Relationship between Body mass index (BMI) and body fat percentage, estimated by bioelectrical impedance, in a group of Sri Lankan adults: a cross sectional study

Background: Body Mass Index (BMI) is used as a useful population-level measure of overweight and obesity. It is used as the same for both sexes and for all ages of adults. The relationship between BMI and body fat percentage (BF %) has been studied in various ethnic groups to estimate the capacity of BMI to predict adiposity. We aimed to study the BMI-BF% relationship, in a group of South Asian adults who have a different body composition compared to presently studied ethnic groups. We examined the influence of age, gender in this relationship and assessed its' linearity or curvilinearity. Methods: A cross sectional study was conducted, where adults of 18-83 years were grouped into young (18-39 years) middle aged (40-59 years) and elderly (>60 years). BF% was estimated from bioelectrical impedance analysis. Pearsons' correlation coefficient(r) was calculated to see the relationship between BMI-BF% in the different age groups. Multiple regression analysis was performed to determine the effect of age and gender in the relationship and polynomial regression was carried out to see its' linearity. The relationships between age-BMI, age-BF % were separately assessed. Results: Out of 1114 participants, 49.1% were males. The study sample represented a wide range of BMI values (14.8-41.1 kg/m(2), Mean 23.8 +/- 4.2 kg/m(2)). A significant positive correlation was observed between BMI-BF%, in males (r = 0.75, p < 0.01; SEE = 4.17) and in females (r = 0.82, p < 0.01; SEE = 3.54) of all ages. Effect of age and gender in the BMI-BF% relationship was significant (p < 0.001); with more effect from gender. Regression line found to be curvilinear in nature at higher BMI values where females (p < 0.000) having a better fit of the curve compared to males (p < 0.05). In both genders, with increase of age, BMI seemed to increase in curvilinear fashion, whereas BF% increased in a linear fashion. Conclusions: BMI strongly correlate with BF % estimated by bioelectrical impedance, in this sub population of South Asian adults. This relationship was curvilinear in nature and was significantly influenced by age and gender. Our findings support the importance of taking age and gender in to consideration when using BMI to predict body fat percentage/obesity, in a population.