The effects of a high protein diet on indices of health and body composition--a crossover trial in resistance-trained men.
Level 2 - randomized trial
Individual randomized crossover trial
PubMed 26778925 · doi:10.1186/s12970-016-0114-2
What was done
Twelve healthy resistance-trained men (mean age 25.9 ± 3.7 years, 7.6 ± 3.6 years of lifting experience) participated in a randomized crossover trial. Subjects completed two 8-week dietary phases: their habitual normal-protein diet and a high-protein diet target of >3 g/kg/day, for a total study duration of 16 weeks. Eleven participants completed most assessments. Dietary intake was monitored using 3-day food diaries each week (two weekdays, one weekend day), and training diaries tracked total work performed. Body composition, exercise performance, and clinical safety markers (blood lipids, glucose, comprehensive metabolic panel including renal and hepatic markers) were measured at baseline and after each 8-week period.
What was found
Protein intake was 2.6 ± 0.8 g/kg/day during the habitual phase and 3.3 ± 0.8 g/kg/day during the high-protein phase (4-month mean: 2.9 ± 0.9 g/kg/day). Total caloric intake and protein intake were significantly higher in the high-protein phase (p < 0.05), with no differences in other dietary measures. There were no significant changes in body composition, exercise performance, blood lipids, glucose, or markers of renal and hepatic function in either group, with no adverse side effects reported.
Why it matters
This crossover trial shows that consuming dietary protein at >3 g/kg/day (over four times the RDA) for 8 weeks did not adversely affect clinical markers of renal, hepatic, or cardiovascular health in resistance-trained men.
Limits
The sample size is very small (n = 12 enrolled, 11 analyzed) and limited exclusively to young, resistance-trained men. Baseline habitual protein intake was already high (2.6 g/kg/day), limiting the contrast between phases. Dietary and training data were self-reported via diaries, and findings cannot be generalized to sedentary individuals, females, or populations with impaired renal or hepatic function.
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