Protein and Exercise
Protein is a popular macronutrient among athletes. In fact, many athletes believe they need to consume large amounts of dietary protein to perform well. Understanding the structure and function of protein, as well as its metabolism in the body is essential to educating athletes on the truths, and myths, of protein requirements for athletic performance.
Structure, Basic Function, and Food Forms
Protein is composed of carbon, hydrogen, and oxygen, but unlike fat and carbohydrate, protein also contains nitrogen. Protein is composed of at least one or more long chain of amino acids, and these individual amino acids, along with the protein’s structure, determine the protein’s function. There are 20 different amino acids that make up proteins. Amino acids are typically classified according to their essentiality or nonessentiality; that is, nonessential amino acids, referred to as dispensable amino acids, are those that can be synthesized by the liver. Essential, or indispensable amino acids, are those the body must obtain from food sources because it cannot make these amino acids on its own. Conditionally indispensable amino acids are those that the body can synthesize under normal, healthy conditions, but due to disease state or nutrient deficiency the body must obtain these amino acids from the diet. See Table 4.4.
Protein can be classified according to its quality, that is, its content and proportionality of indispensable amino acids. A high-quality, or complete, protein contains all the indispensable amino acids in the amounts needed by humans. Food sources of complete proteins include eggs, milk, yogurt, meat, fish, poultry, and soy. Incomplete proteins are those that do not contain sufficient amounts of indispensable amino acids. Incomplete proteins are found in plant foods such as grains, legumes, and vegetables. A diet containing only incomplete protein sources, if consumed in insufficient quantities, may not provide adequate amounts of indispensable amino acids, which may impair the body’s ability to make certain proteins required for health, recovery, and muscle growth.
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Contrary to popular belief, most plant-based proteins do contain all of the essential amino acids (EAA); however, their amino acid profile often falls short of that of the reference protein (albumin). Thus they are considered lower quality. Nonetheless, since individuals rarely consume single food items at a meal or even over the course of the day, assuming the individual is consuming a variety of plant-based proteins and consuming
Dispensable, indispensable, and conditionally indispensable amino acids enough total protein, meeting amino acid requirements will not be difficult. Additionally, by pairing certain plant-based protein foods, known as complimentary proteins, the amino acids profile becomes complete and provides adequate amounts of all of the indispensable amino acids. For example, legumes that are low in tryptophan and methionine can be paired with grains containing greater amounts of these amino acids; the isoleucine and lysine found in legumes complement the low content of these amino acids in grains. Rice and beans, peanut butter and whole wheat (bread), and hummus with pita bread are all examples of complementary proteins. These foods do not need to be paired together in the same meal but rather consumed throughout the day.
Protein has many functions in the human body. Proteins serve as catalysts in the form of enzymes, and support immune function as immunoglobulins. Proteins act as chemical messengers in the form of hormones, and provide a structural role in contractile proteins and collagen. Proteins help transport nutrients and other substances throughout the body such as albumin and lipoproteins. Proteins function as both an intra- and extracellular buffer. This is not even an exhaustive list, as proteins serve many other functions. However, especially as it pertains to athletes, proteins are essential for maintaining fluid balance, and they serve as a structural component of bone and connective tissue, and help repair tissue damage and build muscle. So, while many athletes often know of protein’s role in muscle protein synthesis, it is important they understand protein’s diverse functions in the body.
Categorizing the various sources of protein into plant and animal sources can be helpful when making food choices. Animal sources of protein include meat, fish, and poultry; eggs; and dairy. Plant foods that provide contributable amounts of protein are legumes that include soy and soy products, nuts and seeds, and whole grains. Athletes should understand that different types of protein come in different packages and thus offer diverse nutritional benefits. For example, an animal source of protein such as chicken will provide protein as well as iron, vitamin B12, and zinc. Kidney beans, on the other hand, contain protein, but they also provide potassium, magnesium, and folate.
It is a misconception that animal protein sources are “superior” to plant proteins. As mentioned previously, complementary proteins can be combined to provide the indispensable amino acids that athletes need. While the digestibility and thus bioavailability is greater in animal protein sources, vegetarian athletes can still meet their protein needs through careful menu planning. Considerations for vegetarian athletes will be addressed in Chapter 8. Athletes who are not vegetarian should not rely exclusively upon animal protein foods, but rather should include diverse plant and animal protein sources to ensure a wide variety of nutrients obtained from these foods.
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