Chapter 12: Protein

An Introduction to Protein

Protein makes up approximately 20 percent of the human body and is present in every cell. The word protein is a Greek word, meaning “of utmost importance.” Proteins provide the body with structure and perform many functions. You can stand, walk, run, skate, and swim, because of protein-rich muscles. Proteins are necessary for proper immune system function, digestion, hair, and nail growth. It is estimated that humans have more than one hundred thousand different proteins! This chapter describes the components of proteins, important roles of protein, risks and consequences of consuming too much or too little protein, and where to find healthy sources of protein in food.

Like carbohydrates and lipids, proteins are composed of elements carbon, hydrogen, and oxygen. Importantly, proteins also contain nitrogen; they are the only macronutrient examined here with this important element. (Nucleic acids, DNA and RNA, also contain nitrogen but they are not generally considered among the nutritional macronutrients.)

Proteins are macromolecules composed of amino acid building blocks, which form chains like beads on a string. Protein molecules have different structures and functions based on the amino acid sequences. Each amino acid consists of a central carbon atom connected to a side chain, a hydrogen, a nitrogen-containing amino group, and a carboxylic acid group—hence the name “amino acid.” (Figure 12.1) Amino acids differ from each other by which specific side chain is bonded to the carbon center.

 

It’s All in the Side Chain

The side chain of an amino acid, also called the “R” group, can be as simple as one hydrogen or as complex as a six-carbon ring. Humans use twenty different types of amino acids which are classified into four groups: nonpolar, polar, acidic, and basic (Figure 12.2).

Essential and Nonessential Amino Acids

Eleven of the twenty types of amino acids are synthesized by the human body. These are called nonessential amino acids (Table 12.1). The nine amino acids we cannot synthesize in sufficient amounts, essential amino acids, must be obtained from the diet. Under some conditions including infancy, periods of rapid growth, illness or healing, the body cannot synthesize enough of one or more nonessential amino acids. These are called conditionally essential amino acids. See the following list:

• Essential amino acids: Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, Valine
• Nonessential amino acids (conditionally essential marked with asterisk): Alanine, Arginine*, Asparagine, Aspartic acid, Cysteine*, Glutamic acid, Glutamine*, Glycine*, Proline*, Serine, Tyrosine*

The plants, animals, and fungi that we eat are also made of proteins. All organisms have slightly different amino acid profiles in their proteins. Because we need to eat the essential amino acids, foods that contain more of these have high nutritional value to us.

Types of Proteins

The variety of one hundred thousand different proteins in the human body is possible because the twenty different amino acids combined in different sequences. A chain of amino acids is called a polypeptide (poly- meaning “many” and -peptide from the type of bond formed between amino acids). Polypeptide chains fold into three-dimensional shapes; this folded form is called a protein. (Figure 12.3) Polypeptide chains differ in sequence and in length; the average length of a human polypeptide is about 400 amino acids. The hormone insulin, which regulates blood glucose, is composed of only fifty-one amino acids; whereas collagen, a protein that acts like glue between cells, consists of more than one thousand amino acids. Titin is the largest known protein. It gives strength to muscles and consists of more than twenty-five thousand amino acids!

Sequence variation of proteins is possible because the twenty amino acids can be arranged in any order. Think about how all music is derived from the notes C, D, E, F, G, A, B, and the half-steps between them. There is great variety in music and songs, all from arranging the notes in different orders. Similarly, the twenty amino acids can be linked together in different sequences. For example, if an amino acid sequence for a protein is 104 amino acids long, the possible combinations of amino acid sequences is equal to 20¹⁰⁴, which is 2 followed by 135 zeros! In reality, humans have approximately 20,000 protein-coding genes. Because genes can code for more than one protein, educated estimates for the total number of protein types in the human body range from 30,000 to 100,000.^[1]

Building Proteins with Amino Acids

Building proteins requires three processes: transcription, translation, and protein folding (Figure 12.4). Each is a complex, multi-step process. Transcription involves copying information from DNA into a messenger RNA (mRNA) molecule. The mRNA carries the genetic information in a form that can be read by ribosomes, the cells’ protein-manufacturing structures. In the process called translation, a polypeptide chain is made by the ribosome, using information in the mRNA. Amino acids are linked in a chain in the correct order to make the protein. The peptide bond forms between the carboxylic acid group of one amino acid and the amino group of another, releasing a molecule of water (Figure 12.3).

The third step, protein folding, is based on the amino acid sequence and the environment of the cell (temperature, pH, salt content). When all are correct, the polypeptide spontaneously folds into a particular shape. Changes in amino acid sequence can lead to changes in shape, with possible change in the function of the protein. Some changes are associated with genetic diseases such a phenylketonuria and sickle cell disease. Other changes may improve the protein’s function, or lead to variants such as pigment variations in hair or eyes.

 

Protein Organization

Proteins are like carbohydrates and lipids in that they are polymers of smaller units, but proteins are more complex because of the variety in the smaller units. There are four levels of protein structure. (Figure 12.5)

Primary structure is the one-dimensional sequence of amino acids held together by peptide bonds. Carbohydrates and lipids also are one-dimensional sequences of monomers; they may be branched, coiled, fibrous, or globular, but their sequence is more random.

Secondary structure depends on the chemical interactions between amino acids, which cause the protein to fold into three-dimensional shapes including a helix (like a coiled spring) or sheet.

Tertiary structure is the three-dimensional folding based on chemical interactions between the side chains (R groups) of amino acids. They either repel or attract each other, resulting in the folded structure. Thus, the specific sequence of amino acids directs the protein to fold into a specific, organized shape.

Quaternary structure refers to the combination of more than one polypeptide or protein into a larger functional protein. Hemoglobin is a protein with an important quaternary structure. It is composed of four peptides that bond together to form a functional oxygen carrier.

The structure of proteins in food affects how easy it is to digest. Large fibrous proteins are more difficult to digest than smaller proteins; some, such as keratin, are indigestible. If digestion is not complete, some amino acids may not be absorbed, which decreases the nutritional value of that protein.

Reference

1. Kim MS, Pinto SM, Getnet D, et al.. A draft map of the human proteome. Nature. 2014 May 29;509(7502):575-81. doi: 10.1038/nature13302. PMID: 24870542; PMCID: PMC4403737.

Proteins in Foods: Cooking and Denaturation

Proteins provide food with structure and texture and binds water. Proteins foam when agitated; whisking egg white proteins create bubbles, which are useful in meringue and angel food cake. Yogurt is thicker than the milk it is made of because of the coagulation of milk protein called casein. Cooked proteins add color and flavor to foods. As amino group binds with carbohydrates during cooking, the brown color of cooked meat, sautéed onions and baked bread is produced. The proteins in cake batter aggregate into a network during mixing and baking that gives cake structure.

When proteins are heated or exposed to acid, high salt, alcohol, or mechanical agitation, they are denatured. Weak chemical forces that hold quaternary, tertiary, and secondary protein structures together are disrupted, and the complicated folded structures unravel, returning it to a long strand of amino acids. The unraveled strands may stick together, forming an aggregate (or network). Denatured proteins are no longer functional.

Protein Digestion and Absorption

How are proteins from foods processed into amino acids that cells can use to make new proteins? The digestive system breaks protein into amino acids, which are absorbed into the bloodstream and made available to cells (Figure 12.7). Eggs are a good dietary source of protein and will be used as our example to describe protein digestion and absorption. An egg supplies about six grams of protein.

Mouth to Stomach

The first step in digestion involves chewing, mechanical breakdown into pieces that can be swallowed. The salivary glands provide saliva to aid swallowing and the passage of the partially mashed egg through the esophagus. The mashed egg pieces enter the stomach through the gastroesophageal sphincter. The stomach releases gastric juices containing hydrochloric acid and the enzyme, pepsin, which initiate the breakdown of the protein. The acidity of the stomach denatures, or unfolds, three-dimensional structure and disrupts protein aggregates formed during cooking. Pepsin, secreted by the cells that line the stomach, clips peptide bonds, to dismantles the protein chains into smaller and smaller fragments. Egg proteins are large globular molecules, and their chemical breakdown requires time and mixing. The powerful mechanical stomach contractions churn the partially digested protein into chyme. Protein digestion in the stomach takes longer than carbohydrate digestion, but less time than fat digestion. High-protein meals are slow to pass from stomach to small intestine, making you feel full longer.

Stomach to Small Intestine

The stomach empties the chyme containing denatured protein into the small intestine, where most protein digestion occurs. The pancreas secretes digestive juice that contains more enzymes (called proteases) that break down the protein fragments. The two major pancreatic proteases are chymotrypsin and trypsin. Cells that line the small intestine release additional enzymes that clip protein fragments into individual amino acids.

Muscle contractions of the small intestine mix and propel the digested proteins to the absorption sites in the lower parts of the small intestine. Amino acids are transported from the intestinal lumen through the intestinal cells to the blood using transport proteins and the cellular energy molecule, adenosine triphosphate (ATP). Once amino acids are in the blood, they are transported to the liver. As with other macronutrients, the liver is the checkpoint for amino acid distribution and further breakdown. Amino acids contain nitrogen; when amino groups are removed, the result is nitrogen-containing ammonia, a toxic compound. The liver detoxifies it by transforming it into urea, which contains two nitrogen atoms and is highly soluble in water. Urea is transported to the kidney and excreted in urine. However, most amino acids from food are not broken apart, but are re-used to make proteins in the body.

Amino Acids Are Recycled

Amino acids are recycled to make new proteins. Since cells continually break down proteins and build new ones, over 250 grams of protein in the body are dismantled daily and 250 grams of new protein are built. Amino acids from food and those from protein destruction which are now available for use are called the amino acid “pool” (Figure 12.8) When an amino acid is required to build another protein, it can be acquired from the amino acids in the body. It is important to maintain a healthy amino acid pool by consuming high-quality proteins; if dietary intake is too low, amino acids for new proteins will be obtained from other tissues within the body, especially muscle. This amino acid pool is less than one percent of total body-protein content. The body does not store protein as it does with carbohydrates (as glycogen in the muscles and liver) and lipids (as triglycerides in adipose tissue). Amino acids are also used to build other biological molecules containing nitrogen, such as DNA, RNA, some neurotransmitters and other signaling molecules. Protein can be burned to produce energy as well.

Functions of Proteins

Proteins are the “workhorses” of the body and have different sizes and shapes to perform many different functions. Much of the structure of the body depends on protein; they also perform as enzymes, hormones, and antibodies.

Structure and Motion

The most abundant structural protein in the human body is collagen, found in bone, tendons, ligaments, cartilage, skin, and muscle. Collagen is a strong, fibrous protein with a high percentage of two amino acids, glycine, and proline. The shape of these amino acids allows three peptide strands to twist around each other like a rope. These collagen ropes overlap with others, leading to a quaternary structure that makes bones strong and flexible. Collagen fibers in the skin’s dermis provide it with structure; accompanying elastin protein fibrils make it flexible. Pinch the skin on your hand and then let go; collagen and elastin proteins 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. Closely-packed collagen fibrils in tendons and ligaments allow for mechanical movements of bones and muscle and for these tissues to spring back after moving. Another strong, fibrous structural protein is keratin in skin, hair, and nails.

Enzymes

Enzymes are molecules that are important for specific chemical reactions; they provide a site for a chemical reaction and lower the amount of energy and time needed for the reaction (Figure 12.9). Hundreds of thousands of chemical reactions occur in our cells every minute and most rely on enzymes. Enzymes are specific and use only particular substrates that fit into the active site, like the way a lock is opened by a specific key. Nearly every chemical reaction requires a specific enzyme. Fortunately, an enzyme can fulfill its role as a catalyst many times, although eventually it will fall apart and be replaced.

Hormones

Hormones are chemical messages produced by the endocrine glands; most are made from proteins. They travel through the blood to target cells and communicate a message to initiate a specific reaction or cellular process. For example, after a meal, in response to increased blood glucose, the pancreas releases the hormone insulin. Insulin allows the cells to take glucose out of the blood, to use for making energy, store, or build macromolecules. Many hormones turn enzymes on and off, participating in the complex regulation of the body.

Fluid and Acid-Base Balance

An important function of proteins in the body is maintenance of consistent internal conditions, called homeostasis. Fluid balance and proper pH are two of these conditions.

Fluid balance refers to maintaining the distribution of water in the body. If too much water moves into a tissue, the tissue may swell, and cells may die. Drying out, or dehydration, can inhibit a cell’s ability to perform its normal functions. To keep the water evenly distributed between blood and cells, proteins circulate in the blood. The most abundant protein in blood is the butterfly-shaped protein albumin.

Water will travel across membranes from an area of high concentration of water to one of a low concentration. As a result, water moves toward areas that have higher concentrations of solutes, such as salt, protein, or glucose. Albumin’s presence in the blood keeps protein concentration in the blood similar to that in cells, which keeps the fluid levels similar as well.

Protein is also essential in maintaining proper pH balance, the measure of the acidity of fluid. Healthy blood pH is 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. One of these is the circulating albumin. Albumin is slightly acidic, and because it is negatively charged it balances the many positively charged molecules, such as protons (H⁺), calcium, potassium, and magnesium also circulating in the blood. Albumin acts as a buffer against abrupt changes in the concentrations of these molecules, thereby stabilizing blood pH. The protein hemoglobin also participates in acid-base balance by binding and releasing protons.

Transport

Albumin and hemoglobin also play a role in molecular transport. Albumin binds hormones, fatty acids, some vitamins, essential minerals, and drugs, and transports them throughout the circulatory system. Red blood cells contain hemoglobin molecules that bind oxygen in the lungs and transport it to the tissues. The plasma membrane of the cell regulates how substances get into cells. Many of the channels and transporters that move nutrients and other molecules in and out of cells re made of proteins. Some require energy to function.

Protection

Antibodies are immune system molecules which bind to invading bacteria and viruses and signal to the body to destroy them, to keep us from becoming sick. Antibodies have other roles in the body as well. Antibodies are made of two types of protein chains, called the heavy chains and the light chains. This complex quaternary structure has one end with binding sites to identify specific structures on microbial invaders or damaged cells (Figure 12.10).

Antibodies trigger other factors in the immune system to seek and destroy unwanted intruders. Some of these are proteins as well. The enzyme lysozyme secreted in saliva 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.

Wound Healing and Tissue Regeneration

Proteins are involved in the three phases of wound healing: inflammation, proliferation, and remodeling. If you pricked your finger with a needle, the flesh would turn red and become inflamed. Within a few seconds bleeding would stop. The healing process begins with protein hormones such as bradykinin, which dilates blood vessels at the site of injury. A protein in the blood, fibrin, secures platelets that form a clot to stop the bleeding. In the proliferative phase, cells move in and mend the injured tissue by installing fibers of another protein, collagen. The collagen fibers help pull the wound edges together. In the remodeling phase, more collagen is deposited, forming a scar. Scar tissue is only about 80 percent as functional as normal uninjured tissue. If a diet is insufficient in protein, the process of wound healing is markedly slowed.

Tissue regeneration is a continual process throughout the lifespan. Tissue regeneration is the creation of new cells by cell division, which requires many different proteins including structural proteins and enzymes that synthesize them. It differs from wound healing in that regeneration produces a structural and functional copy of the lost tissue. Worn-out tissue is not replaced with scar tissue but with brand new, fully functional tissue. Some cells (such as skin, hair, nails, and intestinal cells) have a very high rate of regeneration. Others, including heart-muscle cells and nerve cells, do not regenerate at appreciable levels. The cells lining the intestine regenerate every three to five days. Protein-inadequate diets impair tissue regeneration, causing many health problems and, visibly, poor hair and nail growth.

Energy Production

Amino acids in proteins can be disassembled and used to make energy (Figure 12.11); about 10 percent of dietary proteins are used this way. The liver can break down amino acids to carbon skeletons which can be fed into the citric acid cycle. This is the same pathway that extracts energy from glucose. If the diet is low in carbohydrates and fat, the body will use amino acids to make energy. This can compromise the synthesis of new proteins and can even destroy muscle proteins. If the diet contains more protein than the body needs, extra amino acids will be broken down and transformed into fat.

Protein Malnutrition

Low protein intake has health consequences; a severe lack of protein in the diet can cause death. Although severe protein deficiency is rare in the United States, it is estimated that more than half of the elderly in nursing homes are protein-deficient. The Acceptable Macronutrient Distribution Range (AMDR) for protein for adults is 10 to 35 percent of kilocalories, a wide range. Malnutrition and its health consequences will occur if the percentage is less than 10 percent; this type of diet is often low in other nutrients as well as total kilocalories. In this section we will discuss the health consequences of low or high protein intake and dietary choices that can lead to consuming the right amount of high-quality protein.

Health Consequences of Protein Deficiency

Severe protein deficiency is a leading cause of death in children in some parts of the world. Two syndromes associated with protein deficiency are kwashiorkor and marasmus.

Kwashiorkor affects millions of children worldwide. When first described in 1935, more than 90 percent of children with kwashiorkor died. Prompt and appropriate treatment are crucial and can lower this outcome. The name kwashiorkor comes from a word in Ghanaian language meaning “rejected one.” The syndrome most commonly occurs in children who have recently been weaned from breastmilk, usually because another child had just been born. If these children are primarily fed a food based on grain (such as wheat, corn, or rice), their protein intake is much lower than when drinking breastmilk. Kwashiorkor is characterized by swelling (edema) of the feet and abdomen, poor skin health, growth retardation, low muscle mass, fatigue, and liver malfunction. Edema, which leads to the characteristic swollen belly appearance, develops when there is insufficient protein in the blood (Figure 12.12). One of the functions of albumin, an important blood protein, is to hold water in the blood vessels. Low concentrations of blood albumin results in water moving out of the blood vessels and into tissues, causing swelling. Severe protein deficiency in addition to other micronutrient deficiencies, such as folate (vitamin B9), iodine, iron, and vitamin C all contribute to the many health manifestations of this syndrome.

Children and adults with marasmus lack both protein and total calories in their diets. Marasmus affects mostly children below the age of one in poor countries. Body weights of children with marasmus may be up to 80% less than that of unaffected children of the same age. Marasmus is a Greek word, meaning “starvation.” The syndrome affects more than fifty million children under age five worldwide. It is characterized by an emaciated appearance, poor skin health, and growth retardation (Figure 12.13). The symptoms are acute fatigue, hunger, and diarrhea.

Kwashiorkor and marasmus often coexist as a combined syndrome termed marasmic kwashiorkor. Children with the combined syndrome have variable amounts of edema and the characteristics of marasmus. Although organ functions are compromised by undernutrition, the ultimate cause of death is often infection. Undernutrition is intricately linked with the immune system; undernourished children commonly die from severe diarrhea and/or pneumonia from bacterial or viral infection. The United Nations Children’s Fund (UNICEF) reports that undernutrition causes at least one-third of deaths of young children (Figure 12.14). In 2008, 26 percent of children under five worldwide were underweight. That percentage of declined less than 5% in the next eighteen years despite the Millennium Development Goal of halving the proportion of people who suffer from hunger by 2015.

 

 

Health Consequences of Too Much Protein in the Diet

Diets considered to be “high in protein” generally derive more than 30 percent of kilocalories from protein. Many people follow high-protein diets because marketers tout protein’s ability to stimulate weight loss. It is true that high-protein diets increase weight loss in some people. However, the number of individuals who remain on this type of diet is low and regaining weight is common. Experts disagree about possible health consequences of long-term high-protein diets. Observational studies suggest that high animal protein intake, especially red meat, are linked to higher risks for kidney stones, kidney disease, liver malfunction, colorectal cancer, and osteoporosis^[1]. However, red meat is also high in saturated fat and cholesterol; further studies are needed to draw firm conclusions.^[2]

Some scientists hypothesize that high-protein diets may accelerate bone-tissue loss because amino acids block absorption of calcium in the gut and promote calcium loss from bone. Results from studies are not consistent. Results from the Nurses’ Health Study suggest that women who eat more than 95 grams of protein each day have a 20 percent higher risk for wrist fracture.^[3]

High-protein diets can restrict other essential nutrients. The American Heart Association (AHA) states that “High-protein diets are not recommended because they restrict healthful foods that provide essential nutrients and do not provide the variety of foods needed to adequately meet nutritional needs. Individuals who follow these diets are therefore at risk for compromised vitamin and mineral intake, as well as potential cardiac, renal, bone, and liver abnormalities overall.”^[4]

As with any nutrient, protein must be eaten in proper amounts. Moderation and variety are key strategies to achieving a healthy diet.

References

1. Chan DS, Lau R, et al. Red and Processed Meat and Colorectal Cancer Incidence: Meta-Analysis of Prospective Studies. PLoS One. 2011; 6(6), e20456.https://doi.org/10.1371/journal.pone.0020456 . Accessed April 11, 2025.
2. Protein – The Nutrition Source. Harvard School of Public Health. https://nutritionsource.hsph.harvard.edu/what-should-you-eat/protein/ Accessed April 11, 2025.
3. Barzel US, Massey LK. Excess Dietary Protein Can Adversely Affect Bone. J Nutr. 1998; 128(6), 1051–53.https://doi.org/10.1093/jn/128.6.1051. Accessed April 11, 2025.
4. St. Jeor ST, et al. Dietary Protein and Weight Reduction: A Statement for Healthcare Professionals from the Nutrition Committee of the Council on Nutrition, Physical Activity, and Metabolism of the American Heart Association. Circulation. 2001; 104, 1869–74. https://doi.org/10.1161/hc4001.096152. Accessed April 11, 2025.

Dietary Choices Regarding Proteins

How Much Protein Does a Person Need?

The Recommended Daily Allowance (RDA) and AMDR for protein for different age groups are listed in Table 12.1. A Tolerable Upper Intake Limit for protein has not been set.

Table 12.1 Dietary Reference Intakes for Protein. Source: Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids. Institute of Medicine. September 5, 2002. Accessed March 3, 2026. Used under fair use guidelines.

Age Group	RDA (or AI*) (g/day)	AMDR (% calories)
Infants (0–6 mo)	9.1*	Not determined
Infants (7–12 mo)	11.0	Not determined
Children (1–3)	13.0	5–20
Children (4–8)	19.0	10–30
Children (9–13)	34.0	10–30
Males (14–18)	52.0	10–30
Females (14–18)	46.0	10–30
Adult Males (19+)	56.0	10–35
Adult Females (19+)	46.0	10–35

Adequate protein intake will replace protein used and excreted. The RDAs for protein were determined by assessing nitrogen balance; this is accurate because nitrogen is found in amino acids but not in carbohydrates or lipids. When amino acids are broken down, nitrogen is released in the form of ammonia. Ammonia is converted to nontoxic, nitrogen-containing urea, transported to the kidneys, and excreted in urine. Some urea is excreted in the feces. Proteins are also lost in sweat and used as hair and nails grow. For healthy adults, the amount of protein needed daily has been calculated as 0.8 grams of protein per kilogram of body weight.

To calculate based on weight expressed in kilograms, for adults use this formula:

To calculate based on weight expressed in pounds, for adults use this formula:

Note that if a person is overweight, the amount of dietary protein recommended can be overestimated; a better estimate uses a healthy weight for a person.

A person is in negative nitrogen balance when the amount of excreted nitrogen is greater than that consumed, indicating that the body is breaking down protein to meet its demands. This state of imbalance can occur in people who have certain diseases, such as cancer or muscular dystrophy. Someone who has a low-protein diet may also be in negative nitrogen balance as they are taking in less protein than they need.

Positive nitrogen balance occurs when nitrogen excreted is less than nitrogen taken in by the diet. This can happen when the body needs protein to build tissue, such as during pregnancy or when a child is growing rapidly. A person healing from severe wounds may be in positive nitrogen balance because protein is needed for tissue repair.

Dietary Sources of Protein

High-protein foods include meat, seafood, poultry, eggs, soy, dry beans, peas, and seeds. According to the Harvard School of Public Health, “When we eat foods for protein, we also eat everything that comes alongside it: the different fats, fiber, sodium, and more. It’s this protein “package” that’s likely to make a difference for health.”^[1]

Protein sources differ in their additional components. Animal-based foods commonly have high amounts of B vitamins, vitamin E, iron, magnesium, and zinc. Seafood often contains healthy fats, but some animal-based protein-rich foods have an unhealthy amount of saturated fat and cholesterol. Plant sources are good sources of fiber, folate, and minerals such as phosphorus, magnesium, and manganese.

For instance, a hamburger patty made from 80 percent lean meat contains 22 grams of protein, 5.7 grams of saturated fat, and 77 milligrams of cholesterol. A burger made from 95 percent lean meat has the same amount of protein, but only about half as much saturated fat. A cup of boiled soybeans contains 29 grams of protein, 2.2 grams of saturated fat, and no cholesterol. For more comparisons of protein-rich foods, see Table 12.2. To find out the complete nutrient package of different foods, search the USDA Food Composition Databases (https://fdc.nal.usda.gov/).

Table 12.2 Sources of Dietary Protein

Food	Protein Content (g)	Saturated Fat (g)	Cholesterol (mg)	Calories
Hamburger patty 3 oz. (80% lean)	22.0	5.7	77	230
Hamburger patty 3 oz. (95% lean)	22.0	2.3	60	139
Top sirloin 3 oz.	25.8	2.0	76	158
Beef chuck 3 oz. (lean, trimmed)	22.2	1.8	51	135
Pork loin 3 oz.	24.3	3.0	69	178
Pork ribs (country style, 1 piece)	56.4	22.2	222	790
Chicken breast (roasted, 1 c.)	43.4	1.4	119	231
Chicken thigh (roasted, 1 thigh)	13.5	1.6	49	109
Chicken leg (roasted, 1 leg)	29.6	4.2	105	264
Salmon 3 oz.	18.8	2.1	54	175
Tilapia 3 oz.	22.2	0.8	48	109
Halibut 3 oz.	22.7	0.4	35	119
Shrimp 3 oz.	17.8	0.2	166	84
Shrimp (breaded, fried, 6–8 pcs.)	18.9	5.4	200	454
Tuna 3 oz. (canned)	21.7	0.2	26	99
Soybeans 1 c. (boiled)	29.0	2.2	0	298
Lentils 1 c. (boiled)	17.9	0.1	0	226
Kidney beans 1 c. (canned)	13.5	0.2	0	215
Sunflower seeds 1 c.	9.6	2.0	0	269

The USDA recommends eating a variety of lean protein-rich foods. Lean meats include round steak, top sirloin, extra lean ground beef, pork loin, and skinless chicken. The USDA suggests eating eight ounces of cooked seafood every week (typically as two 4-ounce servings) to assure people are getting the healthy omega-3 fatty acids that have been linked to a lower risk for heart disease. Plant-based meals can include dry beans (hydrated and cooked, of course), peas, or soy products as a main dish. Familiar menu choices include chili with kidney and pinto beans, hummus on pita bread, and black bean enchiladas. To include nuts, use them to top a salad, add crunch to yogurt, stir-fry, or steamed vegetables. A plant-based diet can provide all the protein you need, with a little planning. Variety increases the probability of consuming all the essential nutrients.

Protein Quality

Completeness: Amino Acid Composition

Human beings need to eat the amino acids that the body cannot make; these essential amino acids are present in different amounts in different types of foods. The amino acid profile of different foods is therefore one component of protein quality. Complete protein contains all the essential amino acids in the proportions needed by the human body. Foods that contain low amounts of one or more of the essential amino acids are called incomplete protein sources. Foods that are complete protein sources include animal foods such as milk, cheese, eggs, fish, poultry, and meat, and a few plant foods, such as soy and quinoa. The only animal-based protein that is not complete is gelatin, which is made of the protein, collagen.

Most plant-based foods are deficient in at least one essential amino acid and therefore are incomplete protein sources. For example, most grains are deficient in the amino acid lysine, and legumes are deficient in methionine or tryptophan. Because grains and legumes are not deficient in the same amino acids, combining them leads to a dish that has all essential amino acids. This combination is called complementation. Examples of complementary protein foods are given in Table 12.3. Complementary protein sources do not have to be consumed at the same time— consumption in the same day is sufficient.

Table 12.3 Complementing Protein Sources the Vegan Way

Foods	Limiting Amino Acids	Complementary Food	Complementary Menu
Legumes	Methionine/Cysteine	Grains, nuts, and seeds	Hummus and whole-wheat pita
Grains	Lysine	Legumes	Cornbread and kidney bean chili
Nuts and seeds	Lysine	Legumes	Stir-fried tofu with cashews

Digestibility

Not all protein sources are equally digested. Most animal-based proteins are easily broken down during digestion, whereas plant-based proteins are not. This is because some proteins are contained in the plant’s fibrous cell walls and these pass through the digestive tract unabsorbed by the body.

Protein Digestibility Corrected Amino Acid Score (PDCAAS)

The PDCAAS is a score protein quality that includes amino acid composition and digestibility. Foods are ranked against milk protein, egg whites, whey, and soy, which all have a ranking of one. Other foods’ ranks are listed in Table 12.4. A low score does not mean that a food is unhealthy or undesirable.

Table 12.4 PDCAAS of Various Foods

Food	PDCAAS* (1 = highest rank, 0 = lowest)
Milk protein	1.00
Egg white	1.00
Whey	1.00
Soy protein	1.00
Beef	0.92
Soybeans	0.91
Chickpeas	0.78
Fruits	0.76
Vegetables	0.73
Whole wheat	0.42

Protein Needs: Special Considerations

Some groups may need to examine how to meet their protein needs more closely than others. We will take a closer look at protein considerations for vegetarians, the elderly, and athletes.

Vegetarians and Vegans

People who follow variations of the vegetarian diet and consume eggs and/or dairy products can meet their protein requirements by consuming adequate amounts of these foods. Vegetarians and vegans (those who do not consume animal products) can attain their recommended protein intakes by focusing on high-quality plant-based protein sources. Certain amino acids are low in plant-based foods; lysine, for example, is low in grains, nuts, and seeds. Legumes (beans, peas, peanuts) are adequate sources of lysine, but may be low in cysteine. Combining different types of foods is important to ensure that all essential amino acids are consumed. Because the digestibility of many plant-based protein sources is lower than animal proteins, eating large enough servings is important.

The Dietary Guidelines Advisory Committee (DGAC) has suggestions for how three plant-based dietary patterns can meet nutrient needs.^[2]

• Plant-based. Fifty percent of protein is obtained from plant foods.
• Lacto-ovo vegetarian. No animal products except eggs and dairy.
• Vegan. No animal products.

These diets are compared to the common dietary pattern of Americans (USDA Base) in Table 12.5, which shows the percentage of the “meat and beans group” of foods that come from different sources. For example, a lacto-ovo vegetarian diet includes more eggs, soy products, nuts, seeds, dry beans, and peas than the base diet, to make up for the protein missed by avoiding meat and fish.

Those who choose to obtain proteins solely from plants should include foods fortified with vitamins B12, D, and calcium. Other nutrients of concern may be omega-3 fatty acids and choline. If guidelines are followed, these dietary patterns do not significantly affect nutrient adequacy.

Table 12.5 Percentage of “Meat and Beans Group” Components in the USDA Base Diet, and Three Vegetarian Variations. *The dry beans and peas are in the vegetable food group of the base diet. Source:  Vegetarian Food Patterns: Food Pattern Modeling Analysis. US Department of Agriculture. Appendix E-3.3. Accessed September 28, 2017. Public domain.

Food Category	USDA Base (%)	Plant-Based (%)	Lacto-Ovo Vegetarian (%)	Vegan (%)
Meats	44.6	10.5	0	0
Poultry	27.9	8.0	0	0
Fish (high omega-3)	2.2	3.0	0	0
Fish (low omega-3)	7.1	10.0	0	0
Eggs	7.9	7.6	10.0	0
Soy products	0.9	15.0	30.0	25.0
Nuts and seeds	9.4	20.9	35.0	40.0
Dry beans and peas	n/a*	25.0	25.0	35.0
Total	100.0	100.0	100.0	100.0

The Elderly

Muscle mass declines with age, in a process called sarcopenia. A person is sarcopenic when muscle mass is significantly lower than average for a healthy person of the same age. This is associated with weakness, movement disorders, and a poor quality of life. It is estimated that half the US population of men and women above the age of eighty are sarcopenic. A review published in the September 2010 issue of Clinical Intervention in Aging states that higher intakes (1.2 to 1.5 grams per kilogram of weight per day) of high-quality protein may prevent aging adults from becoming sarcopenic.^[3]

Currently, the RDA for protein for elderly persons is the same as for the rest of the adult population, but clinical trials focused on the amount of protein needed to prevent significant loss of muscle mass in older adults may lead to changes in this recommendation.

Athletes

Muscle tissue is rich in protein and has a high turnover rate. During extended exercise, muscle tissue is broken down and some amino acids used as fuel. To avoid using muscle protein for energy, dietary protein is important. Intense exercise such as strength training, stresses muscle tissue so that the body adapts and builds bigger, stronger, and healthier muscle tissue. The body requires protein post-exercise to accomplish this, however, eating excessive protein does not further stimulate muscle-protein synthesis. Nutrition experts recommend that athletes consume protein within one hour after exercise, along with water and carbohydrates at a ratio of 4 grams of carbohydrates to 1 gram of protein.

The RDA for protein is not higher for athletes, but the Academy of Nutrition and Dietetics, the American College of Sports Medicine, and Dietitians of Canada recommend higher protein intake for athletes. 1.2 to 2.0 grams of protein per kilogram. The lower end of the range is recommended for endurance athletes and the higher end of the range for strength athletes.^[4]

For an athlete weighing 170 pounds, calculate the protein range needed as follows.

This much protein yields 372 – 616 kcals. For a 3,000-kcal diet, this represents 12 to 21% of the kcals, well within the AMDR of 10 – 35% for protein.

Table 12.6 Snacks for Exercise Recovery

Foods	Protein (g)	Carbohydrates (g)	Calories
Whole grain cereal with nonfat milk	14	53	260
Medium banana with nonfat milk	10	39	191
Power bar	10	43	250

Protein Supplements

Protein supplements include powders rich in amino acids derived from whey, soy, pea protein or other protein sources. Because the need for high amounts of protein is rare, what are appropriate uses for these products? Protein supplements have not been shown to improve exercise performance or increase strength.^[5] Average protein consumption for Americans in the years 2001-2014 was reported to be 14 – 16% of daily kcals, which is above the minimum recommendation but is not high.^[6]

Many physically active individuals use protein or amino acid supplements. Branched-chain amino acids, such as leucine, are often touted as a way to build muscle tissue and enhance athletic performance.^[7] High-quality protein foods, however, are a good source of branched-chain amino acids; a 3-ounce serving of chicken breast (approximate 2021 price = $0.40) contains the amount of branched-chain amino acids in an 8-gram “serving” of BCAA powder (approximate 2021 price = $0.70). Food sources can frequently provide the amino acids plus additional nutrients at a lower cost.

There is evidence that high-quality dairy proteins (casein and whey) and soy proteins positively impact muscle recovery after hard training. The recommendation is to drink a protein shake containing 20 to 25 grams of protein after an intense workout. High-quality protein foods will give the same benefits and provide other essential nutrients.

What about the numerous protein shakes and protein bars on the market? Read the label, be selective, and don’t use them to replace meals, but rather as exercise-recovery snacks now and then. Some protein bars have high amounts of added sugars. A low-cost alternative after an intense workout is a peanut butter sandwich on whole-grain bread with sliced banana.

References

1. Protein – The Nutrition Source. https://nutritionsource.hsph.harvard.edu/what-should-you-eat/protein/ Accessed April 11, 2025.
2. Jacobs DR, et al. Food, Plant Food, and Vegetarian Diets in the US Dietary Guidelines: Conclusions of an Expert Panel. Am J Clin Nutr. 2009; 89(5). https://doi.org/10.3945/ajcn.2009.26736C.
3. Waters DL, et al. Advantages of Dietary, Exercise-Related, and Therapeutic Interventions to Prevent and Treat Sarcopenia in Adult Patients: An Update. Clin Interv Aging. 2010; 5, 259–70. https://pmc.ncbi.nlm.nih.gov/articles/PMC2938033/  
4. American College of Sports Medicine, Academy of Nutrition and Dietetics, and Dietitians of Canada. Joint Position Statement: Nutrition and Athletic Performance. Med Sci Sports Exerc. 2009; 41(3), 709-31. http://journals.lww.com/acsm-msse/fulltext/2009/03000/Nutrition_and_Athletic_Performance.27.aspx. Accessed September 28, 2017.
5. American College of Sports Medicine, Academy of Nutrition and Dietetics, and Dietitians of Canada. Joint Position Statement: Nutrition and Athletic Performance. Med Sci Sports Exerc. 2009; 41(3), 709-31. http://journals.lww.com/acsm-msse/fulltext/2009/03000/Nutrition_and_Athletic_Performance.27.aspx.
6. Berryman CE, Lieberman HR, Fulgoni VL 3rd, Pasiakos SM. Protein intake trends and conformity with the Dietary Reference Intakes in the United States: Analysis of the National Health and Nutrition Examination Survey, 2001-2014. Am J Clin Nutr. 2018 Aug 1;108(2):405-413. doi: 10.1093/ajcn/nqy088. PMID: 29931213.
7. Gleeson, M. Interrelationship between Physical Activity and Branched-Chain Amino Acids. J Nutr. 2005; 135(6), 1591S–5S. https://doi.org/10.1093/jn/135.6.1591S. Accessed October 1, 2017.

Attributions

Adapted by Pattie S. Green, Ph.D. and Jonathan E. Pottle, Ph.D. at Tacoma Community College from Human Nutrition by University of Hawai‘i at Mānoa Food Science and Human which is licensed under a Creative Commons Attribution 4.0 International License. Specifically, the following parts were adapted here:

• Chapter 6. Protein Introduction
• Defining Protein
• Protein Digestion and Absorption
• Protein’s Functions in the Body
• Diseases Involving Protein
• Proteins, Diet, and Personal Choices