12. Indispensable amino acid requirements for the anabolic drive

As already indicated, there is a substantial amount of circumstantial evidence to suggest that height growth in children reflects the overall level of protein intake (GOLDEN, 1985), and can be assumed therefore to be one important target of the anabolic drive. However, few studies have addressed the question of whether the amino acid quality of dietary protein is important in this role. In one study from South India, comparing the ability of different cereals to support height growth over six months, children fed rice-based diets (n = 10) grew in height at a faster rate than others fed isonitrogenous, isoenergetic wheat-based diets (BEGUM et al., 1970). On the basis of the amino acid compositions of the diets, with lower levels of lysine threonine and isoleucine in the cereal diets, the authors suggested that height growth was limited by IAA supply. However, as pointed out by VAGHEFI et al., (1974) in their review of this and other nutritional studies with cereal-based diets, the possibility of other nutrients limiting height growth must be considered, especially since riboflavin deficiency was actually described in some of the children in these studies. Indeed, GOLDEN, GOLDEN and BENNETT (1988) have argued that in most nutritional studies relating to child growth, even in the absence of any clear signs of deficiencies which might occur on these cereal-based diets, interpretation of the responses is often most difficult, because, for several nutrients which limit growth when deficient, this deficiency cannot be detected since no unique signs occur apart from growth failure. Thus, GOLDEN et al., (1988) argue that, in addition to protein, any one of the minerals such as zinc, phosphate, potassium, magnesium and sodium could be growth-limiting due to its low content and/or bioavailability. Certainly, several supplementation studies in young children (HAMBIDGE, 1991) have indicated an important role for zinc deficiency in inadequate height growth in children.

The evaluation of the extent and mechanisms by which the anabolic drive of dietary protein and IAAs does regulate height growth is, in my view, a most important research task. In fact we know little about the relationship between IAA intakes and any regulatory responses which comprise the anabolic drive. One possibility relates to the role of amino acids in the regulation of the secretion of the anabolic hormones. It has long been known from studies on rodent pancreatic islet control, that the potency of amino acids as insulin secretagogues varies markedly (FAJANS and FLOYD, 1972) and that, with the exception of arginine (the most powerful secretagogue), the IAAs are generally more potent than the dispensable amino acids. Thus, an influence of protein quality on insulin secretion and consequent changes in the levels of the other anabolic hormones regulated by insulin, is one possible mechanism. However, given the fact that, in humans, insulin secretion is less dependent on amino acids than in rodents (see MILLWARD, 1990a), some caution is required in adopting such an explanation.

Animal studies are not particularly informative since very few of them have specifically looked at the regulatory influence of dietary protein sources of different quality. In one study, CREE and SCHALCH (1985) reported higher IGF-1 levels in rats fed casein, compared with isonitrogenous wheat gluten. However, in the rat, plasma IGF-1 levels (and insulin levels; JEPSON et al., 1988) increase with increasing age and body weight and, since the casein-fed rats grew faster, it was not possible to disentangle the influences of body weight growth, as opposed to dietary protein source, on the IGF-1 level.