What Is Leucine?

One of several essential amino acids, leucine plays a crucial role in activating muscle protein synthesis. Keep reading to discover more about the purpose of amino acids, foods rich in leucine, and the potential benefits of supplementation.

What exactly is leucine?

Leucine (l-leucine) is a proteinogenic (protein-building) branched-chain amino acid (BCAA) that contributes to various bodily functions. However, what makes leucine unique is its “essential” status. An essential amino acid is one the body needs to function correctly but cannot produce itself. Instead, we must consume external sources, such as leucine-rich foods and supplements, to get the necessary levels.

So, what about leucine makes it essential to physical health? Well, the amino acid's primary role is to stimulate protein synthesis in the muscle. It works alongside two other branch chain amino acids (isoleucine and valine) to develop skeletal muscle mass, potentially increasing endurance and athletic performance.^[1]

The effects of leucine

The role of leucine is widespread, so it's best if we break down the amino acid's abilities into several key areas. Below is a summary of how it may support physical well-being.

Leucine and muscle growth

Muscle growth is essential, regardless of age or fitness goals, but it only occurs after we've damaged the muscle fibres during exercise. A crucial part of building muscle and staying active is the efficiency of the muscle recovery period post-exercise.

A 2015 meta analysis examined the role of leucine-rich protein supplements across 16 studies and 999 participants. The review concluded that "leucine supplementation was found to exert beneficial effects on body weight, body mass index, and lean body mass", but only in older persons.^[2]

That last point is important because a 2021 study suggests that younger individuals may not benefit from increased leucine intake. The researchers found that 6g daily of leucine "does not improve muscle recovery".^[3]

Leucine and muscle breakdown

The human body is designed to move and be active, with muscles operating under a “use it or lose it” principle. Unfortunately, extended bed rest or inactive periods can lead to muscle breakdown, as your body doesn't see the benefit of maintaining something it isn't using. However, this breakdown is unavoidable for many people, especially if they've experienced an injury or have an underlying health condition.

Encouragingly, two studies from 2016 support the notion that leucine could prevent negative effects on muscle metabolism, mass, and function. There are some caveats, though, as the studies suggest the benefit is short-lived (brief periods of bed rest), and the results have only been observed in middle-aged adults so far.^[4],^[5]

Leucine and endurance

One final but essential area of muscle performance is endurance—the ability to perform strenuous activities for longer periods.

In a 2006 study observing the exercise performance of "competitive outrigger canoeists", researchers note that regular supplementation of leucine "significantly improved endurance performance". They add that, of all the branch chain amino acids, leucine was "suggested to be ergogenic for both endurance and strength/power performance”. These findings go hand in hand with a similar study employing a protein–leucine blend to drive muscle performance.^[6],^[7]

Sources of leucine

While there may be some dispute over the effectiveness of additional leucine supplementation, there's no doubt that a regular, balanced intake is essential to normal muscle function and preventing muscle breakdown.

Foods rich in leucine

Most healthy adults will get all the leucine they need from a varied and balanced diet that includes the following:

• Eggs
• Pumpkin seeds
• Oats
• Cottage cheese
• Chickpeas
• Lean beef

For example, one large egg contains roughly 0.6g of leucine, while half a cup of cottage cheese has 1.27g, and 50g of pumpkin seeds has 1.21g.

We'll cover more on the ideal daily intake of leucine shortly, but you don't need to eat much of the foods listed above to maintain balance. If, however, you're on an exclusion diet or simply don't like any foods rich in leucine, supplementation could prove pivotal to physical well-being.

How to dose leucine

Dosing leucine is a tricky subject because it depends on your personal circumstances. Older people may need more than younger people, but the recommendation also depends on overall well-being and underlying health conditions.

• As a general rule, healthy individuals should consume between 3 and 8g of leucine daily.

For exact dosing advice, there are several calculations involving body weight (mg of leucine per kilogram), but even these fluctuate depending on the health organisation. Ultimately, you shouldn't worry too much about the exact dose of leucine you're consuming unless you're living with a condition that affects protein synthesis or muscle breakdown.

Finally, if you want to make the most of your leucine intake, try to consume food sources or supplements during or shortly after exercise. After exercise, your body is busy rebuilding muscle fibres, making it the ideal time to bolster protein synthesis.

Possible side effects of leucine

The body is well-versed in using leucine, so regular intake (within the thresholds listed above) doesn't appear to cause any noticeable side effects. However, it is possible to have too much of a good thing, including a muscle-building amino acid such as leucine.

Excessive doses of leucine can cause the following:

• Fluctuation of blood sugar levels
• Difficulty absorbing B vitamins
• Gastrointestinal problems

To avoid possible side effects, it's best to consult your doctor or physician about the suitability of leucine supplements. Because leucine supplements are fast-acting and contain a concentrated dose, it's much easier to ingest more than your body needs.

Are leucine supplements safe?

When buying leucine supplements, there are a few simple principles that will ensure the best possible experience. First, you should always buy supplements from reputable suppliers who independently test their products. With the risk of excessive leucine doses causing unwanted side effects, you'll want to ensure the supplements contain the necessary ratios.

The second consideration is to incorporate leucine into an existing supplement routine rather than taking it in isolation. Some studies suggest leucine works well alongside protein, but it also blends with other BCAAs, vitamins, minerals, and botanicals.

Overall, leucine is a well-tolerated compound with a pivotal role in protein synthesis. While the efficacy of supplementation remains hotly debated, there's no doubt that a minimum level is needed to ensure your body can carry out essential biological functions and support your physical wellness goals.

Why not try Muscle Health, a wellness powder with leucine, magnesium, vitamin B, and CBD, and see what difference it can make to your physical wellness regime? Or, to learn more about the role of amino acids in human health, visit our Education section.