blood. The most abundant protein in blood is the butterfly-shaped protein known as albumin. The presence of albumin in the blood makes the protein concentration in the blood similar to that in cells. Therefore, fluid exchange between the blood and cells is not in the extreme, but rather is minimized to preserve homeostasis.
Figure 6.12. The butterfly-shaped protein, albumin, has many functions in the body including maintaining fluid and acid-base balance and transporting molecules.
Protein is also essential in maintaining proper pH balance (the measure of how acidic or basic a substance is) in the blood. Blood pH is maintained between 7.35 and 7.45, which is slightly basic. Even a slight change in blood pH can affect body functions. The body has several systems that hold the blood pH within the normal range to prevent this from happening. One of these is the circulating albumin. Albumin is slightly acidic, and because it is negatively charged it balances the many positively charged molecules circulating in the blood, such as hydrogen protons (H+), calcium, potassium, and magnesium. Albumin acts as a buffer against abrupt changes in the concentrations of these molecules, thereby balancing blood pH and maintaining homeostasis. The protein hemoglobin also participates in acid-base balance by binding hydrogen protons.
Transport
Proteins also play vital roles in transporting substances around the body. For example, albumin chemically binds to hormones, fatty acids, some vitamins, essential minerals, and drugs, and transports them throughout the circulatory system. Each red blood cell contains millions of hemoglobin molecules that bind oxygen in the lungs and transport it to all the tissues in the body. A cell’s plasma membrane is usually not permeable to large polar molecules, so to get the required nutrients and molecules into the cell, many transport proteins exist in the cell membrane. Some of these proteins are channels that allow particular molecules to move in and out of cells. Others act as one-way taxis and require energy to function.
Figure 6.13. Molecules move in and out of cells through transport proteins, which are either channels or carriers.
VIDEO: “The Sodium-Potassium Pump,” by RicochetScience, YouTube (May 23, 2016), 2:26 minutes. This tutorial describes how the sodium-potassium pump uses active transport to move sodium ions (Na+) out of a cell, and potassium ions (K+) into a cell.
Immunity
Proteins also play important roles in the body’s immune system. The strong collagen fibers in skin provide it with structure and support, but it also serves as a barricade against harmful substances. The immune system’s attack and destroy functions are dependent on enzymes and antibodies, which are also proteins. For example, an enzyme called lysozyme is secreted in the saliva and attacks the walls of bacteria, causing them to rupture. Certain proteins circulating in the blood can be directed to build a molecular knife that stabs the cellular membranes of foreign invaders. The antibodies secreted by white blood cells survey the entire circulatory system, looking for harmful bacteria and viruses to surround and destroy. Antibodies also trigger other factors in the immune system to seek and destroy unwanted intruders.
VIDEO: “Specific Immunity, Antibodies,” by Carpe Noctum, YouTube (December 11, 2007), 1 minute. Watch this video to observe how antibodies protect against foreign intruders.
Energy Production
Some of the amino acids in proteins can be disassembled and used to make energy. Only about 10 percent of dietary proteins are catabolized each day to make cellular energy. The liver is able to break down amino acids to the carbon skeleton, which can then be fed into the citric acid or Krebs cycle. This is similar to the way that glucose is used to make ATP. If a person’s diet does not contain enough carbohydrates and fats, their body will use more amino acids to make energy, which can compromise the synthesis of new proteins and destroy muscle proteins if calorie intake is also low.
Not only can amino acids be used for energy directly, but they can also be used to synthesize glucose through gluconeogenesis. Alternatively, if a person is consuming a high protein diet and eating more calories than their body needs, the extra amino acids will be broken down and transformed into fat. Unlike carbohydrate and fat, protein does not have a specialized storage system to be used later for energy.
Self-Check:
Attributions:
- “Protein Functions”, section 6.4 from the book An Introduction to Nutrition (v. 1.0), CC BY-NC-SA 3.0
Image Credits:
- Fig 6.9. ”Enzyme , antibody, and hormone” from “Protein Functions”, section 6.4 from the book An Introduction to Nutrition (v. 1.0), is licensed under CC BY-NC-SA 3.0
- Table 6.2. “Protein types and functions” by Tamberly Powell is licensed under CC BY-NC-SA 2.0
- Fig 6.10. “Collagentriplehelix” by JWSchmidt is licensed under CC BY-SA 3.0
- Fig 6.11. “Enzyme activity” from “Protein Functions”, section 6.4 from the book An Introduction to Nutrition (v. 1.0), is licensed under CC BY-NC-SA 3.0
- Fig 6.12. “Albumin” by Jawahar Swaminathan and MSD staff is in the Public Domain
- Fig 6.13. “Protein carriers in cell membranes” by LadyofHats, Mariana Ruiz Villarreal is in the Public Domain
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.
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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
