Leucine

Leucine, together with isoleucine and valine form the BCAAs or branched-chain amino acids. Leucine is the most important of the 3 BCAAs thanks to its unique regulating role in the protein synthesis within the muscle cells. (1) Unlike all other amino acids, L-leucine is immediately metabolized in the muscle cells. (2) In addition, l-leucine is capable of stimulating protein synthesis by itself. Smaller doses (2,5g and more) stimulate the synthesis of proteins immediately after your training or after waking up. (3)

Leucine is the key amino acid in preventing muscle breakdown immediately after power and endurance sports. (4)

Human biologists at the university of Maastricht have discovered that adding leucine to a post-workout shake increases the anabolic effect of the shake with 16%. (5) The researchers concluded that "The additional ingestion of free leucine in combination with protein and carbohydrate likely represents an effective strategy to increase muscle anabolism following resistance exercise". Their research also provides proof of principle that it is possible to stimulate muscle growth after a resistance training when combining proteins with extra leucine.

A couple of grams of leucine added to protein rich meals also stimulates the growth of muscular tissue. This is because leucine is capable of transmitting signals: meals or supplements rich in leucine increase insulin secretion stimuleren and stimulates the molecular machine used by the cell to create muscle fibres. Larger quantities of leucine (at least 5g) increase protein synthesis the same way as full proteins do.
 

Leucine

Leucine also is a strategic supplement for senior sportsmen, because their body reacts slower to the anabolic stimulus of protein meals. (6) The ideal strategy for them is to add leucine to protein shakes and meals, because “leucine enriched shakes” stimulate their anabolic processes.

According to recent research, leucine even preserves your muscle mass when you are not training.  (7) Researches from the Brazilian University of São Paolo have discovered that a leucine supplementation slows down the breakdown of muscle mass in non-active muscles: leucine has an anabolic effect on the muscle cells. This is probably because the cell invests more energy in synthesizing the muscle protein as it ‘sees’ more leucine metabolites.

Leucine is used as a key ingredient in SynChronize, SynTsize RecoveryProXtend and BCAA 300 caps.

References:
1) Kimball ,Jefferson, Signaling pathways and molecular mechanisms through which branched-chain amino acids mediate translational control of protein synthesis. J.Nutrition, 2006, Jan
2) Anthony, Lang , Crozier, Mac Lean, Kimball, Jefferson. Contribution of insulin to the translational control of protein synthesis in skeletal muscle by
Leucine
3) Biolo, Tripton, Wolfe & Klein. An abundant Supply of amino acids enhances the metabolic effect of exercise on muscle protein / Volpi? Kobayeshi, Sheffield-moore, Wolfe Mittendorfer. Essential amino acids are primarily responsible for the amino acid stimulation of muscle protein anabolism in healthy elderly adults; Am J. Clin. Nutrition 2003
4) American Society for Nutrition J. Nutr. 136:533S-537S, February 2006)
5) Am J Physiol Endocrinol Metab. 2005 Apr;288(4):E645-53
6) Am J Physiol Endocrinol Metab. 2006 Aug;291(2):E381-7
7) Muscle Nerve. 2010 Jun;41(6):800-8