Body mass index as an estimate of body fat

Body mass index (BMI) was compared with percentage body fat (%Fat) measured by dual energy X-ray absorptiometry (DXA) in 233 adolescent schoolgirl volunteers and 179 adult female patients. Repeat measurements were made on 67 of the adolescents and 51 of the adults. The correlations between BMI and %Fat were established from the 300 adolescent measurements and the 230 adult measurements. Although highly significant relationships were found between BMI and %Fat, only 58% of the variance in %Fat in adolescents and 66% in adults could be predicted by BMI. At the 95% confidence levels, a BMI of 20 kg m-2 can correspond to a range of 18-33% body fat in adolescents and 13-32% in adults. Without any change in BMI, an adolescent's percentage fat can change by as much as -3% to +7%. For an individual adult the same BMI can correspond to changes in fat of +/-5%. Since the strength of prediction of percentage body fat from BMI is poor, caution should be exercised in its use for eating disorders research.