PURPOSE: The purpose of the study was to assess the association of protein intake with change in lean mass (LM) during a 9-mo. resistance training (RT) protocol.
METHODS: Normal/overweight sedentary, previously untrained young adults (n = 78, age ~22 yrs. BMI ~ 25 kg/m2) completed a 9-mo., supervised efficacy trial (1 or 3- sets RT, 9 exercises, 3 d/wk.) Participants were required to complete ≥80% scheduled RT sessions and asked maintain usual ad-libitum diets. Body composition (DEXA) and dietary intake (digital photography plus recall) were assessed at baseline, 4.5 and 9 mos. Multiple linear regression models were used to examine the associations between protein intake and changes in LM. Intake variables from the assessment periods were aggregated over the 9-month intervention. Protein intake was examined by using the nutrient residual energy-adjustment method, in which the protein residuals obtained by regressing absolute protein intake on total energy intake are added to mean protein intake and used as the independent variables Models were adjusted for age, sex, race, randomization group, baseline LM, and height. To allow determination of whether the associations were independent of change in overall mass, models were also adjusted for changes in FM.
RESULTS: Participants completed 92 ± 6% of scheduled RT sessions. LM increased significantly from baseline to 9 mos. (1.2 ± 1.7 kg, p <0.0001) with high inter-individual variability (range = - 2.0 to 6.2 kg). Grams of total protein (β=0.01 SE=0.01, p=0.34), animal protein (β=0.02 SE= 0.02, p=0.15), vegetable protein (β=-0.03 SE=0.04, p=0.44), and isoleucine (β=3.4 SE=1.97, p=0.09), were not associated with changes in total LM per unit of energy-adjusted protein intake. However, leucine (β= 1.8 SE=1.2, p=0.03) and valine (β= 3.7 SE=1.4, p=0.01) were positively associated with changes in total LM per unit of energy-adjusted protein intake.
CONCLUSIONS: There was no association with total protein intake and changes in LM in young adults enrolled in a 9-month RT intervention. However, there was a positive association with two of the branch chain amino acids, leucine and valine. Thus, the type of protein may be more important than total protein intake for increasing LM during a long-term RT intervention.