Leucine

Leucine is one of the three branched-chain amino acids (BCAAs) and the single most important amino acid for triggering muscle protein synthesis. It works by activating mTOR, the cellular signal that tells your muscles to start building. Studies have identified a leucine threshold of about 2.5 to 3 grams per meal, which is what most research uses to define a 'protein-effective' meal. That works out to roughly 25-30 grams of high-quality animal protein, or about 35-40 grams of plant protein, per meal. Hitting the threshold matters more for older adults whose muscles show anabolic resistance — the same dose triggers less synthesis than in younger people, so they may need 35-40 grams of protein per meal to clear it. Leucine is naturally rich in whey, eggs, beef, fish, and dairy. Plant sources like soy, lentils, and quinoa contain leucine but at lower concentrations, so plant-eaters may need 10-20 percent more total protein to reach the same threshold consistently. Pure leucine supplements exist but rarely outperform whole protein at sufficient doses. The protein calculator automatically suggests 3-6 meal splits that hit the leucine threshold without requiring you to count amino acids manually.