7.3: Sources and Requirements of Proteins
- Page ID
- 80536
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)How Much Protein Does a Person Need in Their Diet?
Protein recommendations for adults are 0.8 grams of protein for every kilogram of body weight per day (0.36 grams protein per pound).1 Based on these recommendations, the Recommended Daily Allowance (RDA) and AMDR for protein for different age groups are listed in Table \(\PageIndex{1}\). A Tolerable Upper Intake Limit for protein has not been set, but it is recommended that you not exceed the upper end of the AMDR.
| Age Group | RDA (g/day) | AMDR (% calories) |
|---|---|---|
| Infants 0 to 6 months | 9.1* | Not determined |
| Infants 7 to 12 months | 11.0 | Not determined |
| Children 1 to 3 years | 13.0 | 5 to 20% |
| Children 4 to 8 years | 19.0 | 10 to 30% |
| Children 9 to 13 years | 34.0 | 10 to 30% |
| Males 14 to 18 years | 52.0 | 10 to 30% |
| Females 14 to 18 years | 46.0 | 10 to 30% |
| Adult Males 19 years and older | 56.0 | 10 to 35% |
| Adult Females 19 years and older | 46.0 | 10 to 35% |
| * Denotes Adequate Intake | ||
Source: Institute of Medicine.
Figure \(\PageIndex{1}\): "Calculator, Pen and Calendar" by photosteve101 is licensed under CC BY 2.0.
Nitrogen Balance
The appropriate amount of protein in a person’s diet maintains a balance between what is taken in and what is used. The RDAs for protein were determined by assessing nitrogen balance. Nitrogen is one of the four basic elements contained in all amino acids. When proteins and amino acids are are broken down, nitrogen is released. When the liver breaks down amino acids, it produces ammonia, which is rapidly converted to nontoxic, nitrogen-containing urea, which is then transported to the kidneys for excretion. Most nitrogen is lost as urea in the urine, but urea is also excreted in the feces.
Proteins are also lost in sweat and as hair and nails grow. The RDA, therefore, is the amount of protein a person should consume in their diet to balance the amount of protein used up and lost from the body. For healthy adults, this amount of protein was determined to be 0.8 grams per kilogram of body weight.
The IOM used data from multiple studies that determined nitrogen balance in people of different age groups to calculate the RDA for protein (Figure \(\PageIndex{2}\)). A person is said to be in nitrogen balance when the nitrogen input equals the amount of nitrogen used and excreted. A person is in negative nitrogen balance when the amount of excreted nitrogen exceeds that consumed, meaning that the body is breaking down more 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 because they are taking in less protein than they actually need. Positive nitrogen balance occurs when a person excretes less nitrogen than what is taken in by the diet, such as during child growth or pregnancy. At these times, the body requires more protein to build new tissues, so more of what gets consumed gets used up, and less nitrogen is excreted. A person healing from a severe wound may also be in positive nitrogen balance because protein is being used up to repair tissues.
Figure \(\PageIndex{2}\): Nitrogen balance. There are three states of nitrogen balance. When nitrogen input and output are equal, a person is in nitrogen balance (green). If more nitrogen is taken in than excreted, a person is in positive nitrogen balance (orange). If less nitrogen is taken in than excreted, a person is in negative nitrogen balance (red). "Nitrogen Balance" from An Introduction to Nutrition (Zimmerman) is licensed under CC BY-NC-SA 3.0
Dietary Sources of Proteins
Figure \(\PageIndex{3}\): Protein-rich foods include meats, eggs, nuts, and beans. "Protein-rich Foods" by Smastronardo is licensed under CC BY-SA 4.0.
According to MyPlate recommendations from the USDA, the protein food group consists of seafood; meat, poultry, and eggs; beans, peas, and lentils; and nuts, seeds, and soy products.2 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."3 Simply put, different protein sources differ in their additional components, so it is necessary to pay attention to the whole nutrient “package.”
Research suggests that replacing red meats and processed meats with protein-rich foods such as beans, nuts, fish, and poultry can reduce the risk of several diseases, including heart disease and cancer.3 Protein-rich animal-based foods commonly have high amounts of B vitamins, vitamin E, iron, magnesium, and zinc. Seafood often contains healthy fats, and plant sources of protein contain a high amount of fiber. Some animal-based protein-rich foods have an unhealthy amount of saturated fat and cholesterol. As we will discuss later in this chapter, individuals who choose not to eat animal protein can still obtain a complete amount of protein by varying the protein sources in the diet.
When choosing your dietary sources of protein, take note of both the other nutrients and the non-nutrients, such as calories, saturated fat, cholesterol, dyes, and preservatives, to make good selections that will benefit your health. For more comparisons of protein-rich foods, see the USDA data in Table \(\PageIndex{2}\).
| Food | Protein Content (g) | Saturated Fat (g) | Cholesterol (mg) | Calories |
|---|---|---|---|---|
| Hamburger patty, 3 ounces (95% lean) | 22.0 | 2.3 | 60 | 139 |
| Hamburger patty, 3 ounces (80% lean) | 22.0 | 5.7 | 77 | 230 |
| Top sirloin, 3 ounces | 25.8 | 2.0 | 76 | 158 |
| Beef chuck, 3 ounces (lean, trimmed) | 22.2 | 1.8 | 51 | 135 |
| Pork tenderloin, 3 ounces | 22 | 1.0 | 60 | 120 |
| Pork spareribs, 3 ounces | 24 | 9 | 100 | 330 |
| Chicken breast (roasted, 3 ounces) | 25 | 2 | 70 | 170 |
| Chicken thigh (roasted,3 ounces) | 21 | 4.5 | 80 | 210 |
| Chicken leg (roasted, 3 ounces) | 23 | 2.5 | 70 | 180 |
| Salmon, 3 ounces | 18.8 | 2.1 | 54 | 175 |
| Tilapia, 3 ounces | 22.2 | 0.8 | 48 | 109 |
| Halibut, 3 ounces | 22.7 | 0.4 | 35 | 119 |
| Shrimp, 3 ounces | 17.8 | 0.2 | 166 | 84 |
| Shrimp (breaded, fried, 6-8 pieces) | 18.9 | 5.4 | 200 | 454 |
| Tuna, 3 ounces (canned) | 21.7 | 0.2 | 26 | 99 |
| Kidney beans, cooked 1/2 cup | 8 | 0.1 | 0 | 113 |
| Sunflower seeds, 1 cup, edible portion | 9.6 | 2.0 | 0 | 269 |
| Peanut Butter, 2 tablespoons | 7.2 | 3.0 | 0 | 191 |
Tools for Change
The USDA provides some tips and tools for choosing your dietary protein sources. Their motto is “Vary your protein routine.” The overall suggestion is to eat a variety of protein-rich foods to benefit health. The USDA recommends lean meats, such as round steaks, top sirloin, extra lean ground beef, pork loin, and skinless chicken. Additionally, a person should consume 8 ounces of cooked seafood every week (typically as two 4-ounce servings) to ensure they are getting the healthy omega-3 fatty acids that have been linked to a lower risk for heart disease. Another tip is to choose to eat beans, peas, or soy products as a main dish. Some of the menu choices include chili with kidney and pinto beans, hummus on pita bread, and black bean enchiladas. You could also enjoy nuts in a variety of ways. You can put them on a salad or stir-fry or use them as a topping for steamed vegetables instead of meat or cheese. If you do not eat meat, the USDA has much more information on how to get all the protein you need from a plant-based diet. When choosing the best protein-rich foods, pay attention to the whole nutrient package and remember to select from a variety of protein sources to get all the other essential micronutrients.
There are so many options for incorporating a variety of proteins into your diet. Use the links below to investigate your favorite protein sources and find new ways to try healthy proteins in your diet.
- Investigate the whole protein "package" in your favorite foods at the USDA What's In the Foods You Eat Search Tool.
- Find tips and information about protein-rich foods at USDA MyPlate: Protein Foods.
- Try some new, healthy, protein-rich, plant-based foods by visiting the Harvard School of Public Health.
Protein Quality
Although a wide variety of foods contain protein, it differs in quality. High-quality protein contains all the essential amino acids in the proportions the human body needs. The amino acid profile of different foods is, therefore, one component of protein quality. Foods that contain some of the essential amino acids are called incomplete protein sources, whereas those that contain all nine essential amino acids are called complete protein sources, or high-quality 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 consists of the protein collagen.
Figure \(\PageIndex{4}\): Complete and incomplete protein sources. Milk, cheese, eggs, meat, and soy are complete protein sources. Corn, whole wheat bread, and nuts are incomplete protein sources. Image Source: "Edamame" by UnitedSoybeanBoard is licensed under CC BY 2.0. "Milk jug with cottage cheese and eggs on burlap" by wuestenigel is licensed under CC BY 2.0. "Fresh salmon fillet on a cutting board" by Jakub Kapusnak is marked with CC0 1.0. "Raw rib-eye steak of beef with herbs and spices on cutting board" by wuestenigel is licensed under CC BY 2.0. "corn" by Muffet is licensed under CC BY 2.0. "light wheat bread" by Stacy Spensley is licensed under CC BY 2.0. "STOP & SHOP Deluxe Mixed Nuts" by s58y is licensed under CC BY 2.0.
Most plant-based foods are deficient in at least one essential amino acid and, therefore, are incomplete protein sources. For example, grains are usually deficient in the amino acid lysine, and legumes do not contain methionine or tryptophan. Because grains and legumes are not deficient in the same amino acids, they can complement each other in a diet. Incomplete protein foods are called complementary foods because, when consumed in tandem, they contain all nine essential amino acids at adequate levels. Some examples of complementary protein foods are given in Table \(\PageIndex{3}\). Complementary protein sources do not have to be consumed at the same time—as long as they are consumed within the same day, you will meet your protein needs.
Tools for Change
Explore some ethnic grocery markets to find new high-protein foods for your diet or experiment with much cheaper plant-based protein sources. See Table \(\PageIndex{3}\) for complementing protein sources the vegan way.
| Foods | Lacking Amino Acids | Complementary Food | Complementary Menu |
|---|---|---|---|
| Legumes | Methionine, tryptophan | Grains, nuts, and seeds | Hummus and whole-wheat pita |
| Grains | Lysine, isoleucine, threonine | Legumes | Cornbread and kidney bean chili |
| Nuts and seeds | Lysine, isoleucine | Legumes | Stir-fried tofu with cashews |
Protein Digestibility Corrected Amino Acid Score (PDCAAS)
The second component of protein quality is digestibility, as not all protein sources are equally digested. In general, animal-based proteins are completely 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 they pass through the digestive tract unabsorbed by the body. The Protein Digestibility Corrected Amino Acid Score (PDCAAS) is a method adopted by the US Food and Drug Administration (FDA) to determine a food’s protein quality. It is calculated using a formula that incorporates the total amount of amino acids in the food and the amount of protein in the food that is actually digested by humans. The food’s protein quality is then ranked against the foods highest in protein quality. Milk protein, egg whites, whey, and soy all have a ranking of one, the highest ranking. Other foods’ rankings are listed in Table \(\PageIndex{4}\).
| Food | PDCAAS* |
|---|---|
| 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 |
| Legumes | 0.70 |
| Cereals | 0.59 |
| Whole wheat | 0.42 |
| *1 is the highest rank, 0 is the lowest | |
Attributions
- Zimmerman, "An Introduction to Nutrition (Zimmerman)," CC BY-NC-SA 3.0. Figures were updated and the flow of information was slightly changed. Several sections in the original source were moved to other sections in this text.
References
- Lonnie M, Hooker E, Brunstrom JM, et al. Protein for life: review of optimal protein intake, sustainable dietary sources and the effect on appetite in ageing adults. Nutrients. 2018;10(3):360. doi:10.3390/nu10030360.
- Protein foods. US Department of Agriculture. myplate.gov. Accessed September 11, 2023. https://www.myplate.gov/eat-healthy/protein-foods.
- The nutrition source: protein. Harvard T.H. Chan School of Public Health. Accessed September 11, 2023. https://www.hsph.harvard.edu/nutritionsource/what-should-you-eat/protein/.