Summary

There is huge variation in human energy expenditure world-wide; in this chapter, an attempt is made to see whether any of it can be attributed to plasticity. Most of the variation is due to differences in body size and physical activity. The former is self-evident, and can clearly be seen as an outcome of growth and development, while the second is largely behavioural, and is not subject to plasticity. More subtle factors, including variation in body composition, increased efficiency of muscular activity and increased metabolic efficiency at the cellular level, are the subjects of considerable research and debate, and are considered here in relation to the hormonal mechanisms that operate in growth faltering. The lower than expected basal metabolic rate in some populations cannot be attributed to body composition changes (i.e. plasticity) which arise in the course of growth faltering. However, economization of energy expenditure through increased muscular efficiency could arise during growth faltering. Low basal metabolic rate due to increased metabolic efficiency at the cellular level could be an indirect outcome of the lower food intake required to maintain energy balance at lower body weight. Both are likely to be mediated by thyroid hormones, which have a wide field of operation, and are directly involved in the growth faltering process itself.

Introduction

There is considerable variation in human energy expenditure, within and between individuals, and between populations. One accommodation to low energy intake in adults is small body size (Blaxter & Waterlow, 1985). If plasticity is the irreversible development of a range of characteristics in the course of growth and development, it is reasonable to question which nutritional adaptations, if any, could be outcomes of this process.