In a recent study, subjects did resistance training for 45 minutes. After, one group of them supplemented with carbohydrates, another group with carbohydrates and protein, and the last group supplemented with carbs, protein and leucine.
Researchers found that the carbohydrate/protein/leucine supplement group reduced muscle protein breakdown and increased muscle protein synthesis to a greater degree than the carbohydrate/protein group, and to a much greater degree than the carbohydrate-only group9. Thus, more leucine might equal more muscle when added to protein and carbs!
As a free-form supplement, leucine gets absorbed into circulation much faster than when it’s part of a whole protein. Even with a fast-digesting protein such as whey, it can take hours for the leucine to be liberated from the protein and enter circulation. Therefore, leucine concentrations in the blood never spike to high levels. An isolated leucine supplement, however, would be quickly absorbed into circulation, thus spiking blood leucine levels and drastically increasing intracellular leucine concentrations and activating anabolic pathways.
For those who are dieting but want to maintain maximum muscle mass, a leucine supplement may help alleviate some of the catabolic effects of a calorie-deficit diet. Leucine is more anabolic than other amino acids. While this hasn’t been studied in detail, it might be possible to substitute something like 5g of leucine for 30-40g of dietary protein and still have the same or greater stimulatory effect on muscle protein synthesis.
Again, this is a hunch of mine based on current research and my own PhD work. With a leucine supplement, you may be able to maintain high levels of protein synthesis and knock an extra 100 calories off your diet. Can you say, “shredded?” (You don’t have to say it. Just be it.)