Stratification of Fat-Free Mass Index Percentiles for Body Composition Based on National Health and Nutrition Examination Survey III Bioelectric Impedance Data

Background: Loss of protein mass and lower fat-free mass index (FFMI) are associated with longer length of stay, postsurgical complications, and other poor outcomes in hospitalized patients. Normative data for FFMI of U.S. populations do not exist. This work aims to create a stratified FFMI percentile table for the U.S. population using the large bioelectric impedance analysis data obtained from National Health and Nutrition Examination Surveys (NHANES). Methods: Fat-free mass (FFM) was calculated from the NHANES III bioelectric impedance analysis and anthropometric data for males and females ages 12 to >90 years for 3 race/ethnicities (non-Hispanic white, non-Hispanic black, and Mexican American). FFM was normalized by subject height to create an FFMI distribution table for the U.S. population. Selected percentiles were obtained by age, sex, and race/ethnicity. Data were collapsed by race/ethnicity before and after removing obese and underweight participants to create an FFMI decile table for males and females 12 years and older for the healthy-weight U.S. population. Results: FFMI increased during adolescent growth but stabilized in the early 20s. The FFMI deciles were similar by race/ethnicity, with age group remaining relatively stable between ages 25 and 80 years. The FFMI deciles for males and females were significantly different. Conclusions: After eliminating the obese and extremely thin, FFMI percentiles remain stable during adult years allowing creation of age- and race/ethnicity-independent decile tables for males and females. These tables allow stratification of individuals for nutrition intervention trials to depict changing nutrition status during medical, surgical, and nutrition interventions.