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

SummaryObjectiveThe 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.MethodsBody 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/m2, and 12,044 females, with a mean BMI of 25.7 ± 5.1 kg/m2. Multiple regression analysis was used.ResultsBMI 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.ConclusionThe 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.