Leucine: The Most Important Amino Acid for Building Muscle

Key Takeaways

• Leucine is the essential amino acid most directly responsible for triggering muscle protein synthesis (MPS).
  
• Leucine works by activating the mTOR pathway, the cellular signal that initiates muscle building.
  
• Leucine only works fully when taken alongside all eight other essential amino acids—not in isolation.
  
• Research suggests leucine-enriched EAA formulas (around 40% leucine) support MPS and recovery more effectively than standard EAA ratios.
  
• Lower leucine content is one reason many plant proteins stimulate less muscle growth than animal proteins.
  
• A complete, leucine-forward EAA supplement like Kion Aminos delivers leucine in the context of all nine EAAs.

Amino acids—the building blocks of protein—are the catalyst for nearly every chemical process in your body.

They support muscle protein synthesis, muscle gain, brain function, hormone regulation, metabolism, and more.

Of the 20 total amino acids, nine are classified as essential—and of those nine, one is the star player when it comes to muscle protein synthesis.

The MVP of essential amino acids (EAAs)... is leucine.

Leucine is one of the nine essential amino acids and the primary trigger for muscle protein synthesis—the process your body uses to build and repair muscle.

Undoubtedly the most effective amino acid when it comes to building muscle and athletic recovery, leucine supports these processes in a number of ways:

• Increasing the rate of muscle protein synthesis¹
• Regulating the production of anabolic endocrine hormones like IGF-1²
• Stimulating the release of insulin, which enhances the uptake of other amino acids and suppresses muscle protein breakdown³
• Modulating the mTor pathway, which triggers muscle hypertrophy⁴

In fact, the lack of leucine may be why plant-based proteins don’t have as profound of an effect on building muscle when compared to leucine-rich animal proteins.

What does the research say about leucine?

In the last decade alone, there have been a number of high-quality, clinical trials conducted on leucine, especially when it comes to protein synthesis and muscle growth:

In one study, leucine taken with EAAs increased muscle protein synthesis by 33% more than traditional EAAs.⁵

Another randomized controlled trial showed that supplementation with as little as 3g of EAAs enriched with leucine resulted in increased muscle mass, strength, and physical function in post-stroke patients with sarcopenia.⁶

In another study of male college athletes, those who took leucine achieved significantly greater gains in fat-free mass and muscle mass and significant decreases in fat mass compared to placebo groups.⁷

In terms of athletic recovery, the research is also very promising:

A recent study showed that EAAs enriched with leucine significantly suppressed exercise-induced muscle tissue damage.⁸

In another recent study from 2020, leucine-enriched amino acids were shown to preserve muscle force production and reduce muscle soreness more than placebo, even when combined with an already high protein diet.⁹

All that to say, the research is pretty clear: Supplementing with leucine can help you have more lean muscle mass, reduced soreness after exercise, and faster recovery.

 

How much leucine do you need to build muscle?

To maximally stimulate muscle protein synthesis, research points to a leucine-forward profile—roughly 40% leucine within a complete EAA blend. This is higher than the proportion found in muscle itself, because some leucine is oxidized for energy as it enters the cell before it can be used for muscle building.

Here's how we got there:

If your goal is to enhance muscle protein synthesis, or build more muscle, mimicking the amino acid composition of muscle is a good place to start. Human skeletal muscle contains 6.3 grams of leucine per 100 grams (or 20%).

However, when you consume amino acids, some of them get used up before they can be incorporated into skeletal muscle. In fact, that’s exactly what happens to leucine, which tends to get rapidly oxidized by muscle as it enters the cell.

In order to make sure you’re actually getting adequate amounts of leucine—which, as you now know, is the star of the muscle-building show—you should really be supplementing with MORE than the 20% found in skeletal muscle.

You actually need about double the amount of leucine you’ll find in skeletal muscle, especially if you’re concerned about lean muscle and athletic recovery.

Essential amino acids with 40% leucine are found to be more effective in a number of clinical studies when it comes to protein synthesis and muscle building.¹⁰ ¹¹ ¹²

What's the best way to get more leucine?

Before we dive in, let's go back to the studies mentioned above and pick out a few key phrases:

• "Leucine taken with EAAs..."
• "EAAs enriched with leucine..."
• "Leucine-enriched amino acids..."

Yes, the one thing all of these studies have in common is that leucine was not taken in isolation.

To get all of the muscle-building, recovery-enhancing effects of leucine, it must be taken with all eight other EAAs.

	Leucine alone	BCAAs	Leucine within complete EAAs
Triggers MPS?	Yes, but incompletely	Partially	Yes
Provides building blocks to complete MPS?	No	No (only 3 of 9)	Yes (all 9)
Best for muscle building?	No	No	Yes

 

The key is to "bump up" the amount of leucine, and the easiest way to do this is by taking a leucine-enriched amino acid supplement...

What should you look for in a leucine-enriched EAA supplement?

A high-quality leucine-forward EAA supplement should include:

• All nine essential amino acids, not leucine or BCAAs alone
  
• A meaningful leucine proportion (research points to around 40%)
  
• A balanced, research-informed ratio of the remaining EAAs
  
• No added sugar, artificial sweeteners, or fillers
  
• A format that's easy to take consistently

What makes leucine-enriched Kion Aminos different?

We've always known our EAAs were something special. Nearly all of us at Kion start our day with a serving or two of Kion Aminos—for an AM cognition boost, a powerful pre-workout, or to enhance recovery.

But, as EAA supplements have gained popularity, more and more research became available, and after seeing some pretty remarkable new studies, we knew we had to get to work.

 

We wanted to ensure we had the most effective, absorbable amino acids on the market that were formulated based off of the most recent science.

So, after hiring a third party independent research firm to conduct a meta-analysis of all aminos research available, we landed on a seriously upgraded, more potent, even better-tasting formula

Kion Aminos has...

• An LEAA profile with 40% leucine.
• A meticulously crafted amino acid ratio proven to maximize muscle protein synthesis and athletic recovery:
• L-Leucine: 2000 mg (40%)
• L-Lysine: 850 mg (17%)
• L-Isoleucine: 550 mg (11%)
• L-Valine: 500 mg (10%)
• L-Threonine: 475 mg (10%)
• L-Phenylalanine: 350 mg (7%)
• L-Methionine: 200 mg (4%)
• L-Histidine: 71.5 mg (1%)
• L-Tryptophan: 3.5 mg (<1%)
• All nine essential amino acids (including histidine).
• Easy-to-swallow, natural, plant-based capsules. 
• And, just like always, no added sugar, artificial ingredients, chemicals, stimulants, or other nasty fillers.

Not only that, while they were pretty tasty before, our new Mixed Berry and Cool Lime Aminos Powders were meticulously taste-tested and crafted using only natural ingredients and flavorings.

The result is a flavor profile that allows you to sip, and actually enjoy, your amino acid supplement—rather than chugging it down as fast as you can.

Scientific Research

1. https://pubmed.ncbi.nlm.nih.gov/26255285/
2. https://pubmed.ncbi.nlm.nih.gov/23724889/
3. https://pubmed.ncbi.nlm.nih.gov/15930468/
4. https://pubmed.ncbi.nlm.nih.gov/26255285/
5. https://pubmed.ncbi.nlm.nih.gov/21775557/
6. https://pubmed.ncbi.nlm.nih.gov/30273819/
7. https://pubmed.ncbi.nlm.nih.gov/32118385/
8. https://www.jstage.jst.go.jp/article/jpts/31/1/31_jpts-2018-338/_article
9. https://pubmed.ncbi.nlm.nih.gov/32290521/
10. https://pubmed.ncbi.nlm.nih.gov/29183324/
11. https://pubmed.ncbi.nlm.nih.gov/26081485/
12. https://pubmed.ncbi.nlm.nih.gov/25827594/