Leucine is often called the most important amino acid for building muscle. While that’s not entirely wrong, it’s also an oversimplification. Here’s what leucine actually does, why it’s important, and how it fits into the bigger picture of muscle growth.
Leucine is one of the 20 amino acids used by the body to build proteins. More specifically, it is one of the nine essential amino acids (EAAs), meaning the body cannot synthesize it in sufficient quantities to meet its physiological requirements. As a result, leucine must be obtained through the diet (Wu, 2013).
Leucine also belongs to a subgroup known as the branched-chain amino acids (BCAAs), along with isoleucine and valine. Unlike many other amino acids, BCAAs are metabolized extensively in skeletal muscle and can be used as an energy source.
Beyond these shared characteristics of BCAAs, leucine has another important function that makes it unique.
Leucine also acts as a nutrient signal.
Following a protein-rich meal, blood leucine concentrations rise. Muscle cells detect this increase, helping activate signaling pathways centered on mechanistic target of rapamycin complex 1 (mTORC1).
mTORC1 regulates several processes involved in initiating translation and muscle protein synthesis or MPS for short (Anthony et al., 2001; Kimball & Jefferson, 2006). Put simply, it helps switch on the cellular machinery used to build new muscle proteins.
This signaling role is why leucine is often described as a “trigger” for MPS.
Not necessarily.
Leucine appears to have a threshold-like effect on muscle protein synthesis. If leucine intake is below the amount needed to fully stimulate MPS, increasing leucine can produce a greater response. Once that threshold is reached, however, adding more leucine does not necessarily increase MPS further. The amount of leucine needed to reach that threshold, however, can vary.
The studies exploring this are revealing, but they need some context before their results can be compared.
Before looking at those studies, one distinction is important because it affects how their results should be interpreted.
Some studies use free-form EAA mixtures, while others use intact proteins such as whey, soy, egg or beef.
Free-form formulations can differ in both the total amount of EAAs provided and the proportion contributed by leucine and the other amino acids. Intact protein sources also have different amino acid profiles, including different amounts and proportions of leucine and other EAAs.
Just keep in mind that composition is only part of the difference. The form in which those amino acids are consumed also affects how they are delivered to the body.
Free-form EAAs do not require digestion before absorption, while intact proteins must first be broken down.
Together, these differences can affect both the amount of amino acids delivered and how quickly leucine and other EAAs become available in the blood. With that context in mind, the threshold studies become easier to interpret (Gwin et al., 2020).
Glynn et al. (2010) compared 10 g of EAAs containing either 1.8 or 3.5 g of leucine in healthy young adults at rest. Both mixtures increased MPS to a similar extent, despite greater leucine availability and anabolic signaling with the higher-leucine mixture. Under those conditions, about 1.8 g of leucine appeared to be enough (in this particular case).
Pasiakos et al. (2011) found something different. They compared 10 g of EAAs containing 1.87 or 3.5 g of leucine during moderate-intensity endurance exercise. In that setting, the higher-leucine mixture produced a greater postexercise MPS response.
Weight training can also influence how much total protein is needed. Macnaughton et al. (2016) found that 40 g of whey produced a greater MPS response than 20 g following whole-body resistance exercise in young trained men. This suggests that a larger exercise stimulus can shift the protein dose-response upward.
Age can alter the response as well. Katsanos et al. (2006) found that older adults responded better to an EAA mixture containing a higher proportion of leucine. Similarly, Dickinson et al. (2014) found that 10 g of EAAs containing 3.5 g of leucine prolonged the post-exercise MPS response in older men compared with the same EAA dose containing 1.85 g of leucine, even though the initial response was similar.
Despite differences in how amino acids are delivered, both free-form EAAs and intact proteins show a saturable MPS response.
Dose-response studies using free-form EAAs, whey, egg and beef show that MPS eventually reaches a point where additional amino acids or protein do not further increase the acute response (Cuthbertson et al., 2005; Moore et al., 2009; Symons et al., 2009; Witard et al., 2014).
The amount needed to reach that point is not always the same tho. In older adults, for example, Yang et al. (2012) found that 40 g of soy protein increased post-exercise MPS while 20 g did not, suggesting the smaller dose had not yet reached an effective anabolic threshold.
The practical point is that leucine can increase MPS when leucine availability or leucine-sensitive signaling is still limiting the response. Once enough leucine and EAAs are available, however, leucine is no longer the limiting factor.
At that point, increasing the protein dose may provide more leucine and more EAAs without producing a greater MPS response if another physiological factor has become limiting. This is consistent with studies showing that larger protein servings do not always increase MPS further once an effective dose has been reached (Moore et al., 2009; Symons et al., 2009; Witard et al., 2014).
Taken together, these findings suggest that there is no single leucine threshold that applies to everyone. Age, exercise and the amount and availability of EAAs can all influence how much leucine or protein is needed to maximize the response.
For a closer look at how EAAs work together to support protein synthesis, see EAAs 101: Essential Amino Acids Explained.
Once again, not necessarily.
Most leucine studies measure muscle protein synthesis over several hours. Muscle hypertrophy, by contrast, reflects the cumulative effects of resistance training, nutrition and recovery over weeks, months and years.
A larger acute muscle protein synthesis response may be favourable, but it does not guarantee greater long-term muscle growth (Phillips et al., 2014; Damas et al., 2016).
More long-term resistance-training research is needed to determine whether increasing leucine beyond an otherwise adequate protein intake produces a meaningful additional hypertrophy benefit.
Most of the research on leucine focuses on muscle protein synthesis, but one longer-term study also looked at strength.
Ispoglou et al. (2011) gave novice men 4 g of supplemental leucine per day during 12 weeks of resistance training. The leucine group increased total 5-RM strength across eight exercises by 40.8%, compared with 31.0% in the placebo group.
However, the leucine group did not gain significantly more lean tissue. So, while supplemental leucine may improve strength under some conditions, this study does not show that an additional 4 g per day produces greater muscle growth.
Leucine is found in virtually every protein-containing food, although some sources provide considerably more than others.
Excellent sources include:
These foods not only provide leucine but also the remaining essential amino acids needed to support muscle protein synthesis.
Most nutrition apps stop at calories, protein, fat, and carbohydrates. Diet Maker goes much further.
Diet Maker displays leucine and the other eight essential amino acids for every food, recipe, meal and daily meal plan.
