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Review
. 2018 Mar;52(6):376-384.
doi: 10.1136/bjsports-2017-097608. Epub 2017 Jul 11.

A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults

Robert W Morton  1 , Kevin T Murphy  1 , Sean R McKellar  1 , Brad J Schoenfeld  2 , Menno Henselmans  3 , Eric Helms  4 , Alan A Aragon  5 , Michaela C Devries  6 , Laura Banfield  7 , James W Krieger  8 , Stuart M Phillips  1
Affiliations
Review

A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults

Robert W Morton et al. Br J Sports Med. 2018 Mar.

Erratum in

Abstract

Objective: We performed a systematic review, meta-analysis and meta-regression to determine if dietary protein supplementation augments resistance exercise training (RET)-induced gains in muscle mass and strength.

Data sources: A systematic search of Medline, Embase, CINAHL and SportDiscus.

Eligibility criteria: Only randomised controlled trials with RET ≥6 weeks in duration and dietary protein supplementation.

Design: Random-effects meta-analyses and meta-regressions with four a priori determined covariates. Two-phase break point analysis was used to determine the relationship between total protein intake and changes in fat-free mass (FFM).

Results: Data from 49 studies with 1863 participants showed that dietary protein supplementation significantly (all p<0.05) increased changes (means (95% CI)) in: strength-one-repetition-maximum (2.49 kg (0.64, 4.33)), FFM (0.30 kg (0.09, 0.52)) and muscle size-muscle fibre cross-sectional area (CSA; 310 µm2 (51, 570)) and mid-femur CSA (7.2 mm2 (0.20, 14.30)) during periods of prolonged RET. The impact of protein supplementation on gains in FFM was reduced with increasing age (-0.01 kg (-0.02,-0.00), p=0.002) and was more effective in resistance-trained individuals (0.75 kg (0.09, 1.40), p=0.03). Protein supplementation beyond total protein intakes of 1.62 g/kg/day resulted in no further RET-induced gains in FFM.

Summary/conclusion: Dietary protein supplementation significantly enhanced changes in muscle strength and size during prolonged RET in healthy adults. Increasing age reduces and training experience increases the efficacy of protein supplementation during RET. With protein supplementation, protein intakes at amounts greater than ~1.6 g/kg/day do not further contribute RET-induced gains in FFM.

Keywords: growth; hypertrophy; performance. leucine; supplement; whey.

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Conflict of interest statement

Competing interests: SMP has received grant support, travel expenses, and honoraria for presentations from the US National Dairy Council. This agency has supported trials reviewed in this analysis.

Figures

Figure 1
Figure 1
PRISMA flow chart.
Figure 2
Figure 2
Forest plot of the results from a random-effects meta-analysis shown as mean difference with 95% CIs on one-repetition-maximum (1 RM; kg) in untrained and trained participants. For each study, the circle represents the mean difference of the intervention effect with the horizontal line intersecting it as the lower and upper limits of the 95% CI. The size of each circle is indicative of the relative weight that study carried in the meta-analysis. The rhombi represent the weighted untrained, trained and total group’s mean difference. Total: 2.49 kg (0.64, 4.33), p=0.01, untrained: 0.99 kg (−0.27, 2.25), p=0.12 and trained: 4.27 kg (0.61, 7.94), p=0.02.
Figure 3
Figure 3
Forest plot of the results from a random-effects meta-analysis shown as mean difference with 95% CIs on lean or fat-free mass (FFM; kg) in untrained and trained participants. For each study, the circle represents the mean difference of the intervention effect with the horizontal line intersecting it as the lower and upper limits of the 95% CI. The size of each circle represents the relative weight that study carried in the meta-analysis. The rhombi represent the weighted untrained, trained and total group’s mean difference. Total: 0.30 kg (0.09, 0.52) p=0.007, untrained: 0.15 kg (−0.02, 0.31), p=0.08 and trained: 1.05 kg (0.61, 1.50), p<0.0001.
Figure 4
Figure 4
Random-effects univariate meta-regression between age and the mean difference in fat-free mass (FFM) between groups. Each circle represents a study and the size of the circle reflects the influence of that study on the model (inversely proportionate to the SE of that study). The regression prediction is represented by the solid line (−0.01 kg (−0.02,–0.00), p=0.02).
Figure 5
Figure 5
Segmental linear regression between relative total protein intake (g/kg body mass/day) and the change in fat-free mass (ΔFFM) measured by dual energy X-ray absorptiometry. Each circle represents a single group from a study. Dashed arrow indicates the break point=1.62 g protein/kg/day, p=0.079. Solid arrow indicates 95% CI, (1.03 to 2.20).
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