Human body composition undergoes profound change during the life course. In this pattern our species is unique, indicating that the ontogenetic profile of growth and development is a key feature of human biology. Increases and decreases in body fat content imply changes in the relative value of adipose versus lean tissue at different ages, and between the sexes. In old age in particular, there is substantial variability in fatness between populations, yet this can be interpreted as an interaction with variability in energy availability, mediated by declines in lean mass.

The aim of this chapter is to describe the changes in body composition from fetal life to old age. As was the case for the between-species and within-species comparisons reviewed in Chapter 2, ontological data have been collected with a variety of methods. However, ignoring the varying prevalence of obesity, the ontological pattern of development of body composition shows a relatively high degree of consistency between populations. Although subtle differences between populations are likely to occur, reflecting similar variability in growth rate, it is possible to use data primarily from industrialised populations to illustrate the main changes, provided that analysis is restricted to those within the normal range of weight. Indeed, age-associated changes in the regional adipose tissue distribution characteristic of obesity themselves carry clues about the ontogeny of adiposity (Wells, Cole, and Treleaven 2008). Thus, although further data from non-Western populations remain much needed, the overall life-course pattern of adiposity is reasonably well established.