The relationship between BMI and percent body fat, measured by bioelectrical impedance, in a large adult sample is curvilinear and influenced by age and sex.

OBJECTIVE

The study aimed to establish the effects of age, gender and age-gender interactions on BMI-% fat relationships over a wide range of BMI and age. It also aimed to examine controversies regarding linear or curvilinear BMI-% fat relationships.

METHODS

Body composition was measured using validated bio-impedance equipment (Bodystat) in a large self-selected sample of 23,627 UK adults aged 18-99 (99% ≤70) years, of which 11,582 were males with a mean BMI of 26.3±4.7 (sd) kg/m(2), and 12,044 females, with a mean BMI of 25.7±5.1 kg/m(2). Multiple regression analysis was used.

RESULTS

BMI progressively increased with age in women and plateaued between 40 and 70 years in men. At a fixed BMI, body fat mass increased with age (1.9 kg/decade), as did % fat (1.1-1.4% per decade). The relationship between BMI and % fat was found to be curvilinear (quadratic) rather than linear, with a weaker association at lower BMI. There was also a small but significant age-gender interaction.

CONCLUSION

The association between BMI and % body fat is not strong, particularly in the desirable BMI range, is curvilinear rather than linear, and is affected by age.