absorbed). Differences between estimates of true and apparent digestibility are larger when dietary protein concentrations are low, because obligatory fecal losses make up a larger proportion of total fecal nitrogen loss. Very few data on protein digestibility (apparent or true) are available for protein sources fed to nonhuman primates.
Robbins and Gavin (1966) fed a commercial monkey diet containing ground wheat and corn, soybean meal, alfalfa meal, and lactalbumin as protein sources to rhesus monkeys and found that the apparent digestibility of total dietary protein was 83.8%. Hodson et al. (1967), using chimpanzees, estimated the apparent digestibility of protein in diets containing ground wheat, dehydrated alfalfa meal, ground corn, dried skim milk, and soybean meal, and providing 12-18% protein. Apparent digestibility was 63-66%. Liquid diets (1.5-8.5% protein) formulated primarily with purified casein and fed to infant capuchin monkeys had an apparent protein digestibility of about 88.3% (Gallina and Ausman, 1986). Protein in diets fed to Saguinus fuscicollis had an apparent digestibility varying from 72.9% to 87.1% as dietary protein concentration increased. When fed in increasing percentages to Callithrix jacchus, apparent digestibility of dietary proteins increased from 76.6 to 86.8% (Flurer and Zucker, 1985). Thus, the apparent digestibilities of dietary proteins (purified or natural sources) fed to five species of monkeys were found to be 63-88%.
Protein requirements of primates do not appear markedly different from those predicted from studies of other mammals. Table 4-1 summarizes the estimated protein requirements of several species of primates, including humans.
Requirements for juvenile to adult primates, expressed as grams of protein per kilogram of body weight (BW) per day, range from 0.59 g·BWkg-1·day-1 for adult humans to 4.3 g·BWkg-1·day-1 for juvenile squirrel monkeys; most adult primates (when there were sufficient data) required less than 3 g·BWkg-1·day-1. When the daily energy intakes of the species were considered, protein concentrations needed to support requirements were 4.6-7.5% of ME calories or 6.4-8% of dietary dry matter. There were insufficient data on adult rhesus macaques and chimpanzees to fix requirements exactly.
Five primate species have been studied from infancy through adulthood: a squirrel monkey, a cebus monkey, two species of macaques, and humans. In each species, protein requirements, expressed as above, decreased as growth rates declined and animals matured.
The nutritional quality of a protein is heavily influenced by its amino acid composition. Mitchell and Block (1946) suggested that the quality of a protein is inversely proportional to its percent deficit in essential amino acids; that is, “limiting” amino acids determine the quality amino acid score of the protein. Given the chemically determined pattern of amino acids in a reference protein, such as that of whole egg, and in a test protein, the amino acid score of the test protein can be calculated without using live animals. Later, other measures of protein quality such as biologic value (BV), net protein utilization (NPU), and protein efficiency ratio (PER) were popularized in studies with humans or rodents (Pellett and Young, 1980; Rand et al., 1981). The most accurate measure, relative nutritive value (RNV), relies on feeding both a reference protein (or standard) and a test protein at several different growth-limiting concentrations in the same experimental paradigm and comparing animal responses to the test protein and the standard (Hegsted and Worcester, 1966; Rand et al, 1981). The result is expressed as potency (test-protein response as a percentage of reference-protein response). Tests of the RNV of proteins have been conducted with squirrel and cebus monkeys and with humans (Table 4-2). The degree to which an essential amino acid becomes limiting is thought to depend, in part, on the growth rate of the test subject; rapidly growing animals require more amino acids for new tissue growth than do adults.
Data from studies of infant and young squirrel monkeys (Ausman et al, 1979) and cebus monkeys (Samonds and Hegsted, 1973; Ausman et al., 1986) indicate that soy protein, limiting in the essential amino acid methionine, has a lower potency than a standard of casein or lactalbumin. It is noteworthy that the addition of methionine in appropriate amounts provided a dietary protein mixture that was not different from the reference protein as judged by nitrogen balance (Ausman et al., 1986). In a final set of experiments, growth and nitrogen-balance assays with growing cebus monkeys indicated that the potency of casein with respect to lactalbumin was 60-70%, reflecting its relative paucity of cysteine. The results were consistent with the lower potency of the same lots of soy protein and casein when assayed with growing rats (Ausman et al., 1986). In comparison, nitrogen-balance experiments with adult humans fed soy protein yielded potencies less than 100% but often not significantly different from the reference protein (Rand et al., 1981). Experiments in which protein quantity and quality are limiting cannot ethically be conducted with infants or children.