How Much Protein for Muscle Gain? The Morton 2018 Meta-Analysis Explained

If you have ever seen protein targets ranging from 0.8 g/kg to 3 g/kg and wondered which one is real, the answer mostly traces back to a single meta-analysis. Morton et al. 2018 pooled 49 studies and 1,863 lifters to find the point where extra protein stops building extra muscle, and that number now underpins most serious protein guidance, including the defaults inside the Scinergy macro calculator. This article walks through the study itself, what it means in practice, and a full worked example you can copy for your own numbers.

Inside the Morton 2018 meta-analysis: 49 studies, 1,863 subjects

The full citation is Morton RW, Murphy KT, McKellar SR, et al., "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," British Journal of Sports Medicine, 2018 (PubMed record; full text, BJSM; correction notice, PMC). The review pooled 49 studies with a combined 1,863 participants, all involving prolonged resistance training with a protein supplementation arm compared against a control arm.

Directly from the abstract: dietary protein supplementation significantly increased one-repetition-max strength by a mean of 2.49 kg, fat-free mass by a mean of 0.30 kg, and muscle fiber cross-sectional area by 310 square micrometers, along with mid-femur cross-sectional area by 7.2 square millimeters (PubMed abstract). The meta-regression found something equally important: the impact of protein supplementation on fat-free mass gains was reduced with increasing age, and was more effective in resistance-trained individuals compared to untrained beginners. That age-related pattern is directly relevant if you are past your 40s or 50s, and it is part of why our macros for perimenopause guide recommends staying closer to the top of the protein range rather than the bottom as you age.

The 1.62 g/kg ceiling and why 2.2 g/kg is the practical anchor

The meta-regression's central and most cited finding is that protein supplementation beyond a total protein intake of approximately 1.62 g/kg/day produced no further resistance-training-induced gains in fat-free mass. In the paper's own words, "protein intakes at amounts greater than approximately 1.6 g/kg/day do not further contribute to RET-induced gains in FFM" (Morton et al. 2018). The 95 percent confidence interval around that breakpoint extends up to approximately 2.2 g/kg/day, which is the statistical basis for the widely cited practical range of 1.6 to 2.2 g/kg/day (secondary analysis, Nutrient Metrics).

This distinction matters and is worth getting exactly right: 1.62 g/kg is the point estimate of the plateau, the central tendency across the pooled data, while 2.2 g/kg represents the upper bound of statistical uncertainty around that estimate. Individuals with higher variance in response, greater training volume, or other amplifying factors could plausibly benefit from intakes up to that higher figure, even though the average subject in the pooled data saw no additional benefit above roughly 1.6 g/kg.

Three reasons support recommending up to 2.2 g/kg rather than stopping strictly at 1.6 g/kg. First, the confidence interval itself extends to 2.2 g/kg, so this is not an arbitrary round number but a statistically grounded upper bound from the same analysis. Second, protein intake above the plateau carries essentially no downside for healthy individuals with normal kidney function, so a modest safety margin above the population-average plateau is low-risk. Third, the Morton meta-regression found the effect of supplementation is blunted by increasing age and enhanced by prior training experience, meaning individual variation in the true optimal intake is real, and a single fixed number will underserve older lifters and highly trained athletes if set too conservatively. The Scinergy macro calculator uses this exact 1.6 to 2.2 g/kg range as its hypertrophy-focused protein band.

What the ISSN 2017 position stand adds to the picture

The International Society of Sports Nutrition's position stand on protein and exercise, led by Jager and colleagues, is the other foundational document in this space (ISSN Position Stand, full text; summary of the 14 key points). Its baseline recommendation is 1.4 to 2.0 g/kg/day for most exercising individuals to build and maintain muscle mass, which falls within the Institute of Medicine's Acceptable Macronutrient Distribution Range of 10 to 35 percent of calories from protein. During hypocaloric, or cutting, periods, the position stand notes resistance-trained individuals may need higher intakes, in the range of 2.3 to 3.1 g/kg of fat-free mass per day, to maximize lean mass retention. It also cites novel evidence at the time suggesting intakes above 3.0 g/kg body weight per day may support fat loss in resistance-trained individuals, though this is a more novel and less established claim than the baseline range.

On distribution, the ISSN recommends 0.25 to 0.40 g/kg of a high-quality protein, roughly 20 to 40 g total, per meal, with 700 to 3,000 mg of leucine per dose, spread across the day every 3 to 4 hours. It also states the anabolic effect of a resistance training session on muscle protein synthesis lasts at least 24 hours, the underlying rationale for the "the window is wider than 30 minutes" framing used across the modern literature, which we return to below.

Per-meal distribution: how many meals, how many grams each

Schoenfeld and Aragon's work on meal frequency and distribution establishes a practical per-meal framework: to reach a minimum total daily intake of 1.6 g/kg/day, consume approximately 0.4 g/kg of protein at each of a minimum of 4 meals; to reach the upper range of 2.2 g/kg/day using the same 4-meal structure, aim for approximately 0.55 g/kg per meal (PubMed, 2018; full text PDF). Complementary research found evenly distributing protein across 3 meals, with an emphasis on an adequately protein-rich breakfast rather than just dinner, produced better muscle mass and body composition outcomes than the typical skewed pattern of low breakfast protein and high dinner protein, even when total daily protein intake was statistically similar between groups (Journal of Nutrition, 2020).

The leucine threshold required to maximally trigger the mTOR pathway and stimulate muscle protein synthesis is generally cited in the 2.5 to 3 g range per serving, achievable through roughly 25 to 35 g of high-quality, particularly fast-digesting animal, protein (Frontiers in Nutrition, 2024). Older adults show anabolic resistance, meaning they need a larger per-meal protein or leucine dose to achieve the same response as younger adults. That same 2024 review states this population benefits from daily intakes above the RDA, generally 1.2 to 1.6 g/kg/day, and that concentrating adequate protein and leucine into fewer, larger doses, at least one meal above 35 g, matters more for older adults than even distribution across many small, low-protein meals.

Is the anabolic window real? What Aragon and Schoenfeld found

The foundational paper here is Aragon and Schoenfeld's "Nutrient timing revisited: is there a post-exercise anabolic window?" (PubMed, 2013), reinforced by Schoenfeld, Aragon, and Krieger's related 2013 meta-analysis on protein timing and muscle strength and hypertrophy (2013 meta-analysis, Taylor and Francis). Their conclusion: when total daily protein intake is statistically controlled for between groups, the timing of protein relative to a training session provides no additional, statistically significant benefit for muscle hypertrophy or strength. Their pooled analysis without controlling for total protein intake found only a small, borderline-significant effect on hypertrophy with no significant effect on strength, and their interpretation is that this apparent timing effect was actually a confound driven by higher total protein consumption in the "timed" groups, not a true timing effect.

Schoenfeld and Aragon revisited this position in a 2018 review, which softened the "timing is irrelevant" framing to "the window, if it exists, is measured in hours rather than a strict 30 minute cutoff," particularly noting the window may matter more for fasted training or when the prior meal was several hours before the session (PubMed, 2018). The practical hierarchy supported across this literature, in order of importance: total daily protein intake in the 1.6 to 2.2 g/kg range, distribution across a minimum of 3 to 4 meals, per-meal leucine adequacy of 2.5 to 3 g, whether a protein-containing meal occurs within a few hours of training, and last, the exact minute-by-minute timing of a post-workout shake, which matters least of all five factors.

Plant vs animal protein, and a worked example for an 80kg trainee

The Digestible Indispensable Amino Acid Score, or DIAAS, is the current gold-standard protein quality metric, replacing the older PDCAAS method (Food Label Maker explainer; British Journal of Nutrition). Plant protein sources generally show lower digestibility than animal sources, with digestibility for sources like maize, oat, bean, pea, and potato ranging from roughly 45 to 80 percent, compared to consistently higher digestibility for dairy, egg, and meat proteins, some of which score above 100 on the DIAAS scale (plant vs animal digestibility overview; meat product DIAAS values). The practical mitigation for plant-forward eaters is combining complementary plant proteins, such as rice and pea protein together, to fill amino acid gaps, or adding a modest 10 to 15 g of high-DIAAS animal protein to a plant-based meal, which measurably improves the overall amino acid and leucine delivery of that meal.

Now the worked example. For an 80kg trainee, using the Morton 2018 derived range of 1.6 to 2.2 g/kg/day: the minimum effective total is 1.6 times 80, or 128 g/day, and the practical upper anchor is 2.2 times 80, or 176 g/day. A reasonable working target for most intermediate lifters sits around 1.8 g/kg, or 144 g/day.

Distributed across 4 meals at approximately 0.4 to 0.45 g/kg per meal, that looks like 32 to 36 g at breakfast, 32 to 36 g at lunch, 32 to 36 g around a workout, and 32 to 36 g at dinner, with an optional fifth feeding of 25 to 30 g before bed if targeting the upper end of the range. This structure keeps every meal above the roughly 25 to 30 g leucine-adequate threshold for a person of this body weight, avoids any single mega-dose beyond the point of diminishing returns, since a 70 g single serving does not produce a proportionally larger muscle protein synthesis response than a well-dosed 30 to 35 g serving (Frontiers in Nutrition, 2024), and comfortably lands within the ISSN's per-serving guidance of 20 to 40 g per meal.

Two common mistakes worth avoiding: skipping protein at breakfast to "save" it for dinner, which is directly contradicted by the 2020 Journal of Nutrition distribution study above, and letting a very high protein intake crowd out fiber, since lean animal protein and powders consumed at the expense of plant foods can quietly undercut the fiber floor covered in our perimenopause macros article. You can build a full 4-meal protein and macro plan around your own body weight in the Scinergy macro calculator, and once your protein target is set, pairing it with an accurate maintenance calorie estimate matters just as much, which is where choosing the right BMR equation and applying a realistic activity multiplier come in.