The body cannot synthesize all twenty of the amino acids. Those that the body can produce are called non-essential; those it cannot produce are called essential. In order to function properly, the body must receive enough essential amino acids from external sources. Such sources of protein include both plants and animals, although the quality of protein differs between the two. The body's ability to absorb proteins also differs from protein to protein. Finally, proteins are in constant flux: the amount of protein losses must be balanced by gains through the diet. Exercise will contribute to the degradation and synthesis of proteins in the body. Regular exercise demands additional protein consumption.
Since animal and plant proteins can differ markedly in their amino acid composition, the quality of the protein we consume can differ as well. For example, the protein found in plants is always deficient in one or more of the essential amino acids. Therefore, plant proteins are considered low quality proteins. In contrast, animal proteins are usually high in essential amino acids and are considered high quality. This difference is especially important for vegetarians, who must obtain all their essential amino acids from plants. When any one of the essential amino acids becomes depleted, protein synthesis comes to a halt. The amino acid in shortest supply is known as the limiting amino acid, because protein synthesis is limited based on its amount in the diet. In order to avoid problems posed by a limiting amino acid, vegetarians must eat a combination of different types of plant protein. Since different essential amino acids are present in different plants, the proteins in different plants can complement each other and provide the necessary nutrients if a variety of fruits and vegetable are consumed during the day. Although essential amino acids are important for the biosyntheses of other proteins that function in the body, only 15% of a person's total protein requirement has to be provided by essential amino acids.
The disease phenylketonuria (PKU) demonstrates the dichotomy between essential and nonessential amino acids. The amino acid tyrosine is nonessential because it can be created from phenylalanine, while phenylalanine cannot be formed from any other amino acid and is therefore considered an essential amino acid. However, a person with PKU cannot convert phenylalanine to tyrosine because he lacks the enzyme that catalyzes this reaction. Therefore, tyrosine becomes an essential amino acid to the individual with PKU and must be consumed in the diet. Furthermore, amounts of phenylalanine must be limited from the diet since the levels of this amino acid and its precursors will increase, causing brain damage in extreme quantities.
The biological value is another indicator of protein quality. It measures how efficiently the protein found in various foods can be turned into body tissues. Recall how urea is synthesized. Excess amino acids are deanimated, or their amino groups are removed and released within the mitochondria of cells. The remaining carbon skeleton can then be used to meet energy needs or be converted to glucose or fat. The free ammonium ion (NH4+) is eventually converted to urea, which contains one carboxyl group surrounded by two NH2 groups (figure 11). Therefore, the amount of protein used for biosyntheses can be determined by measuring the amount of nitrogen retention. Since essential amino acids will have a higher biological value and be more efficiently used in biosyntheses than nonessential amino acids, they will also have greater nitrogen retention. In other words, fewer deanimation reactions will occur and less urea will be produced in the urine.
Biological values are especially important when considering kidney and liver diseases. Since the liver and kidney are constantly metabolizing dietary protein and recycling endogenous protein, any surplus protein from the diet may place an unnecessary metabolic burden on these organs. Therefore, a limited diet of proteins that are high in biological value will limit the production of urea and eliminate excessive strain on the liver and kidney. Sources of proteins that are high in biological value include egg whites and milk proteins.
The Recommended Dietary Allowance of proteins for adults is 0.8 g of protein per kilogram of desirable body weight. Therefore, a 70 kg man would have to consume approximately 56 g of protein every day to meet this allowance. However, this is not an absolute value. If the person is no longer growing, he or she simply needs to consume enough protein to balance protein losses from the urine and feces. This is called being in protein equilibrium. When the body is growing or recovering from illnesses, it is synthesizing more protein than it is excreting. In this situation, protein intake must exceed daily losses in order to supply tissue with enough building blocks. Other examples of positive protein balances occur during pregnancy and athletic training. Negative protein balance is state where protein excretion exceeds protein intake. This state can be caused by a number of reasons including inadequate protein intake, inadequate energy intake, and deficiency of essential amino acids and kidney disease.