Association of body mass index, waist circumference and waist-hip ratio with mortality in the Scottish Health Surveys
Background
Obesity is a major health challenge of the 21st century. Simple anthropometric measurements, including body mass index (BMI), waist circumference (WC) and waist-hip ratio (WHR), are utilised in epidemiological studies to capture (indirectly) summary estimates of body fat proportion. Some representative population surveys have not demonstrated the strong U or J shaped association between BMI and mortality that might be expected.
Objectives
To examine association of body size with mortality using BMI and two gender specific alternatives believed to reflect fat distribution: WC and WHR.
Design
The linked Scottish Health Surveys provide representative population data, collected by stratified and clustered probability sampling, and are linked to subsequent death records.
Participants
22 426 respondents from three survey waves (1995, 1998 and 2003) who consented to data linkage and were between the ages of 18 and 86 years at the time of interview (9924 men, 12 502 women).
Methods
Cox proportional hazards analyses were used to model the relationship between mortality and each of the variables BMI, WC and WHR. Only valid measurements of each were included and each was divided into four categories with the second lowest category (38.4%, 40.3% and 49.3% respectively) as referent. Hazard ratios (HR) for survival time from interview to death, or 31st December 2007, were firstly adjusted for age, gender, smoking status, alcohol consumption and survey year; and then further adjusted by measures of socio-economic status. Sensitivity analyses involved stratification by gender and age.
Results
The prevalence of obesity, according to World Health Organisation definitions BMI (≥30 kg/m2), WC (men ≥102 cm, women ≥88 cm) and WHR (men ≥1.0, women ≥0.85) was 22.2%, 27.2% and 18.7% respectively. There was no increased risk of mortality associated with obesity as defined using BMI (HR 0.93, 95% CI 0.80 to 1.07). In contrast, the HR for subjects with obesity determined by WC was 1.17 (1.02 to 1.34) and by WHR was 1.34 (1.16 to 1.55). BMI and WC identified the association of “underweight” with increased mortality. Results for CVD mortality showed a stronger gradient. Inclusion of socio-economic status in the models attenuated the results to a limited extent. Gender stratification strengthened the association of underweight with mortality for men using BMI (HR 2.90; 1.87 to 4.51) and of obesity with mortality for women using WHR (HR 1.47; 1.19 to 1.83).
Conclusions
BMI may not capture the harmful association of body size with mortality. WC and WHR may more clearly define the health risks associated with excess body fat accumulation.
