Optimizing amino acid and protein supply and utilization in the newborn

Abstract

As has been pointed out already (Casey, 1989) homologous milk appears to promote optimal growth and well-being in the infant, although it is not .entirely clear why this should necessarily be so. The difficulty in fully understanding the basis of the relations is particularly well illustrated for protein and amino acids. Within the framework of our general appreciation of nitrogen and protein requirements (Food and Agriculture OrganizatiodWorld Health Organizatioflnited Nations University, 1985), the N content of human milk is remarkably low, whether expressed in relation to the total energy content, or the amount ingested daily by the infant. It would appear that all the protein in the milk is not digested and absorbed. This may be particularly true for certain classes of proteins such as immunoglobulins, although we do not have any precise, quantitative information on the extent of this malabsorption. On the other hand, about 25% of the N in human milk is not present as protein at all. A portion can be identified as free amino acids, but by far the largest contribution, about 60%, comes from urea-N (Harzer et af. 1984). The urea-N has generally been considered to be metabolically unavailable. If this is so, the available protein would appear to be exceptionally low at a time when the demands for infant growth, development and elaboration are at their highest (Fomon & Macy, 1958). With our present concepts of protein and amino acid metabolism it is not possible to rationalize the apparent contradiction between the N requirements for growth and that available from breast milk. In order to make sense of the observed reality, we have to be prepared to consider alternative frameworks of understanding.In order to usefully approach this problem I have to start from a base of assumptions founded on experimental observations. There are four propositions I think it necessary to accept in order to be able to proceed.

Human Amino Acid and Protein Requirements: Current Dilemmas and Uncertainties

“…Protein intakes of breast-fed infants would only be relevant to the requirements for weaned children if the utilization of breast milk is similar to that of formula feeds (69-74%). This is the age of most rapid growth, and the N in breast milk is utilized with very high efficiency; an indication of the special properties and qualities of breast milk, which are poorly understood (Jackson, 1989). This higher efficiency of utilization of breast milk would explain the discrepancy, at least in part.…”

“…5 ; see Millward & Rivers, 1988, 1989, is that dietary amino acids are shown as serving two different functions. Dietary amino acids entering the free amino acid pool serve as substrates and are metabolized in a variety of pathways, resulting in either gains or losses by or from the organism.…”

Protein Requirements for Catch-up Growth

“…The most rapid rates of postnatal growth are obtained during early infancy. Urea is effectively retained during early infancy, and may make an important contribution to the N economy of the body (Jackson, 1989). It is clearly recognized that a negative N balance accompanies metabolic stress.…”

The perioperative nutritional care of neonates and infants

“…There are at least 8 amino acids which are essential, but more evidence shows that for neonates at least another 4 to 7 are semiessential due to immature amino acid synthetic pathways. These are histidin, cystein, taurine, tyrosine, prolin and glutamine and arginine (24,25). Glutamine is the most abundant amino acid in the body and is an important precursor for nucleic acid biosynthesis, i.e.…”

“…The total body water (TBW) declines from 85-90% in gestational weeks [25][26][27][28] to 70-75% at term (4). In the first week of postnatal life TBW decreases by another 5% which is mainly noted by weight loss.…”