Effect of age on the association between body mass index and all-cause mortality: the Ohsaki cohort study

To clarify the effect of age on the association between body mass index (BMI) and all-cause mortality.

We followed 43 972 Japanese participants aged 40 to 79 years for 12 years. Cox proportional hazards regression analysis was used to estimate hazard ratios (HRs), using the following BMI categories: <18.5 (underweight), 18.5-20.9, 21.0-22.9, 23.0-24.9 (reference), 25.0-27.4, 27.5-29.9, and ≥30.0 kg/m(2) (obese). Analyses were stratified by age group: middle-aged (40-64 years) vs elderly (65-79 years).

We observed a significantly increased risk of mortality in underweight elderly men: the multivariate HR was 1.26 (0.92-1.73) in middle-aged men and 1.49 (1.26-1.76) in elderly men. In addition, we observed a significantly increased risk of mortality in obese middle-aged men: the multivariate HR was 1.71 (1.17-2.50) in middle-aged men and 1.25 (0.87-1.80) in elderly men. In women, there was an increased risk of mortality irrespective of age group in the underweight: the multivariate HR was 1.46 (0.96-2.22) in middle-aged women and 1.47 (1.19-1.82) in elderly women. There was no excess risk of mortality with age in obese women: the multivariate HR was 1.47 (0.94-2.27) in middle-aged women and 1.26 (0.95-1.68) in elderly women.

As compared with the reference category, obesity was associated with a high mortality risk in middle-aged men, whereas underweight, rather than obesity, was associated with a high mortality risk in elderly men. In women, obesity was associated with a high mortality risk during middle age; underweight was associated with a high mortality risk irrespective of age. The mortality risk due to underweight and obesity may be related to sex and age.