The relationship between body size and mortality in the linked Scottish Health Surveys: cross-sectional surveys with follow-up

Abstract

Objective: To investigate the relationship between body mass index (BMI), waist circumference (WC) or waist-hip ratio (WHR) and all-cause mortality or cause-specific mortality.

Design: Cross-sectional surveys linked to hospital admissions and death records.

Subjects: In total, 20,117 adults (aged 18-86 years) from a nationally representative sample of the Scottish population.

Measurements: Cox proportional hazards models were used to estimate hazard ratios (HRs) for all-cause, or cause-specific, mortality. The three anthropometric measurements BMI, WC and WHR were the main variables of interest. The following were adjustment variables: age, gender, smoking status, alcohol consumption, survey year, social class and area of deprivation.

Results: BMI-defined obesity (≥ 30 kg m(-2)) was not associated with increased risk of mortality (HR = 0.93; 95% confidence interval = 0.80-1.08), whereas the overweight category (25-<30 kg m(-2)) was associated with a decreased risk (0.80; 0.70-0.91). In contrast, the HR for a high WC (men ≥ 102 cm, women ≥ 88 cm) was 1.17 (1.02-1.34) and a high WHR (men ≥ 1, women ≥ 0.85) was 1.34 (1.16-1.55). There was an increased risk of cardiovascular disease (CVD) mortality associated with BMI-defined obesity, a high WC and a high WHR categories; the HR estimates for these were 1.36 (1.05-1.77), 1.41 (1.11-1.79) and 1.44 (1.12-1.85), respectively. A low BMI (<18.5 kg m(-2)) was associated with elevated HR for all-cause mortality (2.66; 1.97-3.60), for chronic respiratory disease mortality (3.17; 1.39-7.21) and for acute respiratory disease mortality (11.68; 5.01-27.21). This pattern was repeated for WC but not for WHR.

Conclusions: It might be prudent not to use BMI as the sole measure to summarize body size. The alternatives WC and WHR may more clearly define the health risks associated with excess body fat accumulation. The lack of association between elevated BMI and mortality may reflect the secular decline in CVD mortality.

Figure 1

Functional form of the association of BMI with the relative hazard of death estimated in a Cox proportional hazards model adjusted for age, gender, smoking, alcohol consumption and survey year. The function was fitted using two-term fractional polynomial functions with powers (log, log). The function was standardized such that the HR was 1 at the mean of the desirable weight category for BMI (18.5–<25 kg m⁻²)=22.57 kg m⁻². Dot-dash lines indicate the 95% confidence interval.