Childhood obesity has now become the most prevalent nutritional disease in developed countries. For example, the prevalence of obesity, defined as a body mass index (BMI) equal to or above the 95th centile for children of the same age and sex, now affects 10 – 15% of children and adolescents in the United States (Flegal et al., 1998). When the prevalence of obesity in the United States is compared across nationally representative surveys conducted over the last 30 years, the most rapid increases in prevalence occured between 1980 and 1994. The greatest increases in body weight have occured in children and adolescents in the upper half of the BMI distribution (Troiano & Flegal, 1998). Stated another way, the mean BMI for children of the same age and sex has increased more than the median. These observations suggest at least two possibilities. They may suggest that the genes that predispose to obesity occur in approximately 50% of the population. Alternatively, these observations suggest that the factors that influence the development of obesity are discrete, and act only on half of the population.

Elsewhere in the world, obesity is also increasing rapidly. Nevertheless, the world-wide prevalence of obesity is generally lower than the prevalence observed among children and adolescents in the United States.

The factors that account for the rapid changes in prevalence remain unclear.