The leucine threshold problem with plant protein
Muscle protein synthesis responds in a roughly threshold-like way to the leucine content of a meal, with a per-serving amount often cited near two to three grams needed to maximize the response in younger adults. Many plant proteins are lower in leucine per gram than whey, so a standard plant serving can fall short of that threshold even when total protein looks adequate.
This explains why an otherwise sufficient plant meal can produce a smaller synthesis response.
Topping up leucine instead of switching sources
Adding a small amount of free leucine, or simply eating a larger portion, can lift a plant-based meal over the threshold and bring the synthesis response closer to that of animal protein, without abandoning a plant diet. This is a practical fix for vegans and vegetarians.
This is general educational information, not medical advice. Adding isolated amino acids changes a meal's balance. Consult a qualified professional for individualized protein planning.