The relationship between body mass index and per cent body fat in the severely obese

Background: International standards define clinical obesity according to body mass index (BMI) without reference to age and gender. Recent studies among adults in the normal to mildly obese BMI ranges have shown that the relationship between BMI and per cent body fat (% fat) differs by age and gender. The extent to which age and gender affect the relationship between BMI and % fat among more severely obese individuals is less known. Aim: The aim was to examine the age–gender association between measured BMI and % fat from a large cohort of adults, including a large number of severely obese subjects (1862 with a BMI ≥ 35 kg/m2). Methods: BMI was computed from measured height and weight, and % fat was estimated from bioelectrical impedance in 3068 adults. Two impedance equations, the Sun equation and the Heath equation (specific to severe obesity), were used to calculate % fat. Results: Average age for 991 men and 2077 women was 46 ± 15 vs. 44 ± 14 years respectively (p = 0.0003). The average BMI was 36 ± 9 kg/m2 for men and 39 ± 10 kg/m2 for women (p