Dietary Protein Intake Is Positively Associated with Appendicular Lean Mass and Handgrip Strength among Middle-Aged US Adults

Background Sarcopenia, a progressive loss of skeletal muscle mass and strength, can begin in the 4th decade of life. Protein intake predicts skeletal muscle mass and strength among older adults, but knowledge of similar associations among middle-aged adults is lacking. Objectives We aimed to assess associations between protein intake and skeletal muscle mass, characterized by appendicular lean mass adjusted for BMI [in kg/m(2) (ALM(BMI))], and muscle strength, represented by handgrip strength adjusted for BMI (GSMAX(BMI)), among middle-aged adults. Methods We analyzed cross-sectional data from 1209 men and 1208 women aged 40-59 y in the 2011-2014 NHANES. Protein intake per kilogram actual body weight (BW), assessed by two 24-h recalls, was examined as continuous and categorical parameters [low (<RDA, 0.8 g/kg BW), moderate (>= RDA, <1.2 g/kg BW), and high (>= 1.2 g/kg BW)]. ALM was measured using DXA, and GSMAX was measured using handgrip dynamometer. "Low lean mass" and "weakness" were defined using NIH criteria for ALM(BMI) and GSMAX(BMI), respectively. Linear and logistic regression models were performed. Results Among middle-aged adults, 15.6% of men and 13.4% of women had low lean mass and 3.5% of men and 2.3% of women exhibited weakness. Protein intakes per kilogram BW were positively associated with ALM(BMI) and GSMAX(BMI) among men and women after adjustment for age, race/Hispanic origin, physical activity, and self-rated health. Compared with the moderate protein group, the high protein group had a higher GSMAX(BMI) and the low protein group had a lower GSMAX(BMI) among men and women. The low protein group had a lower ALM(BMI) (women) and had a higher OR for low lean mass (men) compared with the moderate protein group. Conclusions Higher protein intakes were associated with greater ALM(BMI) and GSMAX(BMI) in this representative sample of US middle-aged adults. Our findings highlight the need for further research on dietary protein as a potential modifying factor of sarcopenia risk in middle age.