How much protein can your muscles use at one time?

Short answer

20 g is not the limit. The 20 g limit came from measuring only 4 hours after leg exercise. After whole-body exercise, 40 g raised the rate of muscle building about 20% more than 20 g, and 100 g kept it higher than 25 g for more than 12 hours. For a 70 kg (154 lb) person, about 28 to 39 g per meal is the guide.

How much protein can your muscles use at one time?
'Over 20 g per meal is wasted.' Is that right?
A common claim is that muscles can only use about 20 g of protein at once and the rest is wasted. So eating a lot in one meal is supposedly a loss. We checked where this formula came from and whether it holds.
Where did the 20 g limit come from?
After leg exercise, over 4 hours, it stopped at 20 g
When 6 people ate 0 to 40 g of egg protein after leg exercise, the rate of muscle building peaked at 20 g and did not rise further at 40 g. In a study of 48 people eating protein after one-leg exercise, 20 g raised it 49% and 40 g raised it 56%, and the researchers concluded 20 g was enough. At 40 g, both studies saw more protein burned for energy.
But whole-body exercise was different
40 g raised it about 20% more than 20 g
When 30 people took 20 g and 40 g on separate days after whole-body exercise, the rate of muscle building was about 20% higher with 40 g. There was no difference between people with more muscle and people with less. The researchers attributed the difference to the amount of muscle exercised.
What about 100 g at once?
It raised it more, for more than 12 hours
36 people took 0 g, 25 g, or 100 g of protein after whole-body exercise. Compared with 25 g, 100 g made the rate of muscle building about 20% higher in the first 4 hours and about 40% higher from 4 to 12 hours. The effect of 100 g lasted more than 12 hours.
Is the extra protein really burned off?
Over 85% of the extra went to building the body
In the same study, even with 100 g, the amount burned for energy was very small compared with the amount used to build the body. When the researchers recalculated the earlier studies behind the 20 g limit, less than 15% of the extra protein was burned and over 85% went to building body tissue.
So why did 20 g come up?
Little muscle worked, short measurement, protein drink only
The 20 g limit came from studies that trained only the legs, gave only a protein drink, and measured for just 4 hours. A 2018 review also pointed out that this number came from eating fast-absorbing protein on its own. A study that measured for 12 hours after whole-body exercise found no limit.
How much per meal?
For 70 kg body weight, about 28 to 39 g per meal
The 2018 review suggested splitting daily protein over at least 4 meals, about 28 g per meal and up to about 39 g for a 70 kg (154 lb) person. That is about 112 to 154 g a day. When 100 g, more than this, was eaten in one meal, the rate of muscle building was still higher than with 25 g.
Summary: 20 g is not the limit
Eating more was used for longer
The 20 g limit came from measuring only 4 hours after leg exercise. After whole-body exercise, 40 g raised the rate of muscle building more than 20 g, and 100 g raised it more than 25 g and kept it up for more than 12 hours. All of these were measured within a day, though, and did not compare muscle size months later.