Amino Acids and Proteins

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.

Proteins are abundant in most plants and animals but some sources are healthier choices. For example, a high intake of red meat is linked to colon cancer in some population studies. Furthermore, red meat is high in fat and cholesterol and circulating levels of these compounds may increase the risk of heart disease. On the other hand, plant proteins contain relatively little fat and no cholesterol. If vegetable servings can be eaten with their complementary counterparts so that all the essential amino acids are regularly consumed, they may be a healthier alternative to red meats. However, vegetarians must be aware that they may not be consuming enough iron in their diet, since red meat supplies plenty of iron. Vegetarians can find iron in vegetable sources including peas, legumes and spinach.

Many weight loss strategies place emphasis on increasing the amount of protein in the diet relative to carbohydrates in order to lose weight quickly. Although it is true that amino acids cannot be directly converted to fatty acids and that some amino acids can only form ketone bodies, excess amounts can still be converted to fat when the energy level of the body is high. But high protein diets may create more serious problems. As the body begins to deplete its supply of glucose, it begins to look elsewhere for energy sources. Levels of glucagon and epinephrine increase, stimulating the release of fatty acids into the bloodstream. The breakdown of glucogenicamino acids begin to supplement the supply of glucose in the bloodstream to keep the brain and skeletal muscle functioning properly. Dietary proteins can contribute to gluconeogenesis under these conditions. Amino acids that are normally used in the building and maintenance of body tissues under conditions of adequate carbohydrate intake must now be used to meet the energy requirements of the body. This causes further protein breakdown in tissues such as the heart, skeletal muscle, and other vital organs. Meanwhile, the decrease in blood glucose creates a condition known as ketosis, or the incomplete breakdown of fats. Without sufficient oxaloacetate being produced from glucose, the Krebs Cycle cannot continue, and further metabolism at this point is thwarted. Acetyl CoA units thatare the breakdown products of fatty acids and proteins can only be converted into ketone bodies, which at high concentrations create acidic, toxic conditions for the body. Further glucogenic protein must be broken down from body tissue to produce increasingly scarce supplies of oxaloacetate. Ironically, the dieter thinks the diet is working because he or she is losing weight. Although some of the weight loss may come from adipose tissue, the majority of weight loss comes from skeletal muscle and vital organs.

Despite creating a dangerous physiological state for the dieter, he or she may have a difficult time keeping the weight off once the diet comes to an end. Decreased lean tissue mass causes a decrease in the body's resting metabolic rate. Furthermore, the absence of glucose causes the enzymes governing the breakdown of glucose to slow down in order to conserve energy. In other words, the dieter's cells have adapted to a lower energy environment. As the dieter begins to return to a normal carbohydrate diet, he or she can no longer metabolize the same amounts of glucose. Therefore, large amounts of glucose and protein are converted to fat. In conclusion, the best way for a person to lose weight is through a regular exercise routine and well-balanced meals.