Safe protein-calorie ratios in diets. The relative importance of protein and energy intake as causal factors in malnutrition.

There has been up to now a lack of agreement as to the way in which protein and energy requirements could be combined to give estimates of adequate dietary protein-energy ratios. In particular, the relevance of the simple ratio, average protein--average energy requirement as a basis for assessing diets, has been questioned on the grounds that it fails to take account of individual variability in needs for energy, and of the extent to which these may be independent of variability in protein requirements. The main problem is to evaluate the range over which individuals can adapt either energy intake to suit expenditure, or expenditure to suit intake, without detriment to health or growth. One solution adopted by Beaton and Swiss in a recent paper is to accept the range of observed variability of energy intakes in normal populations as a measure of this. An alterative is to make use of experimental evidence for the minimum energy intake for maintenance of body energy content. These two approaches are compared, and are shown to give quantitatively similar results. The method based on minimum maintenance requirements offers the further advantage that it allows an assessment of dietary situations in relation to the likelihood of occurrence of different forms of protein--energy malnutrition; those situations in which protein deficiency is a secondary consequence of low energy intake are differentiated from those in which the primary cause is an inadequate level of protein in the diet. The adequate "safe" level of protein-energy ratio in the diets of 2- to 3-year old children is close to 5% and since most varieties of cereal grains appear to provide utilizable protein levels of close to this amount, this lends further support to the view that primary protein deficiency is unlikely to be the main factor causing protein-energy malnutrition in communities for which cereals are the cheapest source of energy.