Background: Excessive thinness has been glamorized among Japanese adolescent girls, and unhealthy dieting to lose weight has become a popular practice among them. The prevalence of underweight in contemporary Japanese girls in comparison with that of boys, however, remains to be studied. Aim: The purpose of this study was to determine the prevalence and trends of underweight in contemporary Japanese teenagers of both sexes using nationally representative cross-sectional data. Subjects and methods: Underweight was defined as BMI < 3rd percentile by age and sex of the 1978-1981 reference population as previously reported. These reference values were compared with BMIs of the contemporary population based on the 2001 Cross-sectional National Survey data of Japanese teenagers (215 972 boys and 216 496 girls). Results: The prevalence of underweight was approximately 2-3% in junior high school students of both sexes, 12.5-14.5 years of age, approximately 3% in senior high school students of both sexes, 15.5-16.5 years of age, and approximately 4% in senior high school students of both sexes, 17.5 years of age. Conclusion: The prevalence of underweight decreased among Japanese teenage students of both sexes during the past decades. Female preponderance in the prevalence of underweight was not confirmed.
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