Associations of Body Mass Index and Waist Circumference with 3-Year All-Cause Mortality Among the Oldest Old: Evidence from a Chinese Community-Based Prospective Cohort Study

Abstract

Objective: Current international and national guidelines for body mass index (BMI) and waist circumference (WC) have been recommended to all adults. However, whether recommendations applied to the oldest old (aged 80+) is poorly known. The study objective was to investigate the relation of BMI and WC with 3-year all-cause mortality among the oldest old.

Design, setting, and participants: A total of 4361 Chinese oldest old (mean age 91.8) participated in this community-based prospective cohort study.

Measurements: BMI and WC were measured at baseline in 2011 and were used as continuous variables and as categorized variables by recommendations or by tertiles. Adjusted, sex-stratified Cox models with penalized splines and Cox models were constructed to explore the association.

Results: Greater BMI and WC were linearly associated with lower mortality risk in both genders. The mortality risk was the lowest in overweight or obese participants (BMI ≥ 24.0) and was lower in participants with abdominal obesity. Compared to the upper tertile, those in the middle and lower tertile of BMI had a higher risk of mortality for men [hazard ratio (HR): 1.23 (1.02-1.48) and 1.53 (1.28-1.82)] and for women [HR: 1.21 (1.03-1.41) and 1.35 (1.15-1.58)]; it was also found in participants in the middle and lower tertile of WC for men [HR: 1.21 (1.01-1.46) and 1.41 (1.18-1.69)] and for women [HR: 1.35 (1.15-1.58) and 1.55 (1.32-1.81)] (all the P values for trend <.001). These findings were robust in further sensitivity analyses or when using propensity score matching, in subgroup analyses, or in octogenarians, nonagenarians, and centenarians.

Conclusions: In Chinese oldest old, both higher BMI and higher WC predict better survival in both genders. The finding suggests optimal BMI and WC may be sensitive to age, thus, the current recommendations for the oldest old may need to be revisited.

Keywords: Body mass index; mortality; oldest old; waist circumference.

Figure 1.. The 3-year all-cause mortality in different BMI and WC tertiles by gender

For men, the 3-year all-cause mortality was 40.7%, and were significantly higher in participants with upper BMI and WC level (p<0.001); the mortality was 48.9%, 39.3% and 34.4% for participants with lower, middle and upper BMI (Figure 1a), and 45.7%, 41.0% and 35.8% for participants with lower, middle and upper WC (Figure 1b). For women, the 3-year all-cause mortality was 41.5%, and were significantly higher in participants with upper BMI and WC level (p<0.001); the mortality was 49.4%, 42.3% and 33.0% for participants with lower, middle and upper BMI (Figure 1c), and 49.9%, 43.5% and 32.4% for participants with lower, middle and upper WC (Figure 1d).

Figure 2.. The associations of BMI and WC with mortality for men and women in Cox models with penalized splines

The lines depict the estimated hazard ratio of BMI or WC for 3-year all-cause mortality in men and women, and the dotted lines indicate the 95% confidence intervals. In Cox models with penalized splines, linear associations of BMI and WC with mortality were identified for men and women on the basis of AIC (the AICs were lower for linear association than the non-linear, and the p-values for linear association were less than 0.001 for men and women).