Combined Increases in Muscle-Strengthening Activity Frequency and Protein Intake Reveal Graded Relationship with Fat-Free Mass Percentage in U.S. Adults, NHANES (1999-2004).

Background: Age-related loss of muscle mass and related ailments are of concern due to associations with disabilities and morbidity as well as constituting a substantial healthcare burden. Muscle-strengthening activities and adequate protein ingestion are recommended for all-age adults in an effort to stave off age-related muscle atrophy. Muscle building abilities decline with age but most research focuses on muscle wasting in the elderly. Objectives: To examine the independent and combined associations of protein intake (g.kg(-1).day(-1)) and muscle-strengthening frequency (times.week(-1), MSF) on fat-free mass percentage (FFM%). Design Setting and Participants: This cross-sectional analysis of a population-based sample with data from the non-institutionalized persons in the United States participating in the National Health and Nutrition Examination Survey (cycles 1999-2000, 2001-2002, 2003-2004) consisted of male (n=2,499) and female (n=2,373) participants 20-49 years of age for analyses. Measurements: MSF was determined by self-report and protein intake was calculated from a 24-hour recall. Differences in FFM% from bioelectrical impedance analysis was estimated using multiple linear regression models controlling for education, race-ethnicity, standing height, and total Caloric intake. Results: One unit increase in MSF or protein intake (beta-coefficient, +/- SE) was associated with significantly more FFM% in males (0.6 +/- 0.1%; 3.5 +/- 0.4%) and females (0.4 +/- 0.1%; 5.9 +/- 0.4%). Independent of protein intake, males and females with MSF=0 had mean +/- SE FFM% of 74.4 +/- 0.4 and 60.7 +/- 0.3, respectively, while mean +/- SE FFM% of males and females who met the recommendation of = 2 times per week were 77.9 +/- 0.5 and 63.0 +/- 0.4. Independent of MSF, males and females with protein intakes below the recommended dietary allowance (RDA) of 0.8 g.kg(-1).day(-1) had mean +/- SE FFM% of 74.0 +/- 0.6 and 58.2 +/- 0.6, respectively, while mean +/- SE FFM% of those whose intakes exceeded the recommendation were 75.6 +/- 0.4 and 62.0 +/- 0.4. The subgroup with the highest mean +/- SE FFM% (80.9 +/- 0.73) comprised males with MSF = 2 times per week who also consumed >1.4g.kg(-1).day(-1). Conclusion: The MSF-protein intake dose relationship with FFM% suggests that performing muscle-strengthening activities >2 times per week while consuming protein above the RDA may result in more fat-free mass and slow age-related losses of muscle mass.