The relationship between body mass index and number of days spent in hospital in Scotland

In most countries in the Western world, more than 50% of adults are overweight or obese putting them at increased risk of hypertension, type 2 diabetes, coronary heart disease, stroke and other chronic disorders. It is not clear what impact increasing prevalence of over-weight and obesity has on hospital admissions. The objective of this study was to examine the relationship between body mass index (BMI) and number of days spent in hospital. The study was designed as a retrospective and prospective cohort study using nationally representative Health Survey data linked to NHS hospital admissions data. The study was set in Scotland. The participants were a nationally representative sample of 6968 (45%) men and 8700 (55%) women, of 16-74 years of age, living in private households whose BMI was recorded in the 1995 and 1998 Scottish Health Surveys. The outcome measure was the number of days spent in hospital between 1981 and 2004. The results showed that the proportion of participants in both normal weight (BMI 20-24.9 kg/m(2)) and over-weight (BMI 25-29.9 kg/m(2)) categories was 37%, with 21% in the obese (BMI >= 30 kg/m(2)) and 5% in the under-weight (BMI <20 kg/m(2)) categories. The median number of days spent in hospital between 1981 and 2004 was six. The odds ratios (95% confidence intervals) for spending above the median numbers of days in hospital adjusted for age, sex, socioeconomic status and behavioural factors (i.e. smoking, alcohol drinking and physical activity) were 1.29 (1.06-1.56) for the <20 kg/m(2) group, 1.00 (0.91-1.11) for the 25-29.9 kg/m(2) group and 1.24 (1.10-1.38) for the >= 30 kg/m(2) group compared with the 20-24.9 kg/m(2) group. In conclusion, extremes of BMI category identified at a single point in time are associated with spending above the median number of days in hospital over a 20-year period after adjusting for demographic, behavioural and socioeconomic exposures.