Are the recent secular increases in the waist circumference of adults independent of changes in BMI?

BACKGROUND Several studies showed that the waist circumference of US adults has increased over the past 25 y. However, because of the high correlation between waist circumference and body mass index (BMI; in kg/m²) (r ∼ 0.9), it is uncertain if these trends in waist circumference exceed those expected on the basis of BMI changes over this time period. OBJECTIVE We assessed whether the recent trend in waist circumference was independent of changes in BMI, age, and race-ethnicity. DESIGN We analyzed data from the 1999-2000 through 2011-2012 cycles of the NHANES. RESULTS The mean waist circumference increased by ∼2 cm (in men) and ∼4 cm (in women) in adults in the United States over this 12-y period. In men, this increase was very close to what would be expected because of the 0.7 increase in mean BMI over this period. However, in women, most of the secular increase in waist circumference appeared to be independent of changes in BMI (mean: 0.6), age, and race-ethnicity over the 12-y period. We estimated that, independent of changes in these covariates, the mean waist circumference increased by 0.2 cm in men and 2.4 cm in women from 1999-2000 through 2011-2012; only the latter estimate was statistically significant. CONCLUSIONS Our results indicate that, in women but not men, the recent secular trend in waist circumference is greater than what would be expected on the basis of changes in BMI. Possible reasons for this secular increase, along with sex differences, are uncertain.