Changes in the distribution of body mass index of adults and children in the US population

BACKGROUND: National survey data show increases in mean body mass index (BM I) and in the prevalence of overweight and obesity for adults and children in the United States, indicating a change in the distribution of BMI. OBJECTIVE: To apply graphical methods to describe changes in the distributi on of BMI. DESIGN: BMI values from the third National Health and Nutrition Examination Survey (NHANES III: 1988-94) were compared with data from earlier cross-se ctional nationally representative surveys for adults 29-74y of age and for children and adolescents 6-17 y of age. Tukey mean-difference plots were us ed to investigate the changes in the distributions of BMI within sex-age gr oups. RESULTS: Mean-difference blots allow qualitative visual comparisons of the distributions of BMI between surveys. For ail sex-age groups, there was inc reasing skewness with a greater shift in the upper part of the distribution so that, within each group, the heaviest subgroup was heavier in NHANES II I than in prior surveys. For the youngest children, the lower part of the d istribution showed virtually no change. With increasing age the whole distr ibution tended to shift upward slightly, suggesting an increase in BMI acro ss the entire population. CONCLUSIONS: These changes in the distribution of BMI suggest the combinati on of both profound environmental determinants and a population with a high degree of susceptibility. The reasons for the increasing prevalence of obe sity should be sought in part by seeking to understand the factors causing increases in the population as a whole.