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.