Obesity has increased at an alarming rate throughout the world and has been regarded as a global epidemic disease in light of its close association with a cluster of cardiovascular risk factors, including hypertension, dyslipidemia, and hyperglycemia. This clustering of metabolic disorders is known as the metabolic syndrome, which is associated with insulin resistance (1). BMI is an estimate of total body fat mass and is probably the most useful scale to define obesity. Obesity has been defined as a BMI >30.0 kg/m2 in World Health Organization (WHO) classification (2), but this does not take into account the morbidity and mortality associated with more modest degrees of overweight. A significant increase in risk of death from cardiovascular disease was found for all BMIs of >25.0 kg/m2 in women and >26.5 kg/m2 in men in a prospective study conducted in the U.S. (3). The relation between BMI up to 30.0 kg/m2 and the relative risk of several chronic conditions caused by excess body fat, including type 2 diabetes, hypertension, coronary heart disease, and cholelithiasis, appears to be approximately linear (4). In Japan and most other Asian countries, a pronounced increase in the prevalence of overweight and obesity has been observed during the past two decades (5). Although the Japanese are often considered to be nonobese compared with Caucasians, because of the differences in the prevalence of obesity and BMI distribution, the clustering of cardiovascular risk factors is thought to occur in this relatively lean population as well.

To investigate whether an increment in body weight increases the risk of metabolic complications in the Japanese, we studied the relation of a graded classification of obesity using BMI values based on the WHO classification to components of the metabolic syndrome, including the levels of fasting plasma glucose (FPG), total cholesterol (TC), triglycerides (TGs), high-density lipoprotein cholesterol (HDL-C), blood pressure, and uric acid. In a population-based cross-sectional study of 1,559 healthy adults (1,169 men, 390 women) aged 35–60 years who underwent annual health examinations in 1998, we classified the subjects into four groups: underweight (BMI <18.5 kg/m2, n = 113), normal (18.5–24.9 kg/m2, n = 1,086), preobese (25.0–29.9 kg/m2, n = 323), and class I obese (30.0–34.9 kg/m2, n = 37) based on the WHO classification (Table 1). Venous blood was sampled after an overnight fast for routine laboratory investigations. Comparisons between groups were performed with Bonferroni’s multiple comparison test. In our study the prevalence of BMI ≥25.0 kg/m2 was 23.1%, and that of BMI ≥30 kg/m2 was 2.4%. All but one of the components were significantly higher (only HDL-C was lower) for the preobese group compared with the normal group (P < 0.001). These components were also higher for the normal group than for the underweight group, except for TGs (P < 0.01). No statistically significant differences were observed among any of the parameters except for systolic blood pressure in the preobese and class I obese groups, whereas there were differences in all of the parameters besides TC (P = 0.09) between class I obese and normal groups (P < 0.05) (Table 1). This means that a significant increase in all of the components of the metabolic syndrome was recognized in preobesity defined as BMI 25.0–29.9 kg/m2 in the WHO classification. However, no BMI-related differences in FPG, TC, TGs, HDL-C, diastolic blood pressure, and uric acid were observed between preobesity and class I obesity. Therefore, abnormalities in these parameters seem to reach a plateau before the BMI reaches 30.0 kg/m2, although this finding should be confirmed in a larger population study.

Thus, 1) the risk of metabolic syndrome is significantly related to the degree of obesity, 2) underweight appears to be more preventive against the metabolic syndrome than normal weight, and 3) preobesity in the WHO classification involves increased cardiovascular risk factors for the Japanese. Therefore, a lower BMI at 25.0 kg/m2 should be used for the Japanese population to estimate the prevalence of obesity and to identify the high-risk groups for cardiovascular disease. Racial differences should thus be taken into account when defining obesity, and we propose a BMI of 25.0 kg/m2 as the optimal cutoff point for obesity in Japanese and presumably other Asian populations.

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Address correspondence to Toshinari Takamura, Department of Endocrinology and Metabolism, Kanazawa University Graduate School of Medical Science, 13–1 Takara-machi, Kanazawa, Ishikawa 920-8641, Japan. E-mail: tt@medf.m.Kanazawa-u.ac.jp.