Leucine promotes muscle protein synthesis via two different methods. Both, lead to the activation of mTOR — the fundamental route to all cell synthesis in the body. Studies have shown that adding a leucine supplement to your diet improves the synthesis of muscle protein.
Leucine’s effects on muscles focus on the production of skeletal protein, this means that the additional protein is produced as muscle mass. Leucine works as an anabolic trigger, and is metabolized directly in muscle tissue. Increasing muscle protein synthesis keeps you in a state of positive nitrogen balance, which is what you want to be if you’re trying to build muscle mass.
Leucine is the amino acid that is thought to be most potent at stimulating MPS. Peak plasma leucine concentrations following protein ingestion typically correlate with muscle protein synthesis rates (Pennings, 2011). This supports the notion that protein digestion rate and protein leucine content are important predictors for anabolic effect of a protein source.
This is best illustrated by study (Churchward-Venne, 2014) which compared the muscle protein synthetic response to five different supplemental protocol:
- 25 g whey
- 25 g whey with 2.25 g leucine for a total of 3 g leucine.
- 25 g whey with 4.25 g leucine added for a total of 5 g leucine.
- 25 g whey with 6 g BCAA added (4.25 g leucine, 1.38 g isoleucine for, and 1.35 g valine).
- 25 g whey (contains a total of 3 g leucine).
All five conditions increased muscle protein synthesis rates compared to fasting conditions. As expected from our earlier discussion on the optimal amount of protein, 25 gram of protein increased MPS rates more than just 6.25 gram.
Interestingly, the addition of 2.25 gram leucine to 6.25 gram whey did not further improve MPS. Since this low whey + low leucine amount had the total amount of leucine as 25 g whey (which contains 3 g leucine), it indicates that leucine alone doesn’t determine the muscle protein synthesis response.
The addition of a larger amount of leucine (4.25 gram) to the 6.25 gram whey did further improve MPS, with MPS rates being similar to the 25 gram of whey. This indicates that the addition of a relatively small amount of leucine to a low dose of protein can be as effective as a much larger total amount of protein.
Finally, it’s interesting that the addition of the 2 other branched-chain amino acids (BCAA) appeared to prevent the positive effect of leucine on MPS. Isoleucine and valine use the same transporter for uptake in the gut as leucine. Therefore, it is speculated that isoleucine and valine compete for uptake with leucine, resulting in a less rapid leucine peak which is thought to be an important determinant of MPS rates.
- Consume 5 grams of leucine within 30 minutes of training to support an anabolic stimulus.
- Combine another 5 grams of leucine in your post-workout protein shake.
- Add Leucine to your protein to help reduce calories for maintaining muscle while dieting.
- Leucine ingestion stimulates tissue protein synthesis via a mammalian target of rapamycin pathway (mTOR) in a dose-dependent fashion.,,
- Leucine increases mitochondrial biogenesis.
- Leucine promotes energy partitioning (decreased energy storage in adipocytes and increased fatty acid utilization in muscle).,
- Leucine acts as strong insulin secretagogue when administered in combination with a carbohydrate; increased protein synthesis is facilitated by increased insulin.,
- Leucine is able to extend the postprandial duration of muscle protein synthesis.
- Leucine exerts an antiproteolytic effect.,
- Leucine exerts a thermogenic effect.,
- Leucine augments weight and adipose tissue loss during energy restriction.
- Activation of the mTOR pathway in the brain appears to decrease food intake and body weight.
- Leucine stimulates a rise in plasma leptin. 
- Leucine acts as a direct fuel.
This essential amino acid can also help regulate blood sugar, per the University of Rochester Medical Center. In fact, when leucine was ingested with glucose, it reduced the blood glucose response and strongly stimulated extra insulin production, per a small December 2008 study in Metabolism.
Leucine plays a key role in blood glucose regulation through its effects on pancreatic β cells, liver, muscle and body fat, per a May 2010 review in Nutrition Reviews.
Benefits of Leucine
- Stimulating Muscle Protein Synthesis
- Inhibits Muscle Mass Breakdown
- Aids in Weight loss
- Improved performance
- Promotes muscle recovery