Supplemental Leucine: Fad or Fitness Fixture?

The science behind leucine and muscle growth.

An Introduction to Leucine

Leucine is an essential amino acid, as it’s one of the few amino acids our body can’t manufacture, so it must be obtained as a component of the protein-rich foods we eat. It’s commonly billed as being vital to building bigger muscles, and is now sold as a standalone supplement. But is it actually vital to muscle growth? Let’s look at what the research says about leucine and see if it’s worth your money.

Setting Parameters

While essential to the big picture of scientific research, studies done on rats don’t predict effects in humans because rodents process amino acids differently than we do, allowing us to ignore the outcomes of those studies. Since most 1R readers are relatively young and healthy, we can also ignore studies done on the elderly and very sick. Finally, since recreational leucine supplementation is about building greater strength and bigger muscles, we can focus on studies that involve weight training, and record changes in subjects’ weight, muscle mass, and/or markers of protein synthesis.

Ideally, we would find long-term studies using protocols that mimic the lifestyles and training of strength-focused competitive and recreational athletes. Unfortunately, the realities of research mean we have to fit various clues together for a complete picture. To help you form your own opinion, I’ll try to reference as many free online resources as I can, though for the sake of accuracy I’ll also use pay sources; if you’re in college (or near one) you can find many of the referenced journals in the library or on the university network through sites like EBSCO.

Leucine Studies

It turns out studies demonstrating that leucine significantly improves muscle protein synthesis in humans fall into a few problem areas. Many were performed on the elderly or on endurance athletes. Getting closer to the target, studies on younger individuals involved in strength training produced mixed results depending on how much protein the subjects ate. How important then is extra leucine to someone who eats a lot of protein or someone who follows an “athlete’s diet?”

One recent study looked at this question directly, and confirmed that leucine ramped-up mTOR, which is a protein responsible for (among other things) starting muscle protein synthesis. More importantly, it revealed that increasing leucine boosted this ramp-up to a certain point, after which additional leucine had no effect. Follow-up work found the ideal ratio of leucine needed to maximize protein synthesis was already in common sources of protein and amino acids, and that eating about 1.6 grams of protein per kilogram of bodyweight everyday provided all the amino acids needed to keep protein synthesis high. Simply put, supplemental leucine is unnecessary for anyone who follows common protein recommendations for athletic performance and physique enhancement.

Why then should we be at all excited about leucine? First, leucine seems to be a trigger for protein synthesis, which is an important facet of understanding how muscles grow. Second, leucine may be useful to lots of people who aren’t 1R readers, in particular the elderly, the ill, and serious endurance athletes.

Wrap-up

Leucine may be essential to muscle growth, but if it is, it’s conditional. As an analogy, we can compare leucine to security guards at a construction site. The guards’ job is to let contractors and members of the construction crew enter the site. If the guards aren’t there to open the main gate, the workers mill around outside and nothing gets done. On the other hand, adding more guards only helps if there are lots of contractors and enough room for them on the site; otherwise, the guards are wasted. Finally, if there are no workers around, the guards can hammer a few nails and push some wheelbarrows, though they won’t be as effective as a full crew.

Leucine, by letting other amino acids into the muscle to repair damage and build up cells, behaves similarly to those security guards. Since most 1R readers get a balanced amino profile from eating lots of whole proteins (or, to reuse the analogy, most 1R readers have an ideal number and ratio of guards and construction workers), supplemental leucine won’t aid muscle growth. Conversely, if your diet is such that supplemental leucine will help, it’s easier and more sensible to focus on getting your protein consumption up to par with the use of other protein supplements that you could pick up in the 1R store.

References:

Aragon. Alan Aragon’s Research Review. 2011 Oct.
Katsanos, et al. A high proportion of leucine is required for optimal stimulation of the rate of muscle protein synthesis by essential amino acids in the elderly. AJP – Endocrinology and Metabolism. 2006 Aug; 291(2) E381-7.
Pasiakos, et al. Leucine-enriched essential amino acid supplementation during moderate steady state exercise enhances postexercise muscle protein synthesis. The American Journal of Clinical Nutrition. 2011 Sep; 94(3):809-18.
Koopman, et al. Combined ingestion of protein and free leucine with carbohydrate increases postexercise muscle protein synthesis in vivo in male subjects. AJP – Endocrinology and Metabolism. 2005 Apr; 288(4):E645-53.
Dreyer, et al. Leucine-enriched essential amino acid and carbohydrate ingestion following resistance exercise enhances mTOR signaling and protein synthesis in human muscle. AJP – Endocrinology and Metabolism. 2008 Feb; 294(2):E392-400.
Stock, et al. The effects of adding leucine to pre and postexercise carbohydrate beverages on acute muscle recovery from resistance training. Journal of Strength and Conditioning Research. 2010 Aug; 24(8):2211-9.
Glynn, et al. Excess Leucine Intake Enhances Muscle Anabolic Signaling but Not Net Protein Anabolism in Young Men and Women. The Journal of Nutrition. 2010 Nov; 140(11): 1970–1976.