Increases in body mass index, even within non‐obese levels, raise the risk for Type 2 diabetes mellitus: a follow‐up study in a Japanese population

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Abstract
Aims Obesity, the strongest risk factor for Type 2 diabetes mellitus, is less prevalent in Japanese than in other populations. We investigated the effects of body mass index (BMI, kg/m2) on the incidence of diabetes mellitus in a Japanese population. Methods A follow‐up study in 16 829 men and 8370 women who were apparently healthy at baseline (age 30–59 years, BMI 14.9–43.2 kg/m2). Incident diabetes mellitus was identified by ‘fasting serum glucose ≥ 7.00 mmol/l (126 mg/dl)’ and/or ‘under medical treatment for diabetes mellitus’. Hazard ratio, as an index for risk ratio, for incident diabetes mellitus according to BMI was estimated using Cox's proportional hazard models. Baseline age, smoking, drinking, exercise and education were computed as confounders. Results During mean follow‐up periods of 7.4 years for men and 7.1 years for women, 869 men and 224 women had incident diabetes mellitus. Although the subjects were averagely non‐obese [mean (sd) BMI 23.1 (2.6) kg/m2 for men, 22.3 (2.7) kg/m2 for women], hazard ratio for incident diabetes mellitus increased in parallel with increases in BMI. Multivariate‐adjusted hazard ratios (95% confidence intervals) for increases in BMI of 1 kg/m2 were 1.26 (1.24, 1.29) for men and 1.24 (1.20, 1.29) for women. Conclusion BMI, even within the non‐obese level, is a dose‐dependent risk factor for diabetes mellitus in middle‐aged Japanese. Increases in BMI of 1 kg/m2 (= body‐weight gain of 2.4–2.9 kg) may raise the risk by about 25%.