Proteins derived from animal sources, such as meat, poultry, milk, and fish, are considered to be of high quality due to their balanced composition of essential amino acids (EAAs) necessary for the synthesis of human tissue proteins. Moreover, these proteins are easily digested. However, it should be noted that gelatin made from animal collagen is an exception as it lacks several EAAs and has a low biological value. On the other hand, proteins from plant sources have a lower quality compared to animal proteins. Nevertheless, by combining proteins from different plant sources, it is possible to achieve a nutritional value equivalent to that of animal protein. For example, the combination of wheat (which lacks lysine but is rich in methionine) with kidney beans (which lack methionine but are rich in lysine) results in a higher biological value than either protein alone. It is also worth mentioning that animal proteins can complement the biological value of plant proteins.

Nitrogen balance refers to the equilibrium between nitrogen intake and nitrogen excretion in the body. In a state of nitrogen balance, the amount of nitrogen consumed is equal to the amount of nitrogen excreted through urine (primarily as urinary urea nitrogen), sweat, and feces. Most healthy adults naturally maintain nitrogen balance. A positive nitrogen balance occurs when nitrogen intake exceeds nitrogen excretion, typically observed during periods of tissue growth such as childhood, pregnancy, or recovery from an emaciating illness. Conversely, a negative nitrogen balance occurs when nitrogen loss exceeds nitrogen intake. Inadequate dietary protein, deficiency of essential amino acids, or physiological stresses like trauma, burns, illness, or surgery can lead to a negative nitrogen balance. The calculation of nitrogen balance within a 24-hour period can be determined using the formula: nitrogen balance = protein intake in grams divided by 6.25 minus (urinary urea nitrogen plus 4 grams). The additional 4 grams accounts for urinary loss in forms other than urinary urea nitrogen, as well as losses through the skin and feces.

The amount of dietary protein required varies depending on its biological value. A higher proportion of animal protein in the diet reduces the overall protein requirement. The Recommended Dietary Allowance (RDA) for protein is calculated based on proteins of mixed biological value and is set at 0.8 grams per kilogram of body weight for adults, equivalent to approximately 56 grams of protein for a 70-kilogram individual. Individuals who engage in strenuous exercise on a regular basis may benefit from additional protein intake to maintain muscle mass, with a recommended daily intake of around 1 gram per kilogram of body weight for athletes. Pregnant or lactating women require an additional 30 grams of protein per day on top of their basal requirements. Infants, in order to support growth, should consume 2 grams of protein per kilogram of body weight per day. It should be noted that protein needs may be influenced by certain disease states, such as kidney disease requiring protein restriction or burns necessitating increased protein intake.

Consuming excess protein beyond the body's needs does not offer any physiological advantage. Excess protein 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 often leads to increased urinary calcium, thereby elevating the risk of developing kidney stones (nephrolithiasis) and osteoporosis.
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.