How Much Protein to Build Muscle?

In 2018 a group led by Robert Morton and Stuart Phillips at McMaster University in Hamilton published what is still the largest meta-analysis on protein supplementation and resistance training: 49 randomised controlled trials, 1,863 participants, studies from 17 countries spanning 1962 to 2016.^[1] Among many other analyses, they plotted the change in fat-free mass against total daily protein intake across 42 study arms and 723 people, and fitted a two-phase regression to find the point where more protein stopped helping. That point was 1.62 g/kg/day. It is the origin of the "1.6 grams per kilo" rule that now circulates everywhere.

What almost never travels with the number is the rest of the sentence. The breakpoint carried a 95% confidence interval of 1.03 to 2.20 g/kg/day, the segmental model explained 19% of the variance, and the fit did not reach statistical significance (p = 0.079). The authors were candid about this. "Given that the CI of this estimate spanned from 1.03 to 2.20," they wrote, "it may be prudent to recommend ~2.2 g protein/kg/d for those seeking to maximise resistance training-induced gains in fat-free mass."^[1]

So the famous figure is not a threshold the body recognises. It is the middle of a wide band, drawn through noisy data, by authors who explicitly suggested planning around the top of it.

Before fixing on a target it is worth knowing how much protein can do. In the same analysis, resistance training on its own raised participants' one-rep max by an average of 27 kg and added 1.1 kg of fat-free mass over a typical 13 weeks. Adding a protein supplement to that training increased the strength gain by a further 2.49 kg (95% CI 0.64–4.33) and fat-free mass by a further 0.30 kg (0.09–0.52).^[1] In relative terms protein was worth about 9% on top of the strength gain and 27% on top of the muscle gain — real, measurable, and far smaller than the training.

Who it helps is not uniform. The effect on fat-free mass was concentrated in people who were already resistance-trained (+1.05 kg, 0.61–1.50) and essentially absent in novices (+0.15 kg, −0.02 to 0.31), whose muscle was growing readily on whatever they were already eating. It shrank with age: +0.55 kg in people under 45 versus +0.06 kg in those over 45 — although, as the authors noted, the older participants in those trials were given an average of only 20 g of supplemental protein a day, which is probably too little to test the question properly.^[1]

A single meta-analysis is a single analysis. The encouraging thing about the protein question is that two quite different approaches land in the same neighbourhood.

The first is a larger dose-response synthesis: 105 trials, 5,402 participants, modelling lean body mass against protein intake across a range from 0.5 to 3.5 g/kg/day. Every extra 0.1 g/kg/day was associated with 0.39 kg more lean mass (95% CI 0.36–0.41) below a total intake of 1.3 g/kg/day, and only 0.12 kg (0.11–0.14) above it.^[2] Returns did not vanish past 1.3 g/kg — they fell to roughly a third. (This group's lead authors work for a Japanese dairy company, which is worth knowing even though the direction of the finding — diminishing returns — is not the flattering one for a protein manufacturer.)

The second comes from metabolic tracer work rather than body-composition trials. Using the indicator amino acid oxidation technique, a Canadian team at Western University and Toronto's Hospital for Sick Children measured protein requirement directly in eight experienced male bodybuilders on a rest day, across intakes from 0.1 to 3.5 g/kg/day over 42 separate experiments. The estimated average requirement came out at 1.7 g/kg/day, with an upper 95% confidence limit of 2.2 — roughly 2.6 times the current official recommendation, measured on a day with no training at all.^[3]

For reference, the Canadian and US dietary reference intakes set the protein RDA at 0.80 g/kg/day for all adults (56 g/day for men, 46 g/day for women), with an estimated average requirement of 0.66 g/kg/day.^[4] That figure was derived to prevent deficiency in the general population, not to build muscle in people who lift, and essentially every line of evidence above it says so.

Putting the three together: 1.6 to 2.2 g per kilogram of body weight per day is the defensible range for someone training for muscle. The International Society of Sports Nutrition's position stand puts it slightly lower and wider, at 1.4–2.0 g/kg/day, which it considers sufficient for most exercising people.^[5] The overlap between these ranges is where the real answer lives.

Two practical caveats. First, these are grams per kilogram of body weight, not of lean mass — if you are carrying a lot of excess fat, scaling to body weight will overshoot, and using a goal weight or lean mass is more sensible. Second, protein is not free: calories from protein still count, and in a surplus the protein target sits inside a calorie target, not above it. If you want to see what your intake actually looks like rather than what you assume it looks like, this is exactly what a food log is for — most people estimate their protein intake badly in both directions.

The one situation where protein intake clearly matters more is a calorie deficit. Losing weight means the body is short of energy, and muscle is a candidate fuel; higher protein intake is the main dietary lever against that.

The cleanest demonstration is a four-week trial in 40 young men at McMaster. Everyone ate about 40% below their energy requirement and trained six days a week with a punishing mix of resistance and interval work. One group ate 1.2 g/kg/day, the other 2.4 g/kg/day. The higher-protein group gained 1.2 ± 1.0 kg of lean mass despite the deficit; the lower-protein group gained 0.1 ± 1.0 kg. The higher-protein group also lost more fat: −4.8 ± 1.6 kg versus −3.5 ± 1.4 kg.^[6]

The claim that the body "can only use 30 grams of protein at a time" is the most durable piece of folklore in this area. It comes from a real observation — muscle protein synthesis measured over three or four hours after a fast-digesting protein drink plateaus somewhere around 20–25 g in young adults — and then over-generalises it into a daily ceiling.

Two problems. The first is that the observation was made with isolated, rapidly absorbed protein eaten alone; slower proteins eaten as part of a mixed meal behave differently. After reviewing both acute and long-term data, Schoenfeld and Aragon concluded that intakes above 20 g do increase amino acid oxidation, but not all of the surplus is burned — some is still used for tissue building — and recommended targeting roughly 0.4 g/kg per meal across at least four meals to reach 1.6 g/kg/day, or about 0.55 g/kg per meal to reach 2.2.^[7]

The second is that the three-to-four-hour measurement window was too short. In 2023 a Dutch group at Maastricht fed 36 healthy young men 0 g, 25 g or 100 g of intrinsically labelled milk protein after resistance exercise and followed them for twelve hours using four simultaneous isotope tracers. The 100 g dose produced a greater and much more prolonged anabolic response than 25 g, with myofibrillar protein synthesis about 20% higher in the first four hours and about 40% higher from four to twelve hours. Amino acid oxidation rose only negligibly. Of the amino acids incorporated into muscle protein, 9% came from the meal after 25 g and 27% after 100 g.^[8] The authors' conclusion was blunt: the anabolic response to protein has no apparent upper limit in magnitude or duration.

That does not make 100 g meals a strategy — it means the ceiling people worry about is not where they think it is. Hit your daily total; spread it across the day because that is easier and probably marginally better, not because a single large meal is wasted.

Older muscle is less responsive to the same amount of protein — anabolic resistance. The size of the gap has been measured. Pooling tracer studies from Toronto and McMaster laboratories, muscle protein synthesis plateaued at 0.24 ± 0.06 g/kg of body weight per meal in young men but 0.40 ± 0.19 g/kg in men around 71. Expressed per kilogram of lean mass the gap was wider still: 0.25 ± 0.13 versus 0.60 ± 0.29 g/kg (p < 0.01).^[9] Baseline rates of muscle protein synthesis were identical between the groups; what differed was the response to feeding.

The PROT-AGE position paper recommends at least 1.0–1.2 g/kg/day for people over 65, ≥1.2 g/kg/day for those who are exercising and active, and 1.2–1.5 g/kg/day for most older adults with acute or chronic illness — with an explicit exception for severe kidney disease (eGFR below 30 mL/min/1.73 m², not on dialysis), where intake may need to be limited.^[10]

One important qualifier: protein alone does not do much for ageing muscle. In a meta-analysis of 28 randomised trials in adults over 50, protein supplementation on its own produced no significant improvement in total lean mass, appendicular lean mass, handgrip or knee-extension strength. Only in the trials that also included resistance exercise did it significantly improve appendicular lean mass (+0.54 kg, 0.03–1.05) and grip strength, and the lean-mass effect was confined to sarcopenic or frail participants.^[11] The protein is a permissive factor. The training is the signal. If maintaining muscle into later life is the goal, see our longevity and performance pages for the broader picture.

This is the question people ask most and the one with the clearest answer for healthy adults. A systematic review and meta-analysis pooled 28 randomised trials in 1,358 participants without kidney disease, comparing higher-protein intakes (≥1.5 g/kg/day, or ≥20% of energy, or ≥100 g/day) against normal or lower intakes. Post-intervention glomerular filtration rate was marginally higher on the high-protein diets (standardised mean difference 0.19, 0.07–0.31) — but the change in GFR did not differ between the groups (0.11, −0.05 to 0.27, p = 0.16).^[12] A higher filtration rate on a higher protein load is a normal adaptive response, not evidence of damage.

That finding applies to healthy adults. It does not extend to people with existing chronic kidney disease, where protein restriction can be part of treatment and intake should be set with a physician. It is also not a licence to ignore what the protein arrives with: the saturated fat, sodium and processing that come attached to some protein sources are their own question.

Three honest gaps. First, women are badly under-represented: Morton's analysis included only 14 study groups of exclusively female participants and reported no obvious sex differences while acknowledging how little work had been done. A 2025 meta-analysis of multi-ingredient protein supplementation in 408 women across nine trials found gains in fat-free mass (+0.45 kg, 0.19–0.71) and strength, but nine trials is a thin base for half the population.^[13]

Second, supplements are not magic. When multi-ingredient protein products were compared head-to-head against plain protein, they were no better; their apparent advantage over placebo came mostly from the protein and, in many formulas, the creatine.^[14] Food and powder are interchangeable at the same total intake.

Third, the heterogeneity in this literature is substantial, and the effect sizes we are arguing over — a third of a kilogram of fat-free mass over three months — are small enough that how consistently you train, sleep and show up will dominate the result.