Morton et al. 2018: Protein Supplementation and Resistance Training
Key takeaways
- Protein supplementation significantly increased fat-free mass and strength gains beyond resistance training alone (mean fat-free mass effect ~0.30 kg).
- The dose-response curve plateaus near 1.62 g/kg body mass per day; intakes above this did not produce additional gains in the pooled data.
- The 95 percent confidence interval extends to approximately 2.2 g/kg/day, which is widely cited as a defensible upper bound for athletic populations.
- Effect sizes were larger in resistance-trained subjects than in untrained ones, suggesting protein matters more once the training stimulus is well-established.
- Age moderated the effect, with older subjects showing relatively smaller responses, consistent with anabolic resistance.
- Total protein intake mattered more than supplementation timing or protein source in this pooled analysis.
Purpose
The protein dose-response question for resistance-trained adults has produced a long literature of small trials with conflicting effect sizes. Morton and colleagues set out to pool the available randomized evidence and use meta-regression to identify the daily protein intake at which additional intake stops producing measurable additional gains in muscle mass and strength.
Design
PRISMA-aligned systematic review and meta-analysis of randomized trials in healthy adults that combined resistance training with a protein supplementation arm versus a control arm. Forty-nine trials with 1,863 participants met inclusion criteria. Outcomes were change in fat-free mass (the primary surrogate for muscle hypertrophy in body-composition studies), change in muscle fiber cross-sectional area where reported, and change in 1-repetition maximum strength. The investigators ran meta-regression with total daily protein intake as the predictor to characterize the dose-response shape.
Key Findings
Protein supplementation produced a small but statistically significant increase in fat-free mass beyond resistance training alone (weighted mean difference approximately 0.30 kg). Strength gains were similarly augmented. The meta-regression identified a breakpoint at approximately 1.62 g protein per kilogram body mass per day; below this, additional intake was associated with additional gains, and above it, the slope flattened to non-significance. The 95 percent confidence interval around this estimate extended to approximately 2.2 g/kg/day, which is the figure most commonly cited as a defensible upper bound for athletes seeking maximum hypertrophy.
The effect of supplementation was larger in resistance-trained subjects than in untrained ones — an important nuance, because it means novice lifters can produce substantial gains on lower protein intakes, while well-trained subjects appear to require closer to the upper end of the range. Age moderated the effect, with older subjects showing smaller responses (consistent with anabolic resistance).
Limitations
Most included trials had short durations (typically 8 to 16 weeks), which truncates the long-term picture. Habitual protein intake was estimated from self-report in many trials, which carries known measurement error. Body-composition assessments varied across trials (DXA, BIA, hydrostatic weighing), introducing methodological heterogeneity. The 1.62 g/kg breakpoint is a population estimate; individual variation in optimal intake remains substantial and is poorly characterized by the existing literature.
Takeaway
Morton et al. is the most-cited reference for the practical recommendation that resistance-trained adults seeking maximum hypertrophy should consume approximately 1.6 to 2.2 g protein per kilogram body mass per day. The lower end of this range is sufficient for most lifters; the upper end captures the confidence interval and provides a margin for individual variation, anabolic resistance with age, and aggressive training volumes. Intakes substantially above 2.2 g/kg are not supported by additional benefit in the pooled data.
References
- Morton RW, Murphy KT, McKellar SR, et al. BJSM. 2018;52(6):376-384. · DOI: 10.1136/bjsports-2017-097608
- Bandegan A et al. Indicator amino acid-derived estimate of dietary protein requirement for male bodybuilders on a nontraining day is several-fold greater than the current RDA. Journal of Nutrition. 2017;147(5):850-857.
- Phillips SM, Van Loon LJ. Dietary protein for athletes: from requirements to optimum adaptation. Journal of Sports Sciences. 2011;29(sup1):S29-S38.
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