Background: The desire for thin body stature and unhealthy eating behaviours have spread among Japanese boys. The prevalence of thinness in boys is expected to increase; however, recent trends have not been reported. Aim: To describe changes in the distribution of body mass index (BMI) of Japanese boys, from 2003 to 2012. Subjects and methods: BMI distribution changes were studied in 2599 relatively affluent Japanese children and adolescents, aged 12.5-17.5 years, during 2003-2012. The 2003 and 2004 data were combined and compared to the combined 2011 and 2012 data. Tukey mean-difference plots were used to study the direction and magnitude of shifts in BMI distributions. Results: Prevalence of thinness has progressively increased from 2.8-4.7% in 2003-2004 to 5.1-7.6% in 2011-2012 in Japanese boys. Tukey plots showed that the increases in the prevalence of thinness are related to shifts in the entire distribution of BMI in the sample. Conclusion: Thin body stature was documented in increasing proportion of Japanese male adolescents. BMI z-scores decreased across the entire BMI centile spectrums, indicating that the whole school population may be at risk of thinness.
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