This is the adaptation side of the resistance-training lever, and it is the evidence Challenge #10 flagged as missing when it separated training-for-strength-gain from training-for-mortality (Muscle-Strengthening Activity and Mortality). Two things distinguish it: it is RCT-grade (unlike the observational mortality data), but it is on surrogates (muscle mass and strength), not patient-important outcomes. The headline: resistance training is the driver; protein is a modest adjunct, and only up to about 1.6 g/kg/day.
Protein augments resistance-training gains — modestly
From «Data from 49 studies with 1863 participants» (RCTs, «RET ≥6 weeks»), protein supplementation added to resistance training, versus placebo/no-supplement:
| Outcome | Added effect of protein (MD) |
|---|---|
| 1RM strength | «2.49 kg (0.64, 4.33)» |
| Fat-free (lean) mass | «0.30 kg (0.09, 0.52)» |
| Muscle fibre CSA | «310 µm2 (51, 570)» |
| Mid-femur muscle CSA | «7.2 mm2 (0.20, 14.30)» |
| Fat mass | «−0.41 kg (−0.70,– 0.13)» |
| Maximal voluntary contraction | «SMD: 0.04 (-0.09, 0.16)» — no effect |
| Total body mass | «0.11 kg (−0.23, 0.46)» — no effect |
Read the magnitudes before the significance. +2.5 kg on a 1RM and +0.3 kg of lean mass over weeks of training are small increments on top of what the training itself delivers — protein does not build muscle, it lets the training build slightly more. (And it did not move total body mass or maximal voluntary contraction at all.)
How much protein — a plateau near 1.6 g/kg/day, with real uncertainty
The decision-relevant number is a dose-response knee: «Protein supplementation beyond total protein intakes of 1.62 g/kg/day resulted in no further RET-induced gains in FFM». This is a genuine, mechanism-grounded plateau — «muscle protein synthesis (MPS)… shows a saturable dose-response relationship», so above the point where synthesis saturates, extra protein has nothing to add.
But hold the number loosely (the lesson from Challenge #10): the break point is «1.62 (1.03, 2.20)» g/kg/day — the confidence interval runs from ~1.0 to ~2.2, so “~1.6” is a central estimate with wide uncertainty, not a precise threshold. And the baseline matters: control groups were already eating «pre: 1.4±0.3» g/kg/day, so the useful move is reaching roughly 1.6, not exceeding it — going higher buys no further lean mass.
This is a legitimate located knee in a curve — the kind the dose-response vocabulary predicts where a saturable mechanism exists, and distinct from the nutrition-reduction curves the wiki has found to be monotone. A saturating supply (protein → synthesis) plateaus; a risk-reduction curve need not.
Who it helps more, and who less
- Reduced with age: «reduced with increasing age (−0.01 kg (−0.02,–0.00), p=0.002)» — older adults gain less lean mass from the supplement (their training still works; the added protein does less).
- Greater in the already-trained: «more effective in resistance-trained individuals (0.75 kg (0.09, 1.40), p=0.03)» — the supplement’s edge grows once someone is past the untrained-beginner phase.
The surrogate boundary — this is the mechanism, not the outcome
Muscle mass and strength are surrogates (Surrogate Outcomes), not patient-important endpoints. Morton is the mechanism half of the resistance-training story — RT (plus adequate protein) builds muscle, RCT-grade — while Muscle-Strengthening Activity and Mortality is the outcome half — strength activity associates with lower mortality, observational-grade. Neither shows that protein supplementation reduces mortality; the composite is “RT builds muscle (proven) and strength associates with living longer (associational)”, with protein a small lever on the first half only. Muscle mass/strength do matter directly for function and sarcopenia, which are on the outcome menu — so the surrogate is not worthless, it is just not the mortality endpoint.
Decision relevance
- If you do resistance training, aim for roughly 1.6 g/kg/day of total protein — that is where the
muscle-building benefit plateaus; more is not better for lean mass. Most people who already eat ~1.4
g/kg are close, so the move is topping up, not loading.
- The target is protein quantity, and it silently assumes quality. Morton’s trials were mostly high-quality supplemental protein (whey). Hitting ~1.6 g/kg from low-DIAAS plant sources (peas 64, wheat 40 vs milk 122) delivers fewer digestible indispensable amino acids per gram, so it needs more grams — hence more food mass — or deliberate complementation to be equivalent -> Protein Quality and the DIAAS Score. This is a source caveat on the number, not a change to it.
- The training is the lever; protein is the adjunct. Do not expect protein alone (without the training) to build muscle — every effect here is during resistance training.
- Age and training status modulate it, not whether to do it: a 46-year-old sits near the age boundary (Morton splits at 45) where the supplement’s edge is modestly reduced but the training’s is not.
- During weight loss, adequate protein + resistance training is how you keep lean mass while losing fat (the fat-mass fell and lean mass rose here) — the practical reason it belongs in a weight-loss program, not just a strength one.
Limits
- Surrogates, not outcomes — no mortality/function endpoint; RCT-grade for muscle/strength only.
- Supplementation, not total dietary protein per se — the trials add protein on top of a diet; whole- food vs supplement was mostly supplement (largely whey), so this is not a food-quality (DIAAS) claim -> Protein Quality and the DIAAS Score holds the source-quality dimension.
- The plateau CI is wide (1.03-2.20); the fibre-CSA effect is fragile; healthy adults only.
- One meta-analysis; the moderator model overall explained little variance, so age/training effects are subgroup signals, not a full explanation.