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Table 12 The relative amounts of essential amino acids in current estimates of the amino acid requirements for growth

^a From National Research Council (1978).
^b From Fuller et al (1989).
^c From Holt and Snyderman (1961).
^d From summary of the literature (Davis et al, 1993).
^e This method of normalization was used because of the wide differences in the total protein requirement in the three species.

Table 13 Comparison of the composition of total (protein-bound + free) amino acids in different species milks

^a Mean of values for chimpanzee and gorilla.
Notes:
1. Note the high cysteine/methionine ratio in human and great ape milk.
2. Note the high serine and low proline of rat milk. This reflects the distinct amino acid sequence of rat b-casein
3. Although extensive data are lacking, about 40% of the glutamate seems to be glutamine. Data taken from Davis et al (1994).

It is interesting to compare the relative quantities of total (protein + free) amino acids in milk to the relative quantities of amino acids in body protein. Amino acid composition of milk (ma amino acid/g total amino acid) from various species is shown in Table 13 (taken with permission from Davis et al, 1994), and the ratios of milk amino acids to body amino acids are shown in Table 14. Three main points are of particular interest. First, with the exception of the sulfur amino acids, there is a remarkable commonality among species that differ widely in their postnatal growth rates and functional maturity at birth. Second, three amino acids (leucine, glutamate/glutamine and proline) account for 39.8 ± 1.1% of milk amino acids. Third, as the comparison of milk and body amino acids shows, all milks are markedly 'deficient' in glycine and arginine. Thus, the mixture of amino acids presented to the naturally-fed infant is quite different from its obligatory needs for protein deposition.

The reason for the differences between milk and body amino acid composition is obscure. The additional amino acid needs of the infant for maintenance cannot explain the differences, because maintenance demands relatively large amounts of nonessential amino acids, which are in relatively low concentrations in milk (see below). It could be argued that the unique composition of milk is indicative of a major pathway of amino acid use that has not been identified. What seems more likely is that the amino acid composition reflects a compromise between the evolution of an optimum mixture and the need for specific physical properties such as the maintenance of solubility at < 37°C and in the presence of triacylglycerols.

The central problem in understanding the amino acid needs of human infants arises from their relatively low rate of protein deposition compared with that of other mammals. As a result, amino acid needs for maintenance of existing stores of protein (and amino acid metabolites) are a high proportion of total amino acid needs. As shown in Table 3, it is clear that except for the first month of life, more than half the need is for maintenance.

Continued

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