49 studies. 1,863 participants. And one number at the end: 1.62 g of protein per kilogram of body weight per day. Beyond that threshold, according to Morton et al. in the British Journal of Sports Medicine, gains in lean mass barely increase further.1
You'll find this number in almost every sports nutrition guide. 1.6 g/kg, done, that's all you need. Go higher and you're wasting money. That's the simplified version. But three things are missing: Who this plateau actually applies to. How to distribute your intake across the day. And what changes when your protein source is plant-based?
Key Takeaways
- 1.62 g/kg/day is the evidence-based plateau for muscle mass gains during resistance training. Beyond that, the meta-analysis by Morton et al. (2018, 49 studies) shows no significant additional benefit.1
- Per meal, 0.4 g/kg is more effective than the commonly cited 0.2 g/kg, especially for adults over 60.
- Anabolic resistance increases protein needs per meal with age: older muscle cells respond less to protein and require more to trigger the same MPS response.5
- The ISSN recommends 1.4 to 2.0 g/kg for active adults. The 1.6 g/kg plateau is a well-supported optimum, not a universal ceiling.4
Contents
What do the meta-analyses say? The 1.6 g/kg threshold explained
The most convincing picture of the relationship between protein intake and muscle growth comes from the meta-analysis by Morton et al. (2018): 49 studies, 1,863 participants, consistent design. Above a daily intake of 1.62 g protein per kilogram of body weight, the dose-response curve clearly flattens. Additional protein produces no measurable further gain in lean mass.1
Morton et al. exclusively included studies that combined supplementation with controlled resistance training and assessed results for significance and effect size. The result sounds almost too clear-cut to be interesting.
Morton et al. (2018): Protein intake above 1.62 g/kg/day produces no significant further increase in lean mass among resistance-trained adults. Additional protein intake compared to control groups yielded an average of +0.30 kg more lean mass.1
But here's the catch: the 1.6 g/kg plateau comes from a mixed study population. Plant-based protein sources were underrepresented. And the question of whether older adults can apply the same threshold isn't answered by the number alone.
Nunes et al. expanded on this in 2022. Their meta-analysis confirms the positive effect of increased protein intake but makes the effect size transparent: an average of +1.3 to +1.4 kg lean mass in the intervention group, compared to +0.8 kg in the control group.2
Nunes et al. (2022): Systematic review confirms the effect of increased protein intake on lean mass (SMD = 0.22). The effect is real but moderate. Results apply to healthy, non-obese adults doing resistance training.2
SMD = 0.22. That's a small to moderate effect. Protein is one of several factors: training, sleep, and total energy balance all play a role. But it's a measurable, reproducible factor. That counts.
Current meta-analytic evidence shows: 1.6 g protein per kg body weight per day maximises muscle growth in resistance-trained adults. More doesn't produce proportional additional gains (Morton et al. 2018, SMD +0.22 from additional protein intake, Nunes et al. 2022). The effect is real but moderate.1,2
How much protein per meal? The 0.4 g/kg rule
Total daily intake is the strongest lever, but not the only one. Schoenfeld and Aragon investigated in 2018 how much protein muscle can use per meal for growth and reached a conclusion that clearly debunks the widespread "30-gram limit."3
The "30 g rule" suggests the body can't use more than 30 g of protein per meal for muscle building. Research doesn't support this. Protein is essentially fully absorbed. The real question is how much of it actively stimulates muscle protein synthesis (MPS), rather than being oxidised or used for other purposes.
Schoenfeld & Aragon (2018): For younger adults (~22 years), the effective per-meal dose is approximately 0.2 g/kg. For older adults (~71 years), this rises to ~0.4 g/kg. Four meals at 0.4 g/kg optimally cover the daily 1.6 g/kg target.3
The distribution is straightforward to plan. At 75 kg body weight, 0.4 g/kg per meal means roughly 30 g of protein per serving, spread across four meals: breakfast, lunch, post-workout, and dinner. Factor in leucine content as well and you cover two bases at once: adequate MPS stimulation and steady supply.
A SYNTYZE protein shake with 24 g of protein and 3 g of leucine per serving is an efficient building block: plant-based, no artificial sweeteners, with an enzyme complex containing 5 digestive enzymes.
A daily protein intake of 1.6 g/kg works best when distributed across at least three to four meals. Schoenfeld and Aragon (2018) recommend ~0.4 g/kg per meal. At 80 kg, that's about 32 g per serving, well above the commonly cited 30 g limit.3
Does protein need change with age?
Yes. And the difference is larger than most people expect. With increasing age, skeletal muscle cells become less responsive to protein, a phenomenon researchers call "anabolic resistance." The MPS response after a meal is weaker in older adults, even when the same amount of protein is consumed as a 25-year-old.5
Stokes et al. (2018): Anabolic resistance increases with age. Older adults require a higher protein dose per meal, particularly with regard to leucine content, to achieve the same MPS stimulus as younger adults. Leucine as an mTOR activator becomes the critical threshold trigger.5
This has a direct practical consequence. If you're over 60 or 65, you shouldn't stay at 0.2 g/kg per meal (the guideline for younger adults) but aim for 0.4 g/kg instead. The daily total recommendation of 1.6 g/kg remains, but distributing it into larger individual servings becomes more important.
Leucine moves centre stage here. As the primary mTOR activator, leucine is the trigger for muscle protein synthesis. Research points to a threshold of around 2.5 g leucine per meal, as we discuss in more detail in our article on leucine and muscle growth. For older adults, this means choosing protein sources that deliver sufficient leucine per serving, not just sufficient total protein.
Anabolic resistance is a normal age-related physiological process, not an outlier. Older adults need more protein and more leucine per meal to trigger comparable MPS responses to younger adults. The 1.6 g/kg daily target remains, but the meal structure changes (Stokes et al. 2018, Schoenfeld & Aragon 2018).3,5
How much protein per day do you need? Your formula by goal and weight
In their 2017 position stand on protein and exercise, the ISSN defined three categories based on training goal and energy balance:4
Active Adults
Muscle Growth
Calorie Deficit
Training for muscle growth at normal energy intake? Then 1.6 g/kg is your target. Training in a calorie deficit and want to preserve as much muscle mass as possible? Then it makes sense to aim towards 1.8 to 2.0 g/kg. Not because more protein drives direct muscle growth, but because in a deficit, some protein gets oxidised and is no longer available for MPS.
With plant-based protein, there's an additional consideration. Amino acid availability from individual plant sources is somewhat lower than from animal sources, due to different DIAAS values and anti-nutrients like phytates. For vegan athletes: with a well-combined protein source, such as pea protein and fava bean protein that complement each other's amino acid profiles, this difference becomes minimal. For a safe margin, some researchers recommend a ~10% surcharge, roughly 1.75 g/kg instead of 1.6 g/kg.
Rule of thumb: 1.6 g/kg as your baseline. In a calorie deficit: up to 2.0 g/kg. With plant-based protein and uncertain source combinations: add 10%. Distributing across 3 to 4 meals at 0.4 g/kg is at least as important as the daily total.
In concrete terms for 75 kg body weight:
- Muscle growth (baseline): 1.6 g/kg × 75 kg = 120 g protein per day
- Per meal (4 servings): 0.4 g/kg × 75 kg = 30 g per meal
- In a calorie deficit: up to 2.0 g/kg × 75 kg = 150 g protein per day
Timing, whether you consume your protein within 2 hours after training, is less critical than total daily intake. ISSN data shows that a window around training is useful but not a rigid requirement.4 If you eat regularly and distribute your meals well, you're already in good shape, as we show in more detail in our article on the anabolic window.
FAQ: Too much protein, are there risks?
In healthy adults without pre-existing kidney conditions, there is no solid evidence that protein intake up to 2.0 g/kg/day causes kidney damage. The ISSN confirms this in their position stand (Kreider et al. 2017).4 Anyone with impaired kidney function should discuss protein intake with their doctor, as different recommendations apply.
Yes, with the right source combination. Pea protein and fava bean protein complement each other's amino acid profiles well and, at sufficient total intake, provide all essential amino acids in bioavailable form. A modest 10% surcharge on the recommended amount closes any DIAAS gaps. It's not a concern; it's a planning variable.
There is no clearly defined toxic threshold for healthy adults. The ISSN sets the upper end of their recommendation at 2.0 g/kg for active adults in a calorie deficit. Consistently going well above that, 3.0 g/kg and more, increases oxidative load without additional muscle-building benefit. The extra effort simply doesn't pay off.4
The Bottom Line
1.6 g/kg is a well-supported target, but not a universal plateau that applies to everyone under all conditions. Older adults need more per meal, plant-based protein sources require a slightly higher daily total, and anyone training in a calorie deficit should plan upward. Distributing across 3 to 4 meals at 0.4 g/kg matters at least as much as the daily total. Not as simple as the one number suggests. But simpler than the effort many put into reaching it.
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