Previous - Next
(Discussion leader A.M. PRENTICE, rapporteur B. SCHÜRCH)
Indicators for the extent to which energy requirements are being met could, theoretically, include assessments of (1) energy intake, (2) growth, (3) physical activity, (4) morbidity, and (5) biochemical parameters (e.g., of protein turnover).
In the long term, people must be in energy balance, i.e., energy intakes must more or less equal energy expenditure. Recent estimates of total energy expenditure obtained with the doubly-labelled water method or based on multiples of measured BMR suggest that earlier measurements of apparently very low habitual intakes in certain population groups must be in error. Also, if we take measurements of energy intakes as an indicator, we have to decide what requirement is to be matched. There are many problems associated with this decision.
If we look at various components of growth, growth in length or height probably has to be excluded as an indicator of caloric adequacy because it is a very slow process that depends on many factors. It has been observed that stunted children have comparatively high intakes per kg body weight. Stunted Gambian children maintain the same fat mass as children of the same height in Cambridge. Changes in lean body mass might be worth exploring further. Thus, weight appears to be an appropriate but very gross indicator. It also may not be the first parameter to undergo changes when energy requirements are not being met.
A rough estimate of energy expended in physical activity can be obtained by measuring BMR and total energy expenditure, and by expressing the latter as a multiple of the former. On this basis and per kg body mass, stunted Gambian children do not differ in activity level from Cambridge children of normal height. The doubly-labelled water method does not seem to be sensitive enough to indicate the point at which energy intakes are so low that activity has to be reduced. Activity is also affected by other factors; e.g., by cultural constraint.
There is a relationship between undernutrition and morbidity, but it is difficult to isolate the role of energy deficiency in diets which most often are deficient in various nutrients as well.
Among biochemical parameters, urea kinetics might be worth further consideration.
Children often seem to be able to track along their centiles of growth curves despite their apparently insufficient diet. During infectious disease episodes their growth is interrupted, and the time afterwards, during which the child should be able to catch up, seems to be the critical period, when energy quality, energy density and micronutrient concentrations seem to be of importance. Under these circumstances, recovery often seems unduly delayed. This raises the question as to whether the diet during catch-up should be different from the normal diet and, if so, for how long during the recovery period children should be given such a special diet. It has also been observed that children do not particularly like foods with a high protein:energy ratio.
It is difficult to clearly define a disease episode. Seroconversion to hepatitis B. for instance, can take place without obvious symptoms, but this does not necessarily mean that this subclinical episode has no metabolic consequences. Scrimshaw's chapter refers to metabolic studies in which afebrile, asymptomatic infections with Q-fever or the 17-D strain of yellow fever used for immunizations resulted in significant catabolic losses.
It also still remains to be explained why healthy breast-fed babies grow more slowly than bottle-fed babies. Can a breast-fed baby with 20% fat mass possibly be energy deficient? The problem is that we know little about the bioavailability of these fat reserves, and other nutrients could be limiting growth. Metabolic changes can be observed in most energy-deficient children, but different children may have different capabilities of coping with energy deficiency.