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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 (β-coefficient, ±E) 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 >1.4 g∙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.