The potency of gluten, the major protein in wheat, for infant cebus monkeys was extremely poor—about 15% (Samonds and Hegsted, 1973). It was improved by adding lysine to the diet in an amount equal to that in the reference protein, but performance was still substantially lower than that of the standard. The same situation was found in studies with adult cebus monkeys fed diets containing bread protein and gluten with various amounts of added lysine. Doubling the lysine concentration in the diet was necessary to allow the monkeys to attain their pre-experimental body weights. Additions of threonine and methionine also were helpful in promoting body-weight gain (Ausman and Hegsted, 1980). Experiments with adult humans commonly indicate that the potency of wheat protein, unsupplemented with lysine, is less than 50% of the standard (see the experiment cited in Table 4-2).

Humans rarely consume a diet containing only a single protein source. The exception might be infants that are fed diets containing only milk or soy proteins for the first few weeks of life. Ordinarily, the amino acid composition of the proteins in the diet complement each other. Indeed, the latest edition of Recommended Dietary Allowances (National Research Council, 1989) suggests that no correction for protein quality need be made in protein-requirement values for humans in the United States in as much as the biologic value of a typical mixed-protein diet is not distinguishable from that of reference protein.

Given that both young and adult monkeys are sensitive to protein quality, it is extremely important that semipurified and natural-product diets contain nutritionally balanced amino acid mixtures. Combining grain and legume proteins (limiting in lysine and methionine, respectively) or animal and plant proteins generally accomplishes this. Of course, to be satisfactory, commercial monkey biscuits should be formulated to contain adequate concentrations and appropriate proportions of essential amino acids.

The essential amino acid requirements of monkeys appear to be similar to those of humans. Although data are insufficient to fix the amino acid requirements absolutely, results of experiments in which essential amino acids were limiting produced results as predicted from studies with humans and with growing and adult mammals of other species.

In primate species with significant foregut fermentation, dietary amino acid requirements may vary. The extent of amino acid degradation and microbial protein synthesis in foregut fermenting species are unknown. Research, similar to the extensive studies that have been conducted in ruminants, is needed to elucidate the effect of foregut fermentation on amino acid bioavailability and requirements.

The preceding sections have established that lysine and methionine are essential amino acids needed in appropriate