reveals that the former has taken place. There has been a decrease in the proportion of the population at healthier weights (e.g., BMI <25 was 57.2 percent in 1960–1962, 54.2 percent in 1971–1974, 54.4 percent in 1976–1980, and 48.1 percent in 1988–1991) and a general increase in the group at risk for weight-related problems (see Figure 2-5). The change in the BMI distribution can also reflect changes in the distribution of other behaviors. For example, smoking is highly related to weight and weight gain, and there has been a general decline in the proportion of the U.S. population that smokes (NCHS, 1994). However, adjusting for the effects of changes in smoking prevalence does not account for the increase in the proportion overweight from 1976–1980 to 1988–1991 (unpublished data from NHANES III, Phase 1, 1988–1991, provided by NCHS).
Cross-sectional data give valuable information on the status of the U.S. population at a given point in time, but longitudinal data are needed