Objective: To examine the impact of body mass index (BMI) upon medical care use and its costs in Japan.
Design: A population-based prospective cohort study from 1995 to 1998.
Subjects: A cohort of 41 967 Japanese adults aged 40-79 y. Subjects who died during the first year of follow-up, or who at baseline reported having had cancer, myocardial infarction, stroke or kidney disease were excluded.
Measurements: Medical care use and its costs, actual charges, by linkage with the National Health Insurance claim history files after adjustment of smoking, drinking and physical functioning status.
Results: There was a U-shaped association between BMI and total medical costs. The nadir of the curve was found at a BMI of 21.0-22.9 kg/m(2). Relative to the nadir, total costs were 9.8% greater among those with BMIs of 25.0-29.9 (rate ratio, 1.10; 95% confidence interval (CI), 1.03-1.17), and 22.3% greater among those with BMIs of 30.0 or higher (rate ratio, 1.22; 95% CI, 1.08-1.37). Estimated excess direct costs attributable to overweight (BMI of 25.0-29.9 kg/m(2)) and obesity (BMI of 30.0 kg/m(2) or higher) represent 3.2% of total health expenditure in the present study, which is within the range reported in Western countries (0.7-6.8%).
Conclusion: Our prospective data demonstrate that the impact of overweight and obesity upon medical care costs in Japan is as large as in Western countries, despite the much lower mean BMI in Japanese populations.