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

Emma Loewe
Author:
December 28, 2023
Emma Loewe
mbg Sustainability + Health Director
By Emma Loewe
mbg Sustainability + Health Director
Emma Loewe is the Senior Sustainability Editor at mindbodygreen and the author of "Return to Nature: The New Science of How Natural Landscapes Restore Us."
woman drinking protein shake
Image by istock // VioletaStoimenova
December 28, 2023

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.

If your protein is lacking in the amino acid leucine, you're leaving some important benefits on the table. Here's why any conversation about protein needs to involve leucine too, and how to make sure you're getting enough of it.

Why is leucine so important?

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.

"The amount of dietary protein at a meal required to initiate an anabolic response in skeletal muscle is driven by the leucine content," reads a 2015 paper3 in the American Journal of Clinical Nutrition co-authored by Don Layman, Ph.D., a leading amino acid researcher. In other words, there is a minimum threshold of leucine that you need to hit in a meal in order to activate mTOR and stimulate the production of new muscle proteins.

How much do you need?

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 muscle growth is your goal, you'll also need to pair your leucine-rich protein with resistance workouts. Experts recommend completing multiple resistance workouts a week, and increasing the stress you're putting on your muscles (by doing more reps, adding more weight, etc.) every few weeks in order to build muscle.

How to get it

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.

"We find from a metabolic standpoint, working predominantly with women, that if they get below 100 grams per day, they lose most of the benefits of protein: fatty acid metabolism, insulin sensitivity, weight loss, satiety," Layman explained.

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.

mindbodygreen's grass-fed whey protein isolate+, for example, delivers 25 grams of protein and 2.5 grams of leucine (6 grams of BCAAs) per serving. And while many protein powders are pumped with funky additives, our grass-fed whey contains no added hormones, antibiotics, pesticides, or solvents. Available in chocolate and vanilla, it's flavored with simple ingredients like organic cocoa, organic monk fruit extract, organic vanilla extract—zero fillers or artificial flavors. This clean, pure protein is a delicious way to hit your daily leucine needs and support lean muscle mass.*

The takeaway

If you want to maintain and build muscle (as we all should), it's important to make sure that you are getting enough total protein and leucine in each meal. Learn more about the nuances of protein and how to make sure you're getting enough here.

If you are pregnant, breastfeeding, or taking medications, consult with your doctor before starting a supplement routine. It is always optimal to consult with a health care provider when considering what supplements are right for you.

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