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Protein Quality, Form & Function

by Natalie Digate Muth, MD, MPH, RD on Mar 04, 2009

What athletes should consider in order to achieve optimal physical performance.

High-protein diets—and their promises of muscle gain, weight loss and improved health—appeal to a wide diversity of people, from athletes to dieters. But how much and what kind of protein is best? More important, does the scientific research support the potential health benefits of this macronutrient?

This article will review protein function; highlight the latest research on the health benefits and risks of a high-protein diet; and offer fitness professionals practical tips on how to choose protein sources and appropriately (and within the scope of practice) answer clients’ questions.

Protein Function

Proteins, made from chains of amino acids, form the major structural component of our muscles, brain, nervous system, blood, skin and hair. This important macronutrient serves as the transport mechanism for iron, vitamins, minerals, fats and oxygen within the body; and it is also vital to maintaining acid-base and fluid balance.

Proteins form enzymes that speed up chemical reactions and create antibodies that the body uses to fight infection. In situations of energy deprivation, the body can break down proteins for fuel. With all of protein’s many important functions, our bodies are well served by getting the right kind and the correct amount of high-quality proteins.

Protein Quality

All proteins are made up of some combination of amino acids, most of which the body manufactures. However, there are nine amino acids that the body cannot make; these nine—referred to as essential amino acids—must be consumed in the diet.

A specific food’s protein quality is determined by assessing its essential amino acid composition, digestibility and bioavailability, which is the degree to which any amino acid can be used by the body. Generally, animal products contain all of the nine essential amino acids (called complete proteins), whereas plant foods do not and are thus considered incomplete proteins. One notable exception to this rule is soy, which is a plant-based complete protein. That’s why animal proteins and soy are better sources of quality protein than other plants. However, you can boost protein quality and get all the essential amino acids you need by combining complementary incomplete plant proteins. Excellent combinations of incomplete plant proteins include grains with legumes (e.g., rice and beans); grains with dairy (e.g., pasta and cheese); or legumes with seeds (e.g., a Greek dish called falafel).

Several scales are used to evaluate protein quality. However, the most widely used and most accepted method is called the protein digestibility–corrected amino acid score (PDCAAS). The proteins with the highest (i.e., best) PDCAAS scores are whey, casein, egg, milk and soy proteins (Hoffman & Falvo 2004). Beef comes in next, followed by black beans, peanuts and wheat gluten (Hoffman & Falvo 2004).

Protein Digestion and Absorption

The goal of digestion is to break down dietary protein into individual amino acids that can be absorbed and later used. The body’s need for dietary protein results from the constant breakdown and regeneration of the body’s cells. The immediate supply of amino acids for cell regeneration comes from the body’s free amino acid pool, which comprises amino acids from dietary sources along with recycled amino acids from cell turnover. Because amino acid recycling is inherently inefficient, dietary amino acid intake is necessary to replace losses.

Each protein is determined by its amino acid composition. For a protein to be digested, it must first lose its unique shape; this process is called denaturation. Adding acid, salt or heat to meat products facilitates denaturation, making proteins more available to digestive enzymes (Mahan & Escott-Stump 2000). Vegetable protein is not as well digested as animal protein because it is less available to digestive enzymes. Some plants (e.g., soybeans) contain enzymes that interfere with digestion (Mahan & Escott-Stump 2000). Food processing can damage amino acids and reduce their availability for digestion.

Protein digestion begins in the stomach with the action of the enzyme pepsin, which denatures the proteins and starts to break the bonds that hold amino acids together. The stomach mixes and churns the food and then releases the mixture to the small intestine in little amounts over the course of 1–4 hours. The pancreas releases enzymes into the small intestine, which further breaks down the proteins into amino acids, and these are then absorbed into the bloodstream and carried to the liver. In the liver, two things can happen: either the amino acids are converted into carbohydrate, or they enter the amino acid pool.

In a healthy body, the amount of protein taken in should exactly match the amount of protein excreted in feces, urine and skin. Muscle mass is maintained, with a fraction of muscle protein being destroyed and an equal amount being rebuilt daily using amino acids from the amino acid pool. During periods of muscle growth, protein synthesis exceeds protein destruction and excretion (Mahan & Escott-Stump 2000).

Protein Types

Apart from egg, meat, fish and poultry, the highest-quality protein sources are milk, the milk components whey and casein, and soy.


Whey is one of the two major milk proteins; the other is casein. Whey is the liquid that remains after the milk has been curdled and strained. There are three varieties of whey: whey protein powder, whey protein concentrate and whey protein isolate, all of which provide high levels of the essential amino acids, as well as vitamins and minerals. Whey powder is 11%–15% protein and is used as an additive in many food products (Hoffman & Falvo 2004). Whey concentrate is 25%–89% protein, whereas whey isolate has a protein content of 90% or greater; both concentrate and isolate are commonly used in dietary supplements (Hoffman & Falvo 2004). It should be noted that while the isolate form is nearly pure whey, some proteins can be lost during the manufacturing process. Unlike the other whey forms, isolate is lactose-free (Hoffman & Falvo 2004).

Studies have found that whey protein offers numerous health benefits (see the sidebar “Health Benefits of Whey Protein”). Whey contains high levels of the amino acids that play an important role in muscle hypertrophy (Hayes & Cribb 2008; Krissansen 2007). Whey is rapidly digested and absorbed; it also has a remarkable ability to stimulate muscle protein synthesis, even more so than casein and soy (Hayes & Cribb 2008).


Casein, which gives milk its white color, accounts for 70%–80% of milk’s protein (Hoffman & Falvo 2004). Casein provides a sustained, slow release of amino acids into the bloodstream, sometimes lasting for hours (Hoffman & Falvo 2004). Some studies suggest that combining casein and whey may produce the greatest muscular strength improvements after an intensive resistance training program (Kerksick et al. 2006).


Soy is the most widely used vegetable protein source. It is the only vegetable protein that contains all nine essential amino acids. Similar to whey, soy proteins can be divided into three types: soy flour (50% protein), which is often used in baked goods; soy concentrate (70% protein), which is commonly added to nutrition bars, cereals and yogurts; and soy isolate (90% protein) (Hoffman & Falvo 2004). Soy isolate is highly digestible and easily added to sports drinks, health beverages and infant formulas (Hoffman & Falvo 2004).

Early studies indicated that soy might

  • lower low-density lipoprotein (LDL) cholesterol and blood pressure;

  • protect against breast cancer;

  • maintain bone density; and

  • decrease menopausal symptoms (Hoffman & Falvo 2004).

Unfortunately, subsequent studies have failed to confirm the early research. In fact, the Nutrition Committee of the American Heart Association (AHA) released an advisory in 2006 warning against soy or isoflavone (a component in soy) supplementation (Sacks et al. 2006). However, the AHA did recommend increasing daily intake of soy products (such as tofu, soy burgers and soy nuts), which contain high levels of heart-healthy polyunsaturated fats, fiber, vitamins and minerals and low levels of saturated fat (Sacks et al. 2006).

General Protein Recommendations

Resistance training and cardiovascular exercise induce beneficial muscular and structural damage. Because protein helps the muscles and tissues repair and rebuild themselves, the American Dietetic Association (ADA), Dietitians of Canada (DOC) and the American College of Sports Medicine (ACSM) suggest that athletes have higher protein needs than the general population. These agencies advise endurance athletes to consume about 1.2–1.4 grams per kilogram (g/kg) (0.5–0.6 grams per pound [g/lb]), whereas strength-trained athletes should consume up to 1.6–1.7 g/kg (0.7–0.8 g/lb) (ADA, DOC & ACSM 2000). However, a 2005 report from the Institute of Medicine (IOM) concluded that there was no compelling scientific evidence to support active individuals’ increasing their daily protein intake above the 0.8 g/kg (0.4 g/lb) per day recommended for the general population (IOM 2005).

Since the IOM report was issued, there has been much confusion and misinterpretation of the agency’s protein recommendations, especially as they relate to athletes’ needs. What many critics fail to realize is that the protein intake of 0.8 g/kg (0.4 g/lb) reflects the recommended dietary allowance (RDA), which is the minimum daily intake level that meets the nutrient requirements of nearly all healthy individuals; the RDA is not meant to be an ideal or maximal intake level.

Some who question the IOM findings note that a variety of studies have shown that higher levels of protein intake support muscle mass, strength and function; bone health; maintenance of energy balance; cardiovascular function; and wound health (Wolfe & Miller 2008). These researchers suggest that the ideal protein intake should be based on the acceptable macronutrient distribution range (AMDR), which is 10%–35% of daily energy intake (Wolfe & Miller 2008).

Benefits and Risks of a High-Protein Diet

Research is now emerging as to the benefits and risks associated with higher protein consumption, including whether or not protein induces weight loss, improves athletic performance and can be readily incorporated into a vegetarian diet.

Weight Loss

While low-carbohydrate/high-protein diets, such as the Atkins or South Beach plans, may no longer be the hottest trend, some studies have shown that for weight loss and health benefits these diets are just as good as—and sometimes better than—the standard low-fat/high-carbohydrate diet (Foster et al. 2003; Gardner et al. 2007; Dansinger et al. 2005). Low-carb/high-protein diets contribute to weight loss through several mechanisms. The initial weight loss on these diets is largely attributable to a diuretic effect from the low intake of carbohydrates, which results in rapid early water loss. These diets also contribute to glycogen depletion (which may be detrimental for endurance athletes) and metabolic ketosis, which leads to decreased appetite and decreased caloric intake (St. Jeor et al. 2001).

Early studies have shown that the Atkins diet produced greater initial weight reduction at 3 and 6 months than other diets studied, but a year later, weight loss was the same as it was for the other diets (Foster et al. 2003). A randomized trial that compared the Atkins, Zone, Ornish and LEARN (similar to the federal MyPyramid guidelines) diets found that the Atkins dieters lost more weight and had an improved health profile at 1 year (Gardner et al. 2007). The overall consensus among health experts is that it does not matter what type of diet a person chooses as long as he or she can stick to it, which is difficult to do regardless of which plan you pick (Dansinger et al. 2005).

Athletic Performance

Protein plays important roles in endurance and resistance training exercise. Both modes of exercise stimulate muscle protein synthesis (Phillips 2006), which is further enhanced if protein is consumed around the time of the physical activity (Hayes & Cribb 2008). Consumption of protein immediately after exercising helps in the repair and synthesis of muscle proteins. Protein intake during exercise probably does not offer any additional performance benefit if sufficient amounts of carbohydrate—the body’s preferred energy source—are consumed. However, for endurance athletes who need to consume adequate calories to fuel extended training sessions, or for any exerciser striving to lose weight, research suggests that protein can preserve lean muscle mass and ensure that most weight loss comes from fat rather than lean tissue (Phillips 2006).

While these may seem like great reasons to boost your daily protein intake, it is worth noting that most people habitually consume far more protein than they need (ADA, DOC & ACSM 2000). Protein consumption beyond recommended amounts is unlikely to result in further muscle gains because the body has a limited capacity to utilize amino acids to build muscle (ADA, DOC & ACSM 2000).


With good planning, vegetarians can consume a diet that contains adequate amounts of high-quality proteins. Legumes, dried beans, peas, nuts, soy and meat alternatives provide ample protein; however, few vegetarian foods provide all of the essential amino acids, which is why vegetarians must consume a variety of complementary protein-rich plant foods throughout the day.

Because plant proteins are not as readily digested as animal proteins, vegetarian athletes should consume about 10% more grams of protein than other athletes (ADA, DOC & ACSM 2000). For example, experts recommend that nonvegetarian athletes typically consume a 3,000-calorie diet with 10% of calories gleaned from protein; that works out to be about 300 calories (75g) derived from protein. However, a vegetarian athlete with the same caloric intake per day should consume about 30 extra protein calories (8g), for a total daily consumption of approximately 330 protein calories (300 plus 30).

What About Protein Supplements?

Many athletes use protein supplements to boost their protein intake and to consume a particular protein type or amino acid. The billion-dollar supplement industry has been quick to respond to increased consumer demand for protein products.

However, because the research findings are inconsistent and little is known about the safety of these products, the ADA advises against individual amino acid supplementation and against protein supplementation overall (ADA, DOC & ACSM 2000). While some supplements may in fact provide health benefits, generally speaking, consumers should purchase and use these products cautiously, as they are not closely regulated by the U.S. Food and Drug Administration. Importantly, no matter how safe some dietary products appear to be, fitness professionals should never recommend supplements to clients.

What to Tell Your Clients About Protein

When discussing high-protein diets with your clients, stay within your scope of practice.

  • Never advocate a particular diet to a client!

  • Encourage clients to get their protein from whole foods rather than supplements, as the ADA recommends (ADA, DOC & ACSM 2000).

  • Refer clients to a registered dietitian for help designing a balanced high-protein diet, whenever appropriate.

Keep in mind the following considerations:

  • Total daily protein intake should not be excessive and should be reasonably proportional (~15% of total caloric intake) to carbohydrate (~55% of total caloric intake) and fat (~30% of total caloric intake) (St. Jeor et al. 2001).

  • Not all proteins are created equally. Other than soy, vegetable proteins are incomplete proteins. Vegetarians and those who eat limited amounts of animal products should consume a wide variety of high-protein vegetarian foods.

  • Carbohydrates should not be omitted or severely restricted when upping protein intake, especially by athletes who need large amounts of carbohydrate to fuel optimal performance. A minimum of 100 g of carbohydrate per day is recommended (St. Jeor et al. 2001).

  • Selected protein foods should not contribute excess total fat, saturated fat or cholesterol to the diet (St. Jeor et al. 2001).

  • The eating plan should be safely implemented so as to provide adequate nutrients (St. Jeor et al. 2001).

The Protein Promise

The jury is still out on the best amounts, mechanisms and methods of protein intake. However, a large body of research shows that—when combined with regular exercise and an overall healthy lifestyle—protein can live up to its promises of muscle gain, weight loss and improved health.


American Dietetic Association (ADA), Dietitians of Canada (DOC) & American College of Sports Medicine (ACSM). 2000. Position of the American Dietetic Association, Dietitians of Canada, and the American College of Sports Medicine: Nutrition and athletic performance. Journal of the American Dietetic Association, 100 (12), 1543–56.

Dangin, M., et al. 2002. Influence of the protein digestion rate on protein turnover in young and elderly subjects. Journal of Nutrition, 132, 3228S–33S.

Dansinger, M.L., et al. 2005. Comparison of the Atkins, Ornish, Weight Watchers, and Zone diets for weight loss and heart disease risk reduction. The Journal of the American Medical Association, 293 (1), 43–53.

Foster, G.D., et al. 2003. A randomized trial of a low-carbohydrate diet for obesity. The New England Journal of Medicine, 348 (21), 2082–90.

Gardner, C.D., et al. 2007. Comparison of the Atkins, Zone, Ornish, and LEARN diets for change in weight and related risk factors among overweight premenopausal women. The Journal of the American Medical Association, 297 (9), 969–77.

Harvard School of Public Health. 2009. The Nutrition Source. Protein: Moving closer to center stage.; retrieved Jan. 13, 2009.

Hayes, A., & Cribb, P.J. 2008. Effect of whey protein isolate on strength, body composition, and muscle hypertrophy during resistance training. Current Opinion in Clinical Nutrition and Metabolic Care, 11, 40–44.

Hoffman, J.R., & Falvo, M.J. 2004. Protein: Which Is best? Journal of Sports Science & Medicine, 3, 118–30.

Institute of Medicine (IOM). 2005. Food and Nutrition Board. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty acids, Cholesterol, Protein, and Amino Acids. Washington, DC: The National Academies Press.

Kerksick, C.M., et al. 2006. The effects of protein and amino acid supplementation on performance and training adaptations during ten weeks of resistance training. Journal of Strength and Conditioning Research, 20 (3), 643–53.

Krissansen, G.W. 2007. Emerging health properties of whey proteins and their clinical implications. Journal of the American College of Nutrition, 26 (6), 713S–23S.

Mahan, L.K., & Escott-Stump, S. 2000. Krause’s Food, Nutrition & Diet Therapy (10th ed.). Philadelphia: Saunders.

Phillips, S.M. 2006. Dietary protein for athletes: from requirements to metabolic advantage. Applied Physiology, Nutrition, & Metabolism, 31, 647–54.

Sacks, F.M., et al. 2006. Soy protein, isoflavones, and cardiovascular health: An American Heart Association science advisory for professionals from the nutrition committee. Circulation, 113, 1034–44.

St. Jeor, S.T., et al. 2001. 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, 104, 1869–74.

Wolfe, R.R., & Miller, S.L. 2008. The Recommended Dietary Allowance of protein. The Journal of the American Medical Association, 299 (24), 2891–93.

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About the Author

Natalie Digate Muth, MD, MPH, RD

Natalie Digate Muth, MD, MPH, RD IDEA Author/Presenter

Natalie Digate Muth, MD, MPH, RD, is a registered dietitian and a recent graduate of the UNC School of Medicine. She has made several appearances as a nutrition expert on CW's San Diego 6, been quo...