In developed and developing countries, morbidity and mortality rates are increasing in individuals classified as being obese [1-6]. The higher morbidity and mortality rates of obese people are due to the increased incidence of obesity-related (lifestyle-related) diseases. It is now recognized that different criteria of obesity by BMI are necessary in different ethnic groups and populations [7,8]. In addition to the degree of obesity, other factors which increase the morbidity rate of obese people include abnormal fat distribution such as upper body obesity and visceral obesity [9,10]. Recently, the definition of 'pathological obesity' has been made in Japan [11]. Definition of Obesity and Previous Criteria of ObesityObesity is defined as excessive fat accumulation but not over-weightedness. The average human body usually consists of 82% lean body mass, which is essential for sustaining daily life and physical activities, and 18% body fat which, in essence, is energy store for emergency situations [12]. Thus, obesity can be defined as 'overstorage of body fat beyond 18%'. Usually, body fat above 25% in men and 30% in women is considered to be obese. According to this definition, obesity should be determined by measuring body fat. Although there are presently many methods for measuring body fat, no methods can be conducted easily, accurately and inexpensively.

The aim of this study was to describe the 20-year changes in body mass index (BMI; kgm(-2)) and the prevalence of overweight in Japanese adults. Cross-sectional annual nationwide surveys (National Nutrition Survey, Japan) were carried out with a large probability sample of the Japanese population. Data sets of the 1976-95 surveys, comprising 91983 men and 120822 women (> or =20 years of age), were used. The analyses were carried out in age and gender groups, and by residential area according to the size of the municipality (metropolitan areas, cities and small towns). The mean BMI increaseed in men with an increment of +0.44 kg m(-2) 10 years(-1) and slightly decreased in women, by -0.09 kg m(-2) 10 years(-1), after adjustment for age. A decreasing trend of mean BMI was most significant in the female 20-29 years age-group (-0.38 kg m(-2) 10 years(-1)), in contrast to an increasing trend observed in elderly women (60-69 and 70+ years age-groups). The prevalence of preobese (BMI: 25-29.9kg m(-2)) and obese (BMI > or = 30 kg m(-2)) men increased from 14.5% and 0.8%, respectively, in the time-period 1976-80 to 20.5% and 2.01% during 1991-95. The increasing trend was most evident in the youngest age-group (20-29 years) and in those from small towns. The overall prevalence of preobese and obese women did not change during the 20-year study period. The prevalence in younger women decreased, this trend being more prominent in metropolitan areas. Although the prevalence of overweight (BMI > or =25 kg m(-2)) in Japanese men and elderly women has increased in the last 20 years, the mean BMI in younger women, especially those in metropolitan areas, has decreased. A population approach to control obesity and prevent obesity-associated diseases should be mainly focused on men and women older than 40 years of age.

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