Supplement Performance - Leucine
A Superstar of Amino Acids
"A conservative is a man who sits and thinks, mostly sits." —Woodrow Wilson, 28th
president of U.S. (1856-1924)
The role of amino acids as substrates for protein synthesis (i.e., building blocks of
protein) is well established. However, a function for amino acids in modulating the signal
pathways regulating muscle protein anabolism has only recently been described. The
stimulation of protein synthesis in skeletal muscle produced by an intake of a mixed meal is
due largely to branched-chain amino acids (leucine, isoleucine, valine; BCAAs). Of the three
BCAAs, leucine is clearly the most effective. In fact, a recent study at Baylor College of
Medicine suggests that the postprandial (occurring after meal) rise in leucine, but not
isoleucine or valine, acts as a nutrient signal to stimulate muscle protein anabolism. This
article examines the latest findings on BCAAs, especially leucine.
Leucine Supplementation and Exercise Performance
To my knowledge, there’s only one study examining direct ergogenic effects of
leucine supplementation. Dr. Crowe and co-workers investigated the effects of leucine
supplementation on the exercise performance of outrigger canoeists. Thirteen canoeists
underwent testing before and after six weeks of supplementation with either leucine (45
milligrams per kilogram per day) or placebo (fake supplement). The results revealed that
leucine supplementation significantly improved endurance performance and upper body
power in outrigger canoeists. The authors speculated that the ergogenic effects of leucine
were likely related to a reduction in skeletal muscle damage with training and/or an increase
in skeletal muscle protein anabolism.
Indeed, there’s evidence that BCAA supplementation before exercise decreases the
breakdown of muscle proteins during exercise in humans and that leucine strongly promotes
protein anabolism in skeletal muscle. Furthermore, it’s been reported that BCAA
supplementation prior to resistance training decreased delayed-onset muscle soreness and
muscle fatigue occurring for a few days after exercise, suggesting BCAA supplementation
may be useful for muscle recovery following exercise.
Finally, there’s some evidence suggesting that BCAA supplementation may delay
central fatigue during prolonged exercise, but this isn’t relevant to gym rats, so it’s not
discussed here.
Leucine and Treating Obesity
It is now clear that diets with higher protein and reduced carbs are beneficial for
obese individuals. Numerous studies indicate that a higher-protein/lower-carbohydrate diet
increases fat loss and reduces loss of muscle mass. In fact, a recent meta-regression (a
generalization of subgroup analysis that can be used to investigate the heterogeneity of
effects across studies) published in the respected American Journal of Clinical Nutrition
confirmed that high-protein/low-carb diets favorably affect body mass and composition
independent
("a calorie is not a calorie").
It was recently proposed that leucine is a key to the metabolic advantage of a higherprotein
diet because of its unique role in the regulation of muscle protein anabolism, insulin
signaling and glucose recycling via alanine. Leucine appears to regulate the burning of
glucose by skeletal muscle through stimulation of glucose recycling via the glucose-alanine
cycle. These mechanisms produce muscle mass sparing and provide a stable blood sugar
environment during energy-restricted diets.
Although leucine plays an important role in preservation of muscle protein, it’s not the
only factor behind the metabolic advantage of low-carb/high protein diets. Carbohydrate
restriction dramatically lowers insulin levels and insulin signaling in fat tissue plays a very
important role in fat storage. Dr. Matthias Bluher and co-workers at the Harvard Medical
School created mice with fat-specific disruption of the insulin receptor gene. Interestingly,
their study demonstrated that lack of an insulin receptor in fat tissue produces almost
complete protection against obesity. In other words, fat-specific insulin receptor knockout
mice can eat a shitload of food and still hardly gain any fat mass. Thus, it’s clear that insulin
plays an important role in obesity, independent of energy intake.
Safety of Branched Chain Amino Acids
Concerning safety, toxicity studies using animals have shown that BCAAs are safe
amino acids when the three BCAAs are provided in a ratio similar to that of animal protein
(i.e., a 2:1:1 leucine:isoleucine:valine ratio).
Closing Remarks
Supplements that work through a placebo effect, but have no intrinsic effects,
eventually fall by the wayside and are abandoned by the majority ("This stuff sucks, bro.
Don’t waste your money!"). However, BCAAs are used simply because they work. And even
though there have been attempts to discourage it, the popularity hasn’t waned. Gym rats and
fitness babes already know what science is now showing: BCAA supplementation (especially
leucine supplementation) can decrease muscle protein breakdown and boost muscle protein
anabolism. Also, a high intake of leucine spares muscle mass and provides a stable blood
sugar environment during energy restriction.
of energy intake, supporting the proposed metabolic advantage of these diets
