What role does leucine (a BCAA) play in building muscle?

The essential amino acid leucine continues to be the focus of research, especially in regards to examining the effects of adding supplemental leucine to meals and protein drinks. Whey protein is naturally high in leucine making up about 10% of the amino acids. Thus, a 20 gram serving of whey protein delivers about 2 grams of leucine. While this is an effective dose of leucine, current findings suggest it may not be optimal.

In one study, adults at least 60 years of age consumed 20 grams of whey protein with a small amount of added leucine (3 grams total) or a control milk product matched for calories. They performed a bout of resistance exercise on one leg 15 minutes before ingesting the drinks so they could measure rates of muscle protein synthesis just due to the drink (by examining the rested leg) and in response to exercise (by examining the exercised leg).

They found that consuming the leucine-enriched whey drink resulted in a 47% increase in muscle protein synthesis, whereas the milk drink had a much smaller impact (10%). The increase in muscle protein synthesis in the exercised leg was also higher in the leucine-enriched whey drink (28%) versus the milk drink (3%). These results support the use of whey protein with added leucine as superior to a traditional milk product for stimulating muscle protein synthesis in healthy older subjects.

In a different study, researchers examined whether adding leucine to a drink low in total protein was beneficial. Active men were randomly assigned to ingest one of five whey protein isolate supplements after resistance exercise:
• 25g whey (3g leucine)
• 6.25g whey (0.75g leucine)
• 6.25g whey + extra leucine (3g leucine total)
• 6.25g whey + extra leucine (5g leucine total)
• 6.25g whey + extra BCAAs (5g leucine total) 
They all drank a 241 calorie beverage with 5.6g of fat from coconut oil and 23g of carbohydrate (sucrose) for the whey only group, and 35g of carbohydrate for the other groups to match energy levels across all groups. During the first 1.5 hours after ingestion, all groups experienced similar, significant increases in muscle protein synthesis. From 1.5 to 4.5 hours after ingestion, the group who received 25g of whey experienced the highest rate of muscle protein synthesis (~267%). Interestingly, the group who received 6.25g of whey protein + extra leucine (5g leucine) also experienced a high level of muscle protein synthesis (~220%) despite containing about half the amount of total amino acids. The other groups came in a close second.
The results provide evidence that adding extra leucine to low (suboptimal) protein meals can significantly improve the anabolic effect on muscle. Practically, this means if you want to consume one-fourth the amount of whey and add a teaspoon of extra leucine powder you can get the same benefit as consuming a full 25 grams of protein from whey. 

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