Protein: how much, when, and the leucine threshold
For building or keeping muscle, the evidence points to roughly 1.6 g of protein per kg of body weight per day, rising toward 2.0 to 2.2 g/kg when you are dieting and want to protect lean mass. Spread it across 3 to 4 meals of about 0.4 g/kg each, enough to clear the leucine threshold that flips muscle protein synthesis on. Quality matters at the margins, older adults need more, and in healthy kidneys none of this causes harm.
Key facts
- The largest meta-analysis to date found muscle gains from training plateaued at about 1.6 g/kg/day (95% CI 1.03 to 2.20).1
- In a calorie deficit, lean dieters preserve muscle best at 2.3 to 3.1 g per kg of fat-free mass, roughly 2.0 to 2.2 g/kg total.2
- Each meal should land around 0.4 g/kg to maximise muscle protein synthesis; the trigger is about 2 to 3 g of leucine.34
- Older adults are anabolically resistant and need more, around 1.0 to 1.2 g/kg/day and a higher per-meal dose.5
- High protein does not damage healthy kidneys: a meta-analysis found no difference in filtration rate.6
How much per day
The UK reference nutrient intake is just 0.75 g/kg/day, set by the Scientific Advisory Committee on Nutrition to stop most healthy adults running short.7 That is a floor for avoiding deficiency, not a target for building muscle. For that, the anchor figure comes from Morton and colleagues' 2018 meta-analysis in the British Journal of Sports Medicine, pooling 49 trials and 1,863 people doing resistance training. Adding protein clearly improved strength and lean mass, and the benefit flattened out at about 1.6 g/kg/day.1 Worth noting: the confidence interval was wide (1.03 to 2.20), so 1.6 is a sensible central estimate rather than a hard ceiling, and a little more does no harm.
The one situation that justifies pushing higher is a calorie deficit. When you are eating less to lose fat, protein both spares muscle and keeps you full. Helms and colleagues' review of lean, trained dieters concluded that needs rise to 2.3 to 3.1 g per kg of fat-free mass, which for most people works out near 2.0 to 2.2 g/kg of total body weight, scaling up the leaner and more aggressive the diet.2 If you are recomposing rather than simply cutting, our piece on body recomposition covers how protein and training interact.
Evidence strength, plainly. Daily target near 1.6 g/kg for muscle: strong (large meta-analysis of RCTs). Higher intake in a deficit: moderate (systematic review, smaller trials). Per-meal and leucine thresholds: moderate (controlled acute studies measuring synthesis, not long-term muscle gain). Kidney safety in healthy people: strong (meta-analysis of controlled trials).
When: distribution and the leucine threshold
Total protein matters most, but how you spread it has a measurable effect on the machinery of muscle building. Muscle protein synthesis is not a tap that runs in proportion to intake; it works more like a switch. Below a certain dose it barely responds, then it climbs steeply, then it plateaus, with extra protein in that one sitting largely burned for energy rather than added to muscle. The amino acid that throws the switch is leucine, which activates the mTORC1 signalling pathway. The practical threshold in younger adults is roughly 2 to 3 g of leucine per meal, the amount in about 25 to 30 g of a high-quality protein.4
Translated into protein, the maximally effective per-meal dose is about 0.4 g/kg. Moore and colleagues pooled dose-response data and found synthesis plateaued at around 0.24 g/kg in young men and 0.40 g/kg in older men, which is why the per-meal figure is set near the higher value to cover everyone.3 Because each meal triggers a fresh pulse, spreading intake helps. In a 12-hour study after training, Areta and colleagues compared 8 x 10 g, 4 x 20 g and 2 x 40 g; the moderate, evenly spaced pattern of 4 x 20 g produced the strongest synthesis.8 The lesson is not to obsess over timing but to avoid the common pattern of a tiny breakfast and a huge dinner.
| Goal or group | Daily target | Per meal (70 kg) |
|---|---|---|
| UK floor (avoid deficiency) | 0.75 g/kg7 | not the aim |
| Building / keeping muscle | ~1.6 g/kg1 | ~28 g |
| In a calorie deficit (lean) | 2.0 to 2.2 g/kg2 | ~35 g |
| Older adults (65+) | 1.0 to 1.2 g/kg5 | ~30 g+ |
Quality: plant versus animal
Protein sources differ in two ways: their leucine content and their digestibility. Animal proteins (whey, dairy, eggs, meat, fish) are leucine-rich and highly digestible, so they clear the threshold in a smaller serving. Many plant proteins are lower in leucine and less digestible, which is why a gram-for-gram plant meal can stimulate slightly less synthesis. This is not a barrier, it is an adjustment. Eating a bit more total plant protein, blending sources so their amino acid gaps cancel out (for example pea with rice), and favouring leucine-dense options like soya and pea isolate close the gap. Controlled studies show that when leucine or total dose is matched, plant blends can stimulate synthesis comparably to whey.9 For an evidence-led look at supplements that pair with training, see our insights, and the stack builder helps avoid duplication.
Older adults and anabolic resistance
Ageing muscle responds less to a given dose of protein, a phenomenon called anabolic resistance. The fix is more protein, not less. The PROT-AGE expert group recommends 1.0 to 1.2 g/kg/day for healthy older people, and more (1.2 to 1.5 g/kg) for those who are active or managing illness.5 The per-meal threshold also rises: older adults need closer to 0.4 g/kg and around 2.5 to 2.8 g of leucine to match the response younger people get from less.3 Combining that with resistance training is the single most effective strategy against age-related muscle loss.
The kidney myth
The belief that high protein harms the kidneys comes from advice given to people who already have kidney disease, where protein is restricted. It does not transfer to healthy kidneys. Devries and colleagues' 2018 meta-analysis in the Journal of Nutrition pooled controlled trials and found that higher-protein diets did not change kidney filtration rate compared with lower or normal intakes in healthy adults.6 A higher protein intake does raise filtration acutely, but this is a normal functional response, not damage.
Who should take care. The reassurance applies to healthy kidneys. If you have chronic kidney disease, reduced kidney function, or a single kidney, your protein target should be set with your clinician or renal dietitian, not from general advice. Very high intakes have not been studied long-term in these groups.6
What to ask your GP
- Do my kidney blood tests (eGFR) give any reason to limit protein in my case?
- Given my age and activity, is a higher protein target sensible for keeping muscle?
- If I am losing weight on medication or by dieting, how do I protect muscle?
- Are there interactions with any condition I have before I increase protein?
References
- Morton RW, 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. Br J Sports Med. 2018;52(6):376-384. PMID 28698222.
- Helms ER, Zinn C, Rowlands DS, Brown SR. A systematic review of dietary protein during caloric restriction in resistance trained lean athletes: a case for higher intakes. Int J Sport Nutr Exerc Metab. 2014;24(2):127-138. PMID 24092765.
- Moore DR, et al. Protein ingestion to stimulate myofibrillar protein synthesis requires greater relative protein intakes in healthy older versus younger men. J Gerontol A Biol Sci Med Sci. 2015;70(1):57-62. PMID 24895282.
- Zaromskyte G, et al. Evaluating the leucine trigger hypothesis to explain the post-prandial regulation of muscle protein synthesis in young and older adults: a systematic review. Front Nutr. 2021;8:685165. PMC8295465.
- Bauer J, et al. Evidence-based recommendations for optimal dietary protein intake in older people: a position paper from the PROT-AGE Study Group. J Am Med Dir Assoc. 2013;14(8):542-559. PMID 23867520.
- Devries MC, et al. Changes in kidney function do not differ between healthy adults consuming higher- compared with lower- or normal-protein diets: a systematic review and meta-analysis. J Nutr. 2018;148(11):1760-1775. PMID 30383278.
- British Nutrition Foundation. Protein: dietary reference values and the UK reference nutrient intake (0.75 g/kg/day). nutrition.org.uk, accessed 2026.
- Areta JL, et al. Timing and distribution of protein ingestion during prolonged recovery from resistance exercise alters myofibrillar protein synthesis. J Physiol. 2013;591(9):2319-2331. PMID 23459753.
- Pinckaers PJM, et al. Muscle protein synthesis in response to plant-based protein isolates with and without added leucine versus whey protein in young men and women. Am J Clin Nutr / PMC. PMC11153912, 2024.
This article is educational and does not constitute medical advice, diagnosis, or a treatment recommendation. Medication uses described as “off-label” are not licensed for that purpose in the UK and should only be considered under qualified clinical supervision. Always speak to your GP, pharmacist, or a registered specialist before starting, stopping, or changing any treatment. If you have severe or alarm symptoms - unintentional weight loss, blood in your stool, difficulty swallowing, persistent vomiting, a fever, or severe pain - seek urgent medical care.