Leucine
syn. L-leucine
syn. 2-Amino-4-methylpentanoic acid
PERFORMANCE VALIDATION SCORES
Combined 9
Clinical Support Rating 5
Empirical Evidence Rating 4
Description
Leucine is an amino acid, one of eight that are considered essential components of the diet. Leucine is also one of three branched-chain amino acids (BCAA). These amino acids are highly prominent in skeletal muscle tissue, with leucine accounting for about 8% of the total muscle protein content.** BCAAs are structurally and functionally distinct from other amino acids, and are involved in a variety of biological processes, including skeletal muscle energy and protein metabolism. Leucine stands out from other branched-chain amino acids as the most active at stimulating protein synthesis within muscle cells. Given this role, leucine supplements are widely used in sports nutrition to enhance muscle growth and recovery following intense exercise.
Exactly how muscle cells sense and respond to
leucine is not fully understood. We do know that this amino acid stimulates the mammalian target of rapamycin (mTOR) signaling pathway, which is necessary for the initiation of muscle protein synthesis.
Leucine also seems to work synergistically with insulin to help stimulate protein synthesis and nutrient uptake within muscle cells after intense exercise. It specifically seems to intensify the signal intensity of phosphatidylinositol-3 kinase (PI3K), which is triggered by insulin…
Leucine may also be metabolized
for fuel when needed. Oxidation of this amino acid is increased significantly during exercise, in some direct correlation with whole-body oxygen consumption and energy demands.
While studies may be conflicting, leucine appears to be a strong supporter of muscle energy and anabolic processes. Under some circumstances, its supplementation may impart measurable muscle-building or performance-enhancing effects. It is of note, however, that some of the cellular anabolic pathways responsible for muscle growth appear to be better stimulated
by all three branched-chain amino acids (leucine,
isoleucine, and valine) compared to leucine alone. Complete BCAA supplementation has also been more extensively studied, and its use is fairly well supported in the medical literature. While leucine may indeed have beneficial effects in exercising humans, most bodybuilders and athletes prefer complete BCAA supplementation to pure leucine products.
Promoted Benefits
Leucine is promoted to increase muscle mass and strength, and improve athletic performance.
Clinical Studies
This ingredient has been shown to improve performance in placebo-controlled studies with trained adults.
Its Clinical Support Rating is 5 (5/5).
One placebo-controlled study examined the effects of leucine supplementation in a group of competitive outrigger canoeists. Subjects took a dosage of 45mg/kg of bodyweight per day, which equates to slightly less than 4 grams for a person of about 185lbs in weight. This level of supplementation was continued for a period of six weeks. Exercise evaluations included upper body power testing with a row to exhaustion at 70-75% maximal oxygen consumption. In
this study, subjects taking leucine noticed statistically significant improvements in upper body power and exercise endurance compared to those taking placebo.
Rowing time to exhaustion was increased from 77.6min to 88.3 minutes, and perceived exertion decreased
With regard to protein kinetics, another placebo-controlled study examined the effects of the branched-chain amino acid leucine on protein synthesis and retention following resistance training. The leucine was added to a whey protein/carbohydrate recovery drink. The study also examined the same protein/carbohydrate drink without leucine, as well as a carbohydrate only drink. The exercise protocols consisted of
eight sets each of leg press and leg extensions, which were performed at approximately 80% 1-rep maximum weight for approximately eight reps each. Subjects consumed the supplemental drink every 30 minutes after training, for a total of 6 hours. Measures of protein kinetics were taken at various points over this six-hour window. The leucine-enriched drink resulted in the greatest overall retention of muscle protein.
Studies with leucine have had conflicting results, with some failing to find statistically significant improvements in exercise performance following supplementation. 3.454,45 Whether these inconsistent results are due to individual factors, study methodological difficulties, or specific circumstances in which leucine can
exert tangible ergogenic effects remains unclear. Further research is needed to better understand the potential role of leucine in improving exercise
performance.
Empirical Evidence
Leucine is widely available as a stand-alone nutri-
tional supplement. The feedback on leucine as an ergogenic aid tends to be positive. Leucine supplements are most commonly used with bodybuilders and power athletes looking to support muscle anabolism. In many cases, the user can attribute a visibly greater
level of muscle gain when adding leucine to their regular post-exercise or during-training support drinks. It is of note, however, that many find the supplementation of all three branched-chain amino acids to be superior to leucine use alone. While both seem to provide tangible benefits for a majority of users, the results from complete BCAA supplementation tend to be better. A minority of users fail to notice any
benefits with leucine supplementation (or BCAA supplementation for that matter). It may be that in some circumstances branched-chain amino acid manipulation alone will not provide enough added stimuli for sufficient muscle or performance improvements beyond that provided by optimal training and diet. Overall, leucine has an Empirical Evidence Rating of 4
(4/5).
Effective Dosage
Based on clinical studies, a dosage of 3-10 g per day is recommended.
Side Effects / Safety
Leucine was well tolerated during clinical studies,
with no significant side effects reported. Higher doses may cause gastrointestinal distress.