Leucine Threshold 101: What It Is & Why It's Essential For Building Muscle

Despite what protein powders marketed to gym rats might lead you to believe, we all need to prioritize protein to feel our best—no matter how much time we spend at the weight rack.

Eating enough high-quality protein is essential for muscle mass, brain function, immunity, and much more. But the "high-quality" callout is important here since not all protein is created equal.

Protein is made up of building blocks called amino acids—nine of which are essential, meaning the body can't create them on its own and we need to get them from food. Leucine is one of these nine amino acids, and it's also one of three branched-chain amino acids, which have a unique composition that makes them important supporters of skeletal muscle growth and maintenance.

Having adequate muscle goes far beyond aesthetics or even performance, by the way. It's an important consideration for everyone since those who are under-muscled tend to have a suboptimal quality of life and life span concerns overall, and they're more likely to face challenges related to physical functioning as they get older1.

And research shows that leucine, in particular, is even more important than the other BCAAs for supporting healthy muscles as we age. That's because the amino acid is a key activator of the mTOR pathway2, which controls muscle protein synthesis, or the creation of new muscle.

"You can think of the BCAA leucine as a trigger or 'it's go-time' button for muscle protein synthesis (i.e., the critical cellular process for building muscle)," explains mindbodygreen's vice president of scientific affairs, Ashley Jordan Ferira, Ph.D., RDN.

Leucine also supports endurance and strength, and it may assist with fat loss by increasing the satiety hormone leptin and improving glucose balance, though we need more research to be sure.

The kicker is that you can't just have a little bit of leucine here and a little bit there and call it a day.

Curious about this meal threshold and how to hit it consistently, we invited Layman onto the mindbodygreen podcast to learn more. There, the University of Illinois at Urbana-Champaign professor explained that 2.5 to 3 grams of leucine per meal appears to be a sweet spot for triggering essential muscle pathways.

Research shows that meals containing less than 1.8 grams of leucine4 don't seem to support muscle synthesis. But more isn't always more in this case either. Eating a meal (or BCAA supplement) containing more than 3 grams of leucine doesn't seem to deliver any additional benefits.

So, what do you need to eat to hit this 2.5- to 3-gram leucine target? Animal proteins have a higher percentage of leucine5 than plant proteins, and a standard serving of chicken, beef, or a whey protein supplement should get you there. "In whey protein, leucine is about 12%, so 23 grams of whey protein isolate will trigger it. Whereas in soy protein isolate, it's about 7.8%, so now you need 33 or 34 grams. So all proteins aren't equal,"* Layman said in his episode.

Getting enough leucine as a vegan or vegetarian is a bit more challenging and will likely require eating larger portions and/or supplementing where appropriate. Learn more about the plant and animal foods that are highest in leucine here.

If you're eating enough bioavailable, complete proteins—which contain each of the essential amino acids in the proper amounts—to fulfill your daily protein needs, you should be getting plenty of leucine.

The thing is that your protein needs might be higher than you think. They'll depend on factors like your weight, activity level, sex, and genetics, but most adults will want to get upward of 100 grams of protein each day.

Research suggests that distributing this protein6 in meals throughout the day tends to be best for muscle growth. If you're running around during breakfast or lunch and don't have time to whip up a proper serving, using a protein powder can be a convenient way to fill in gaps.* But again, plant proteins are lower in leucine, so an animal-based product (like whey protein) will typically provide more muscle support.