Associations of age and body mass index with hydration and density of fat-free mass from 4 to 22 years

Background Most body composition techniques assume constant properties of fat free mass (FFM) (hydration and density) regardless of nutritional status, which may lead to biased values. Aim To evaluate the interactive associations of age and body mass index (BMI) with hydration and density of FFM. Methods Data from subjects aged between 4 and 22 years old from several studies conducted in London, UK were assessed. Hydration (H FFM ) and density (D FFM ) of FFM obtained from the four-component model in 936 and 905 individuals, respectively, were assessed. BMI was converted in to z -scores, and categorised into five groups using z -score cut-offs (thin, normal weight, overweight, obese, and severely obese). Linear regression models for H FFM and D FFM were developed using age, sex, and BMI group as predictors. Results Nearly 30% of the variability in H FFM was explained by models including age and BMI groups, showing increasing H FFM values in heavier BMI groups. On the other hand, ∼40% of variability in D FFM was explained by age, sex, and BMI groups, with D FFM values decreasing in association with higher BMI group. Conclusion Nutritional status should be considered when assessing body composition using two-component methods, and reference data for H FFM and D FFM is needed for higher BMI groups to avoid bias. Further research is needed to explain intra-individual variability in FFM properties.