Changes in waist circumference relative to body mass index in Chinese adults, 1993-2009

Although body mass index (BMI) and waist circumference (WC) are correlated, the relationship between WC and BMI may have changed over time.

To describe temporal trends in BMI and WC distributions and quantify the increase in WC at a given BMI over time.

Data on adults aged 20-59 years from two waves (1993 and 2009) of the China Health and Nutrition Survey were used in a pooled cross-sectional analysis. Quantile regression examined age-adjusted temporal trends in the distributions of BMI and WC. Linear regression examined changes in mean WC over time, adjusting for BMI, age at survey and survey year. All models were stratified by gender.

There was a significant increase in BMI and WC over time, particularly at the 95th quantile: on average, men had 2.8 kg m(-2) (95% confidence interval (CI): 2.4, 3.3) and women 1.5 kg m(-)(2) (95% CI: 1.1, 2.0) higher BMI in 2009 compared with their counterparts in 1993. WC increased by 9.0 cm (95% CI: 7.5, 10.1) and 5.0 cm (95% CI: 3.4, 6.6) for men and women, respectively. On average, men and women had a 3.2 cm (95% CI: 2.8, 3.7) and 2.1 cm (95% CI: 1.7, 2.5) higher WC in 2009 compared with their counterparts in 1993, holding BMI and age constant. WC adjusted for BMI increased to a larger extent among obese versus lean individuals and among younger versus older women.

For both genders, BMI and WC increased significantly over time, with particularly greatest increase in magnitude in the upper tail of the BMI and WC distributions. Furthermore, WC at equivalent BMIs was higher in 2009, compared with their counterparts in 1993. Our findings suggest that even if BMI remained constant from 1993 to 2009, adults in 2009 might be at increased cardiometabolic risk as a result of their higher WC.