What the research actually shows
This is the rare nutrition question with a clean answer, because someone pooled every decent trial and looked for the point where the benefit stopped.
Three findings matter for you:
- The ceiling is around 1.6 g/kg/day. Above that, extra protein produced no further gain in fat-free mass. The curve flattens; it does not turn downward.
- The total effect of supplementing is modest. Averaged across all trials, extra protein added 0.30 kg of fat-free mass (95% CI 0.09–0.52) and about 2.5 kg to one-rep max. Protein is a requirement, not a growth drug; the training does the work.
- It works better if you already lift, and worse as you age. The benefit was larger in resistance-trained subjects and shrank with increasing age. This is the unusual case where the finding transfers to trained lifters rather than away from them.
A second, larger review looked at the shape of the curve rather than a single cut-off, and found the same thing from a different angle.
Below about 1.3 g/kg/day, each extra 0.1 g/kg bought roughly 0.39 kg of lean mass. Above it, the same increment bought only 0.12 kg. Same story: steep returns while you're under-eating protein, shallow returns once you're not.
Mixed: untrained and trained adults across both reviews, but the meta-regression specifically tested training status and found the effect was larger in trained lifters. That makes this one of the few protein numbers you can quote at a lifter without a caveat. The age effect is real though: an older trainee gets less out of the same gram of protein.
So why do you hear 2 g per pound?
Because it's harmless, it sounds serious, and nobody ever got fired for over-recommending protein. Roughly 2.2 g/kg (1 g per pound of bodyweight) sits comfortably above the breakpoint, which is why it survives as a rule of thumb. It works by being generously wrong in the safe direction.
There is one situation where deliberately overshooting earns its keep: an aggressive fat-loss phase. In a deficit, higher protein protects lean mass and blunts hunger, and intakes toward 2.2–2.4 g/kg are commonly used by lean athletes cutting hard (Helms 2014). That's a different job than building muscle, and it's why the numbers you hear from bodybuilders in contest prep look inflated compared to the meta-analysis.
If you're worried about the high end: 28 controlled trials found no effect on kidney function in healthy adults.
What to do
- Target 1.6 g/kg/day, about 0.73 g per pound. For an 80 kg lifter that's roughly 130 g.
- Going to 2.2 g/kg is fine, just don't expect more muscle for it. Use the high end while dieting, not while bulking.
- Use bodyweight, not goal weight, unless you're carrying a lot of fat. Then use an estimate of your lean mass or goal weight instead, or you'll chase a number you don't need.
- Hit the daily total before you optimise anything else. Distribution, timing, and source are all downstream of this number . See the 30 g myth and the anabolic window.
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