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In 1997 when WHO initiated the formation of the International Obesity Task Force (IOTF), the Task Force proposed the cut-offs for overweight and obesity as BMI 25 and BMI 30, respectively. If we accept the criteria of BMI ≥ 30 to indicate obesity, it would appear that the prevalence of obesity in Japan of less than 3% has changed little during the last 40 years, and we cannot explain the rapid increase in incidence of obesityassociated chronic diseases such as diabetes, hypertension and hyperlipidemia. Thus, JASSO decided to define BMI ≥ 25 as obesity. This cut-off has been proposed for use in the Asia-Oceania Region, and WHO Western Pacific Region noted this proposal. According to this criterion the prevalence of obesity in Japan would average 20%, with a high of 30% in men over 30 years old, and women over 40 years old. Thus the rates would have increased four times in men and three times in women during these last 40 years. What has caused the increased prevalence of obesity in Japan? Several causes of obesity have been advanced: (i) overeating (ii) errors of eating pattern (iii) inactivity (iv) heredity, and (v) disturbance in thermogenesis. Hyperphagia and inactivity are two major risk factors for obesity. Hyperphagia may be an important factor in individuals. However, the average energy intake in adult people in Japan has not increased; in fact it has declined (2104 kcal/day to 1967 kcal/day) during these 40 years. During this period, the prevalence of obesity has increased three or more times as mentioned above. This indicates that inactivity may be the main cause for the increased incidence of obesity in Japan. Errors of eating pattern (irregular eating, night eating, etc.), including a high proportion of fat to total energy intake (8.7% increased to 26.5%), and a high incidence of β 3-adrenergic polymorphism, might also have contributed to the increased incidence of obesity in Japan.