There are definitely specific requirements for proteins, though the exact amount is somewhat questionable. The Recommended Daily Allowance (RDA) of protein according to U.S. government standards is 0.8 gram per kilogram (1 kilogram equals 2.2 pounds) of ideal body weight for the adult. Ideal body weight is used in the calculation because amino acids are not needed by fat cells, only by the lean body mass. So an adult male who should weigh about 154 pounds, or 70 kilograms, requires 56 grams of protein daily. A female whose best weight is 110 pounds, or 50 kilograms, needs 40 grams a day. The RDA increases by 30 grams per day during pregnancy and 20 grams per day during lactation. During growth, different amounts are needed. For example, 2.2 grams of protein are needed per kilogram of body weight each day in the first six months of life, and 2.0 grams per kilogram for the next six months.
Daily Protein Requirements
|6 months–1 year||2.0|
|19 years and older|| 0.8|
*(in grams per kilogram [2.2 pounds] of body weight)
These requirements are based on maintaining a positive nitrogen balance in children and an even to positive nitrogen balance in adults. Protein is the nitrogen-containing nutrient. As it is broken down for excretion, it must be replaced by dietary nitrogen so protein formation can continue. In the healthy adult, nitrogen equilibrium, or zero balance, is the ideal, while a positive nitrogen balance is needed during times of illness and healing. In children, when growth is occurring regularly, a positive nitrogen balance is necessary, as it is in pregnancy.
As discussed in the previous section, Food Complementarity, the protein requirements are also based on the protein quality, as measured by the biological value (BV). Protein is also measured by the way it supports growth; this measurement, called the protein efficiency ratio (PER), is determined by feeding an animal a particular protein food and measuring its growth.
The reference protein for determining the biological value of foods is that of eggs (ovalbumin), the food with the highest BV at 94 percent (although mother's milk is valued at 100 percent). Next are fish at 75–90 percent, rice at 86 percent, legumes at 70–80 percent, and meats and poultry at 75–85 percent. Corn, an incomplete protein, has approximately 40 percent biological value.