Registered Dietitian Says This Amino Acid Is the “On Switch” for Muscle Growth
Most people know the basics of building muscle: lift heavy and eat your bodyweight in protein. But once you move past the beginner stage, those gains don’t come as easily. At the intermediate and advanced levels, fine-tuning both your diet and your training becomes essential if you want to keep adding size. Some nutritionists even swear by a single amino acid, leucine, as a key tool for dialing in muscle growth.
Found in high amounts in whey protein, leucine has long been recognized as more than just a building block. Amy Shapiro, MS, RD, CDN, and lead nutritionist at ButcherBox, explains that this amino acid acts as a signal to your body, not just raw material.
"While all amino acids contribute to muscle repair and growth, leucine uniquely tells your body when to start building muscle," she says. "It’s the most potent activator of the muscle-building pathway known as mTOR, which regulates muscle protein synthesis. Without enough leucine, that signal is muted, even if total protein intake is adequate."
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How Leucine Signals Your Body to Build Muscle
Simply put, leucine is the trigger that tells your body it’s time to build muscle. When you sit down to a protein-forward meal like chicken, pork, or lean beef paired with carbs such as rice or sweet potatoes, leucine helps activate mTOR and signals that your body has what it needs for repair and growth.
Without enough leucine in the mix, that process stalls. "Without hitting that leucine threshold, muscle protein synthesis remains limited, even if calories and protein are present," Shapiro says.
If you’ve spent any time chasing muscle growth through supplements, you’ve probably heard leucine mentioned alongside the other branched-chain amino acids, isoleucine and valine. While all three play a role in muscle metabolism, they don’t contribute equally. Leucine is the key signal. The others support the process, but don’t actually turn muscle building on.
BCAAs and Whole-Food Proteins
"Essential amino acids (EAAs) are required to actually build muscle tissue, but without sufficient leucine, the muscle-building response is muted," reveals Shapiro. "That’s why isolated BCAA supplements often underperform compared to whole protein sources, which naturally contain leucine plus all EAAs in the right ratios."
So what does that mean in practice? While leucine can be supplemented through certain whey proteins or BCAA powders, Shapiro says most people are better off focusing on real food. Whole-food proteins, especially animal-based options, consistently deliver better results because they provide enough leucine alongside the full spectrum of amino acids your muscles actually need to grow.