Proteins from animal sources: Proteins from animal sources (meat, poultry, milk, and fish) have a high quality because they contain all the EAA in proportions similar to those required for synthesis of human tissue proteins (Fig. 27.19), and they are more readily digested. [Note: Gelatin prepared from animal collagen is an exception. It has a low biologic value as a result of deficiencies in several EAA.] 2.
Proteins from plant sources: Plant proteins have a lower quality than do animal proteins. However, proteins from different plant sources may be combined in such a way that the result is equivalent in nutritional value to animal protein. For example, wheat (lysine deficient but methionine rich) may be combined with kidney beans (methionine poor but lysine rich) to produce a higher biologic value than either of the component proteins (Fig. 27.20). [Note: Animal proteins can also complement the biologic value of plant proteins.]
B. Nitrogen balance
Nitrogen balance occurs when the amount of nitrogen consumed equals that of the nitrogen excreted in the urine (primarily as urinary urea nitrogen, or UUN), sweat, and feces. Most healthy adults are normally in nitrogen balance. [Note: There is, on average, 1 g nitrogen in 6.25 g protein.] 1.
Positive nitrogen balance: This occurs when nitrogen intake exceeds nitrogen excretion. It is observed during situations in which tissue growth occurs, for example, in childhood, pregnancy, or during recovery from an emaciating illness.
2.
Negative nitrogen balance: This occurs when nitrogen loss is greater than nitrogen intake. It is associated with inadequate dietary protein; lack of an essential amino acid; or during physiologic stresses, such as trauma, burns, illness, or surgery.
Nitrogen (N) balance (g Nin – g Nout) in a 24-hour period can be determined by the formula, N balance = protein intake in g/6.25 – (UUN + 4 g), where 4 g accounts for urinary loss in forms other than UUN plus loss in skin and feces.
C. Protein requirements
The amount of dietary protein required in the diet varies with its biologic value. The greater the proportion of animal protein in the diet, the less protein is required. The RDA for protein is computed for proteins of mixed biologic value at 0.8 g/kg of body weight for adults, or ~56 g of protein for a 70-kg individual. People who exercise strenuously on a regular basis may benefit from extra protein to maintain muscle mass, and a daily intake of ~1 g/kg has been recommended for athletes. Women who are pregnant or lactating require up to 30 g/day in addition to their basal requirements. To support growth, infants should consume 2 g/kg/day. [Note: Disease states influence protein needs. Protein restriction may be needed in kidney disease, whereas burns require increased protein intake.] 1.
Consumption of excess protein: There is no physiologic advantage to the consumption of more protein than the RDA. Protein consumed in excess of the body’s needs is deaminated, and the resulting carbon skeletons are metabolized to provide energy or acetyl coenzyme A for fatty acid synthesis. When excess protein is eliminated from the body as urinary nitrogen, it is often accompanied by increased urinary calcium, thereby increasing the risk of nephrolithiasis (kidney stones) and osteoporosis.
2.