Comparison of children's body fatness between two contrasting income groups: contribution of height difference.

OBJECTIVE: To compare measures of growth and body fatness (body mass index (BMI) and % body fat) in children from two contrasting income backgrounds and to examine the contribution of height difference to these measures.
DESIGN: Cross-sectional study.
SETTING: Schools in inner East London ('low income') and West London, Hertfordshire and Cambridgeshire ('high income'), UK. PARTICIPANTS: A total of 2298 children aged 5-14 years. MAIN OUTCOME MEASURES: Height, weight, BMI (weight per height(2)) and percentage body fat (%BF, by bioelectrical impedance analysis).
RESULTS: Children from the 'lower income' background were significantly shorter, heavier and fatter (%BF) with a higher BMI for their age compared with those from a 'higher income' background. Prevalence of overweight/obesity was greater in the 'lower income' group children, assessed on the basis of BMI, and this income group difference was magnified when based on %BF (overfat/obese). Irrespective of the assessment tool used, overweight/overfat/obese children as a group were significantly taller for their age compared with children categorized as normal weight/normal fat. Despite the overfat/obese children being taller for their age, an 'income group' difference in height remained within this category.
CONCLUSION: These findings confirm the income group influence on obesity prevalence. They also illustrate that BMI underestimates the true number of children having excess body fat, particularly in 'low income' children. Exactly why BMI seems to function differently along income group divisions in unclear, but a shorter height-for-age of the 'lower income' group children could be one explanation. These findings raise important questions about the causes and consequences of obesity in children from 'lower income' backgrounds.