In people who regularly lift weights, taking extra protein or essential amino acids can slightly increase muscle thickness—by about 1 to 2 millimeters—if their daily protein intake is under 1.6 grams per kilogram of body weight or if a single meal contains less than 2 to 3 grams of leucine. Beyond about 2.0 grams per kilogram per day, additional protein does not lead to further muscle growth.

We analyzed one assertion on protein supplementation and muscle thickness in resistance-trained adults, and the evidence we’ve reviewed leans toward the idea that extra protein can slightly increase muscle thickness—by about 1 to 2 millimeters—when daily intake is below 1.6 grams per kilogram of body weight or when a single meal lacks 2 to 3 grams of leucine, a key muscle-building amino acid [1]. Beyond about 2.0 grams per kilogram per day, adding more protein does not appear to lead to further increases in muscle thickness [1]. This suggests there may be a point where more protein doesn’t add more benefit, even for people who lift weights regularly. We didn’t find any studies that contradicted this pattern. The effect is small and only noticeable under specific conditions—like when baseline protein intake is low or when leucine isn’t well distributed across meals. We don’t know if this ceiling applies equally to everyone, or how long these changes last. What we’ve found so far is limited to one assertion, and more research could change this picture. For now, if you’re lifting weights and already eating around 1.6 to 2.0 grams of protein per kilogram each day, adding more may not help you build thicker muscles.