How much protein can your body use in a single meal?
There is no hard upper limit above which your body definitively wastes protein: 20-25 grams of fast protein per meal maximises muscle building, but higher doses are not pointless. Spread your intake across a minimum of 4 meals and make sure you consume sufficient calories.
Around 20-25 grams of fast-digesting protein per meal, such as whey protein, appears to largely maximise muscle building in young adults who do resistance training. If you eat more, the extra amino acids are more often burned as an energy source. But the popular idea that your body 'wastes everything above 25 grams' is not entirely correct: a portion of that higher intake is still used for muscle and tissue repair.
The 20-25 gram threshold applies specifically to fast-digesting protein sources. If you choose slower sources such as casein, or eat protein together with carbohydrates and fats, absorption is slower. That may mean your body can extract more from a larger portion as well, but the precise added benefit has not yet been well quantified. This point of reference is provisional and the evidence behind it is thinner than for the 20-25 gram guideline.
More practically: spread your protein intake across a minimum of 4 meals per day. Aim for approximately 0.4 grams per kilogram of body weight per meal. If you weigh 75 kg, that is about 30 grams per eating moment. Across the day, this leads to a minimum of 1.6 grams per kilogram per day. The upper limit cited in the literature is 2.2 grams per kilogram per day, divided over 4 or more meals of at most around 0.55 grams per kilogram.
An important caveat: these recommendations only apply when you are consuming sufficient calories. Under conditions of severe energy deficit, such as during intense military operations, even 20-25 grams of protein per meal proved insufficient to stimulate muscle building or halt muscle mass loss. The body then redirects the amino acids as fuel. Your daily calorie intake is therefore just as important as the amount of protein.
All claims are based on two PMIDs (29497353 and 31785885), with most findings resting on a single source. No meta-analyses were used as a direct source. The strength of evidence is reasonable for the core findings on fast-digesting protein and distribution across the day, but thinner for the effect of slow protein sources and meal composition.