Obesity is an excess of body fat. In adults, values for the ‘normal’ and ‘healthy’ amounts of body fat have not been defined although it is recognized that on average men and women differ in the percentage of body weight as fat (%BF) with women being fatter than men. Percentage BF is also under genetic influence and there are ethnic and familial differences in normal fatness. In childhood the proportion of fat to lean tissue in the body not only varies between boys and girls, but also changes with age and physical maturity making it even more difficult to determine what is physiologically normal either for an individual or for a population (Table 2.1).

Even within populations with similar lifestyles, the range of fatness between individuals is wide. Familial tendencies to obesity are probably a mixture of the effects of shared environment and shared genetic inheritance.

Fattening periods

Table 2.1 shows how estimated fatness as %BF varies with age in boys and girls. The absolute values vary with the method used to determine them and in most cases the values were recorded some years ago when the subject population was considerably thinner than now.

Physiologically the age related changes in body composition suggest the body prepares for periods of vigorous growth (early years, puberty and pregnancy) by laying down fat which can then fuel subsequent growth.

Early infancy

Figure 2.1 illustrates the changing fattening and ‘stretching’ periods of childhood in boys and girls.