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

Abstract

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. To evaluate the interactive associations of age and body mass index (BMI) with hydration and density of FFM. Data from subjects aged between 4 and 22 years old from several studies conducted in London, UK were assessed. Hydration (HFFM) and density (DFFM) 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 HFFM and DFFM were developed using age, sex, and BMI group as predictors. Nearly 30% of the variability in HFFM was explained by models including age and BMI groups, showing increasing HFFM values in heavier BMI groups. On the other hand, ∼40% of variability in DFFM was explained by age, sex, and BMI groups, with DFFM values decreasing in association with higher BMI group. Nutritional status should be considered when assessing body composition using two-component methods, and reference data for HFFM and DFFM is needed for higher BMI groups to avoid bias. Further research is needed to explain intra-individual variability in FFM properties.