Building muscle: how branched-chain amino acids (BCAAs) shape up

Studies of branched-chain amino acids, or BCAAs, popular with fitness enthusiasts, have concluded they are ineffective

In 2012, the British Medical Journal Open published the findings of a team, led by Dr Carl Heneghan of Oxford University, assessing the evidence supporting 431 sports performance-enhancing claims made by companies for 104 different products.

They concluded: “The current evidence is not of sufficient quality to inform the public about the benefits and harms of sports products. There is a need to improve the quality and reporting of research . . . ”

This is especially relevant in relation to sports nutrition products for human consumption. One concern appeared in a UK report entitled, Bodybuilding supplementation and tooth decay, published in the British Dental Journal in 2015. It states that bodybuilding supplements can contain high amounts of sugar: "Supplement users are consuming these products, while not being aware of their high sugar content, putting them at a higher risk of developing dental caries."

The global sports nutrition market is worth billions of dollars, and according to, in Ireland – with more people undertaking weight training, CrossFit and high-intensity exercise – "sports protein powder accounted for half of retail value sales in sports nutrition in 2017".


Of the 20 amino acids which build proteins, nine are “essential” amino acids (EAA), meaning they can’t be made by the body but only consumed through diet. Three of these nine EAAs are the so-called branched-chain amino acids (BCAA) – leucine, isoleucine and valine.

BCAAs are popular with fitness enthusiasts, especially weight trainers and bodybuilders, because they are widely claimed to stimulate muscle protein synthesis.

Is this claim supported by evidence?

In 2017, in a freely available online review in the Journal of the International Society of Sports Nutrition, entitled, Branched-chain amino acids and muscle protein synthesis in humans: myth or reality?, Dr Robert R Wolfe of the University of Arkansas notes that all nine EAAs are needed to produce muscle protein. Since EAAs cannot be made by the body, then consuming only three EAAs in the form of BCAAs means that the only source of the remaining EAAs derives from the breakdown of muscle protein.

Wolfe also cites two studies published in the 1990s which are the only investigations in humans to assess the response of muscle protein when BCAAs alone are administered. He is clear: “We can conclude from these two studies that BCAA infusion not only fails to increase the rate of muscle protein synthesis in human subjects, but actually reduces the rate of muscle protein synthesis and the rate of muscle protein turnover.”

Muscle-boosting claims

So why do BCAA promoters ascribe muscle-boosting claims to their products? One possible reason may arise from a study published in the Journal of Nutrition in 2006 called Branched-chain amino acids activate key enzymes in protein synthesis after physical exercise. However, Wolfe states that even if these key enzymes are activated, they "can only coincide with increased muscle protein synthesis if there are ample EAAs to provide the necessary precursors to produce complete protein".

Wolfe’s overall verdict: “When all evidence and theory is considered together, it is reasonable to conclude that there is no credible evidence that ingestion of a dietary supplement of BCAAs alone results in a physiologically-significant stimulation of muscle protein. In fact, available evidence indicates that BCAAs actually decrease muscle protein synthesis.”

Are there any circumstances in which BCAAs might confer benefit?

In a study in the Journal of the International Society of Sports Nutrition, a team led by Professor Glyn Howatson of Northumbria University, England, published a paper with the long title of: "Exercise-induced muscle damage is reduced in resistance-trained males by branched-chain amino acids: a randomized, double-blind, placebo controlled study. Prof Howatson's team recruited 12 male volunteers (mean age: 23 years) who were familiar with resistance training. They undertook a series of muscle-damaging exercises, such as drop-jumping, under controlled conditions over a 12-day period. Participants received either BCAAs or placebo. The study concluded that: "BCAA administered before and following damaging resistance exercise reduces indices of muscle damage and accelerates recovery in resistance-trained males."

So, can these results be usefully applied by recreational exercisers?

Prof Howatson – applied physiologist and director of research at Northumbria University – is particularly interested in understanding exercise stress, recovery and optimising adaptation from training. He told The Irish Times that "with a well-balanced diet, there should be little or no need for additional protein or, for that matter, BCAAs". However, he added, "in our study we exposed volunteers to very strenuous resistance exercise designed to induce muscle damage. This induces micro tears resulting in reduced muscle function and high levels of soreness that can persist for several days. In this scenario, we believe the demand for BCAA is increased, so providing a greater availability through diet can help repair the damaged muscle at an accelerated rate."


Yet, Howatson is for the most-part unconvinced that additional protein, BCAA or otherwise, is entirely necessary. “A good diet is probably sufficient even during periods of intensified training, but there might be a time and a place when additional dietary protein might be warranted. For example, when entering a new phase of training, such as hypertrophy – resistance training designed for muscle growth and increasing cross sectional area – additional dietary protein could be needed.”

However, Howatson emphasises that in his study the exercise stressor was particularly tough and a somewhat extreme model of when BCAAs might be beneficial.

So current evidence suggests that, for muscle growth, BCAA consumption is unnecessary.

Rather, a healthy diet is best.