Title: The association between different levels of protein intake per day and muscle strength and skeletal muscle mass depending on sex, age, and physical activity level in young and middle-aged adults
Thesis Supervisor: Dr. Darren Warburton
Committee members: Dr. Mahsa Jessri, Dr. Carolyn McEwen
Defence Chair: Dr. Cameron Mitchell
Abstract:
Background: A protein intake above the recommended dietary allowance (RDA) (i.e., 0.8 g∙kg-1∙day-1) might benefit muscle strength (MS) and skeletal muscle mass (SMM). However, whether a higher protein intake would benefit MS and SMM (depending on sex, age group, and physical activity level) at younger ages is still to be determined.
Objective: To examine whether a protein intake above the RDA is associated with a higher MS and SMM depending on sex, age group, and physical activity level in young and middle-aged adults.
Methods: A secondary data analysis from the National Health and Nutrition Examination Survey (NHANES) was conducted. We analyzed data from participants aged 20 to 59, with dietary intake (24 h dietary recall), combined handgrip strength (HGS), dual-energy x-ray absorptiometry-derived appendicular lean soft tissue (ALST), accelerometer-assessed physical activity, among other variables (n = 3406). The protein intake was categorized as <0.8, 0.8 to 1.0, 1.0 to 1.6, and ≥1.6 g∙kg-1∙day-1. HGS and ALST were adjusted for BMI (HGS-BMI, and ALST-BMI, respectively). Physical activity was categorized as low, moderate, and high. The HGS-BMI and ALST-BMI were compared among the protein intake groups, setting the 0.8 to 1.0 g∙kg-1∙day-1 group as the reference. This analysis was carried out for the whole sample, by sex (men, women), age group (young, middle-aged), physical activity level (low, moderate, high), and their corresponding 12 subgroups in one robust model adjusted for confounding variables per outcome variable.
Results: Protein intake per day was positively associated with MS in almost all the subgroups (11 out of 12, p < 0.05), whereas it was associated with SMM in half of the subgroups (7 out of 12), mainly in men (5 out of 6). Overall, the proportional benefit observed with a higher protein intake was greater in MS (≈30% vs the reference group), and it began to be observed at lower doses (starting at 1.0 g∙kg-1∙day-1) than in SMM (≈10% vs the reference group), which was evident above 1.6 g∙kg-1∙day-1.
Conclusions: A high protein intake (>1.0 g∙kg-1∙day-1) would help most people to be stronger, while higher protein amounts (≥1.6 g∙kg-1∙day-1) would be necessary to observe a higher SMM.