The Master Amino Acid for Bodybuilding

Leucine and MPS

Without this amino acid, which your body can't make on its own, the whole process of muscle protein synthesis is hampered.

Your muscles listen to you. If you don't use them much, you tell them to stay the same size. If you don't talk to them at all by being an inactive couch spud, they eventually start to shrink from neglect. But if you give them the right messages, they'll grow. The more physical labor you do, the more they're needed and the more they'll adapt.

In a sense, bodybuilding is a carefully curated collection of movements that simulate physical labor. We push and pull weights and our muscles listen to that message. If they're challenged enough, and if you're feeding them and giving them time to recover, they'll answer you.

That's a pretty way to think about a very complicated process called muscle protein synthesis or MPS. You don't have to know a damn thing about MPS to build muscle. People were building bulging biceps long before we had stable-isotope labeling methods to measure how MPS works. Before we had terms like mRNA, ribosomes, polypeptide chain formation, and protein folding, we knew to bench press and squat.

But if you only know one thing about MPS, it should be this: without enough leucine, the whole process doesn't work. Without adequate leucine, your muscles can't "hear" you.

What is Leucine Exactly?

You know the basics: L-Leucine is one of the nine essential amino acids. It's "essential" because your body can't synthesize it and you have to get it from food or supplementation. This BCAA helps regulate blood sugar and produce growth hormone. It also combats muscle oxidation (burning) from intense exercise and jumpstarts the recovery process after hard workouts.

But most importantly, it's the master amino acid for muscle protein synthesis. When you eat protein-rich foods or take supplements containing leucine, it triggers a cascade of muscle cell activity that ultimately leads to the synthesis of new muscle proteins (big biceps, bro).

It works like this:

  • Activation of mTOR Pathway: Leucine activates the mTOR signaling pathway in muscle cells. mTOR is a key regulator of protein synthesis and cell growth. High leucine levels "turn on" the mTOR pathway.
  • Initiation of Translation: Activation of the mTOR pathway leads to the initiation of protein translation. That's the process where the genetic information encoded in messenger RNA is translated into protein. Leucine kick-starts this process by promoting the assembly of the protein-synthesizing machinery.
  • Increased Ribosomal Activity: In the presence of leucine, ribosomes (cellular structures responsible for protein synthesis) get more active. More ribosomes are engaged in translating mRNA into protein, which speeds up the rate of protein synthesis.
  • Enhanced Recruitment of Amino Acids: Leucine also enhances the uptake and utilization of all the other amino acids.

The Age Factor

As we get older, we get partially "resistant" to leucine. Accordingly, studies show that older adults need twice as much supplemental leucine as their younger counterparts to approximate similar levels of muscle protein synthesis.

I'm Convinced. Where Do I Get Leucine?

Mostly from animal products. (Sorry, vegans.) If it walks, swims, or flies, it's a good source of leucine. Likewise, eggs and dairy products contain leucine.

A serving of sirloin steak provides roughly 2 to 2.5 grams of leucine, a serving of chicken breast has about 1.5 grams, and an egg contains about 0.8 grams. (Cooking time and temperature affect the leucine content, so it's tough to put an exact number on most foods.)

Vegans aren't totally out of luck, because beans, nuts, and seeds contain a bit of leucine too, though certainly not as much as a lean steak.

Among supplements, whey protein isolate has the highest leucine content. Thirty grams contains approximately 3 grams of leucine. That's one of the reasons isolate is the chosen form of whey used in the MD Protein (Buy at Amazon) formula along with micellar casein. Casein is also a complete protein, containing all the essential aminos, including leucine.


Surge Workout Fuel (Buy at Amazon) provides a whopping 5 grams per serving, along with other performance enhancers like beta-alanine, betaine, and HBCD.


By the way, unlike most supplement companies, Biotest only uses fermented leucine in Surge, the same quality used in medical-grade IV drips. Standard leucine made from chicken feathers and hair is cheaper, but, well, that's just gross.


TC, Given the activation of mtor and a recent study showing an association with high leucine intake a certain cancers, do you have concerns.

Likely due to Leucineā€™s insulin raising activity