Problems with body mass index as an index to evaluate physical status of children in puberty.

Background: The aim of the present study was to examine the problems associated with using body mass index (BMI) for evaluating the physical status of children during puberty and the problems associated with establishing international standard values as cut-off points for obesity and thinness in children.

Methods: A cohort study was undertaken on BMI as an indicator of physical status in puberty based on the data in the 2000 Annual Report of School Health Statistics. The subjects were 695,600 children from 5 to 17 years of age.

Results: In puberty, even in those with the optimal weight for their sex and age, BMI increases as height increases; therefore, a considerable variation in the range of BMI is observed in normal individuals of the same degree of fatness. For this reason, there is considerable overlap in the "healthy weight", "overweight" and "obese" groups defined by the CDC using BMI percentile with regard to fatness in puberty. When the pubertal growth patterns are different among children from different countries, it is difficult to determine internationally average BMI by sex and age.

Conclusions: Distinguishing the respective healthy weight, overweight, and obese groups based on cut-off points using BMI percentile during puberty is difficult. If differences exist in the socioeconomic status of the subject cohorts and in the passage through puberty among children from different countries, we must consider that average values of BMI for the same age of children in puberty will have different meanings. With regard to evaluation of physical status in pubertal children, only BMI average values for the same stage of pubertal development are meaningful, rather than individual values.