What the research actually shows
This question has two good meta-analyses, four years apart, and their apparent disagreement is the most useful part.
- No difference in absolute lean mass, and none in strength.
- A small favouring effect for animal protein on percent lean mass.
- In adults under 50, animal protein gained more: 0.41 kg (95% CI 0.08 to 0.74) of absolute lean mass. Resistance training status made no difference to the result.
The authors also flag something that reframes the whole comparison: total protein intake was generally above the recommended allowance in both arms of most trials. When everyone is eating plenty, source has less room to matter.
The larger and more recent analysis found a small overall advantage for animal protein on muscle mass (SMD −0.20; 95% CI −0.37 to −0.03), stronger in adults under 60 and absent in those over 60. Then the subgroup analyses take the finding apart:
- Soy versus milk protein: no difference at all across 17 RCTs (SMD −0.02). This is the comparison most vegan lifters are actually making.
- The gap was against non-soy plant proteins (rice, chia, oat, potato; SMD −0.58, 5 RCTs), and against whole plant-based diets (SMD −0.51, 7 RCTs).
- No difference in strength or physical performance. Consistent with Lim, four years and twenty-five trials later.
So the two analyses don't really conflict. Both say strength is a wash. Both find a small mass advantage that narrows to nothing when the plant source is good.
Heavily skewed toward older adults and sarcopenia research. Reid-McCann's framing is age-related muscle loss, and both analyses include many trials in people over 60 who are not resistance training. Lim tested for a training effect and found none, which helps. But the number of trials specifically in young trained vegan lifters is very small, and that is the population asking the question.
Why the gap exists at all
Two mechanisms, both of which you can work around:
- Leucine. It's the amino acid that triggers muscle protein synthesis, and most plant proteins carry less of it per gram. Soy is the exception, which is exactly why soy matched milk in the data.
- Digestibility. Plant proteins arrive wrapped in fibre and cell walls, so less of what's on the label reaches the bloodstream as usable amino acids.
Both are dose problems, not quality problems in the fatal sense. Eat more total protein and the deficit closes. This is why the standard adjustment for plant-based lifters is to aim slightly above the usual 1.6 g/kg/day.
The "incomplete protein" thing
Largely obsolete. The idea that plant proteins must be combined at the same meal was walked back decades ago. Your liver maintains an amino acid pool that smooths this out across the day. Eat varied plant proteins across a day and combining takes care of itself.
What to do
- Eat more total protein if you're plant-based. Aim ~1.8–2.0 g/kg rather than 1.6, to cover digestibility.
- Lean on soy. Tofu, tempeh, edamame, soy protein isolate, the one plant source that matched milk protein head to head.
- Treat rice, pea, oat and hemp as supporting sources, or blend them. Rice and pea together cover each other's weak amino acids.
- Ignore protein combining at each meal. Variety across the day is enough.
- Watch B12, iron, zinc, omega-3 and creatine. These matter more for a vegan lifter than the protein question does.
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