The dietary supplement industry’s economic impact is in the billions of dollars annually and continually growing. One of the most popular dietary supplements on the market are BCAA supplements. Marketed as beneficial for building and maintaining muscle mass, and for improving exercise performance, they can be found as pure BCAA supplements and in the form of BCAA-enriched products and in energy drinks.
The purpose of this article is to give a thorough overview of the function of BCAAs, and the scientific evidence supporting the use of BCAAs by athletes in general and bodybuilders and strength athletes in particular.
What are BCAA Supplements?
Muscle protein is composed of 20 different amino acids, of which nine are essential (EAA or Essential Amino Acids). Essential amino acids, or indispensable amino acids, are amino acids that cannot be synthesized in the body in significant amounts, but have to be provided through the diet. Three of these essential amino acids are referred to as branched-chain amino acids, or BCAAs, because of their molecular structure.
Unlike the other amino acids, BCAAs are not metabolized in the liver.1 Instead, they are mostly metabolized in the musculature, which makes them a readily available energy source for muscular exercise and able to influence the production of neurotransmitters. The most common reasons for using a BCAA supplement are to stimulate muscle protein synthesis, reduce muscle protein breakdown, and to promote gains in muscle mass when combined with a resistance training protocol.
A common misconception is that EAAs and BCAAs are two different things, but this is not the case. All three of the BCAAs are also essential, EAAs, and cannot be synthesized by the body.
Another common misconception is the notion that BCAA supplements are calorie free. The manufacturers of BCAA products can claim 0 calories on the supplement facts panel through a loophole in the FDA regulations. The FDA regulations for nutrition labeling of dietary supplements state, “protein shall not be declared on labels of products that… contain only individual amino acids”.2 Since a BCAA supplement is not an intact protein the manufacturers can claim that the product contains no energy. This claim is not physiological fact, however, since amino acids are what provides the energy content of protein. The three branched-chain amino acids, leucine in particular, are in fact the most energy-dense of the amino acids. BCAAs provide more than 500 calories per 100 grams, more than sugar or any intact protein.3
BCAAs also stimulate insulin release, especially when combined with carbohydrate. Combining leucine with glucose results in an insulin response almost twice the magnitude of an intake of glucose only.4
BCAAs and Muscle Protein Synthesis
Branched-chain amino acids have long been known to have a stimulatory effect on muscle protein synthesis. Early animal and cell culture studies lay the groundwork that has since been reproduced in numerous human studies.5 When plasma leucine levels increase after the intake of BCAAs, either in the form of protein-rich food or in the form of a protein shake or a BCAA supplement, muscle protein synthesis also increases. It is possible that valine and isoleucine might also be involved in this process, but this is currently unknown.
To produce a robust muscle protein synthetic response, at least 3 grams of leucine has to be consumed in a bolus intake, a mechanism referred to as the “leucine threshold”.6
Smaller intakes of leucine do stimulate muscle protein synthesis, but less so, in a dose-dependent manner. The source of the leucine, if it comes from food or from a supplement, is of minor importance.
Branched-chain amino acids stimulate muscle protein synthesis by activating mTOR-signaling.7 mTOR, or mammalian target of rapamycin, is a protein kinase that regulates protein synthesis in response to nutrient intake and growth factors like physical stress. mTOR nucleates two different complexes, mTORC1 and mTORC2.
Resistance training has profound effects on mTOR-signaling, mainly through activation of an enzyme called p70S6-kinase. Both resistance training and BCAAs independently activate this enzyme, but can also act in synergy when BCAAs are consumed in conjunction with resistance training, leading to a greater combined effect than the sum of the parts.8
Leucine appears to be the primary amino acid that stimulates muscle protein synthesis by activating the mTOR signaling pathway. Removing leucine from an EAA-complex significantly reduces the muscle protein synthetic response.9 The stimulation of protein synthesis is not eliminated, however, indicating that leucine is not necessary to induce at least some anabolic response, although a minor one compared to when leucine is included.
Even though resistance training itself activates signaling mechanisms that control muscle protein synthesis, these effects are enhanced when BCAAs, regardless of source, are consumed following a training session.
BCAAs vs a Complete EAA-supplement or Whey Protein during Resistance Training
A Swedish study from 2016 demonstrated that consuming a solution containing all nine essential amino acids activates mTOR and stimulates protein synthesis more effectively than BCAAs alone.10 The participants in the study were given placebo, leucine, BCAAs or a complete EAA-complex following a resistance training session. The EAA solution activated the mTORC1-complex more than the BCAA supplement, which in turn was more effective than just leucine. The effectiveness of the EAA supplement was mostly potentiated by the branched-chain amino acids, but the inclusion of the other essential amino acids apparently enhanced the effect.
In another recent study, BCAAs activated the signaling pathways for muscle protein synthesis, but only 22 % more effectively than placebo and 50 % less effectively than previously shown following whey protein intake in doses providing equivalent amounts of BCAA.11
The scientific evidence indicates that BCAAs alone do stimulate muscle protein synthesis, but less effectively than a supplement containing all essential amino acids and even less effectively than an intact protein like whey protein.
Do BCAAs Produce Actual Increases in Muscle Mass?
A stimulation of muscle protein synthesis does not actually mean that new muscle protein is synthesized in practice or that amino acids are incorporated into new muscle protein. It only means the activation of signaling pathways for protein synthesis. For actual synthesis of muscle protein to occur, a few preconditions must be met.
All 20 amino acids have to be available in sufficient amounts for muscle protein to be synthesized. Only the nine essential amino acids have to be provided exogenously, through dietary means. The other 11, non-essential, amino acids can be synthesized in sufficient amounts in the body.
If one or more of the essential amino acids are available in insufficient amounts, this will limit the amount of muscle protein synthesized. Non-essential amino acids provided in less than optimal amounts through dietary means can be compensated by endogenous production of these amino acids. That compensation through synthesis of amino acids in the body is not possible if the limiting amino acid is essential.
In the post-prandial state, when all essential amino acids are available in sufficient amounts, muscle protein balance is positive, meaning muscle protein synthesis exceeds muscle protein breakdown. Precursors for protein synthesis are available both in the form of elevated plasma concentrations of amino acids from food and from amino acids released during protein breakdown. This surplus of amino acids leads to increased protein synthesis and a positive muscle protein balance.
In the fasted state, the muscle protein balance is negative and the amino acid concentrations in the blood fall. Amino acids release from the muscle and into the blood and muscle protein synthesis falls, resulting in a catabolic state.
The only source of amino acid precursors for muscle protein synthesis in the fasted state is muscle protein breakdown. Since muscle protein synthesis requires an abundant supply of essential amino acids, an intake of BCAAs in the fasted state can only result in a more rapid incorporation of amino acids from muscle protein breakdown into new muscle tissue at best. This means that BCAA supplementation in the fasted state cannot result in a positive muscle protein balance, with muscle protein synthesis exceeding muscle protein breakdown, since all that is happening is that broken down muscle protein is being recycled. Far from 100% of the amino acids released during muscle protein breakdown are reutilized in this manner.12
The most recognized piece of research showing actual muscular hypertrophy from BCAA supplementation is probably a study presented at a conference in 2009.13 This is a study financed by a manufacturer of dietary supplements, which shows that BCAA supplementation is superior to whey protein during an eight-week long resistance-training period. The BCAA group gained 4 lbs. more fat free mass than the whey protein group during these weeks. Those results in such short amount of time are similar to those possible from high dose anabolic androgenic steroid abuse, and they feel quite improbable.
This study was never published for some unknown reason, and is still only available in the form of the original poster presentation. This makes it inadmissible as any kind of scientific evidence, but this fact has not prevented BCAA manufacturers from using it extensively in marketing.
There are no other long-term studies comparing BCAA to intact proteins where actual hypertrophy has been the subject of the research. A 2011 study gave 26 untrained men 4 grams of leucine in addition to their usual diet during 12 weeks of resistance training.14 The leucine group gained significantly more fat free mass and strength. The problems with these results are the facts that the diet of the participants was quite low in protein, and that there was no group given the same amount of extra leucine in the form of added protein. Even if the leucine supplement was effective, it was only effective in the context of a low protein diet.
The Effects of Leucine or BCAAs Combined with Intact Protein
Several studies have examined if adding leucine of BCAAs to a protein-rich meal enhances the anabolic potential of said meal.
When consuming protein providing all essential amino acids in quantities large enough to stimulate muscle protein synthesis, adding more leucine does not provide additional benefits.15
If the amount of protein in a single meal provides too little leucine for a robust effect on muscle protein synthesis, the meal can be made more anabolic by adding extra leucine. One study demonstrated that adding leucine to 6 grams of whey protein was as effective as a larger dose of 25 grams of whey protein in stimulating muscle protein synthesis.16 However, only the large dose of intact whey protein was able to sustain this increased rate of muscle protein synthesis.
Adding all three branched-chain amino acids to a suboptimal amount of protein does not result in the same additive effects as leucine alone.17 This is probably the result of antagonism between the three added amino acids, with valine and isoleucine competing with leucine for absorption from the gut, resulting in a less prominent increase in plasma amino acid concentrations and reduced activation of signaling pathways related to protein synthesis.
BCAAs and Muscle Protein Breakdown
In normal, untrained adults in energy balance, any gains in muscle mass in the fed state balance the loss of muscle mass in the fasted state. Muscle protein is synthesized and broken down continuously, in a turnover of several hundred grams of protein per day. New muscle protein is synthesized while old proteins are broken down and catabolized.
Adding resistance training to the equation increases muscle protein synthesis to the point where it exceeds muscle protein breakdown, resulting in a positive 24-hour muscle protein balance and, over time, significant hypertrophy. A high protein intake can further enhance this anabolic response.
Both muscle protein synthesis and muscle protein breakdown actively influence the control of human muscle mass. The primary driver of muscular anabolism is protein synthesis, although muscle protein breakdown certainly plays a role in the adaptive responses to exercise and nutrition.
Decades of research have shown that branched-chain amino acids reduce both whole body and muscle protein breakdown.18 However, if BCAA supplementation also affects the elevated rate of muscle protein breakdown following resistance training is currently unknown. Regardless of the fact that it has not been investigated if oral BCAA intake during or following a training session impacts muscle protein breakdown, BCAA manufacturers often claim anti-catabolic effects of BCAAs following exercise as a main sales pitch.
It is currently unclear if it is even desirable to reduce muscle protein breakdown following resistance training through targeted nutritional interventions. Recent research has indicated that it might even be counterproductive to do so.19 An effective protein synthesis process might be dependent on an effective muscle protein breakdown. The increase in the rate of muscle protein breakdown following a training session might represent a cleanup process resulting in the removal of old, damaged muscle, and the re-utilization of amino acids to synthesize new muscle protein.
A major source of amino acids for muscle protein synthesis is broken-down muscle protein. Only about 10% of the amino acids from dietary protein ends up as muscle protein.20 Excessively reducing the natural muscle protein breakdown following resistance training might reduce the amount of amino acids available for synthesis of new muscle protein.
There are currently no practical methods to directly measure muscle protein breakdown during exercise to identify exactly from where the amino acids originated, whether from muscle of from other fat free tissue. There is no evidence that BCAA supplementation reduces exercise-induced muscle protein breakdown, or if it is even beneficial to do so.
BCAAs during Periods of Energy Restriction
Given the purported effects of branched-chain amino acids on muscle protein synthesis and muscle protein breakdown, it would seem prudent to supplement with BCAAs during periods of energy restriction, for example during the cutting phase of a bodybuilding diet, to minimize or eliminate the potential loss of muscle mass.
In a recent study, 17 young, resistance-trained men received either BCAAs or placebo in the form of carbohydrate during 8 weeks of hypocaloric strength training.21 The authors of this study, financed by a manufacturer of BCAAs, concluded that strength-training individuals could minimize the loss of lean mass and performance while losing body fat during periods of energy restriction through BCAA supplementation. They also claimed that the BCAA group lost significant amounts of body fat, while the carbohydrate group did not.
Looking more closely at the results of the study, they show that the carbohydrate group lost an average of 1.4 kilograms of body fat, while the BCAA group lost only 0.6 kilograms of body fat.
The participants in the BCAA group did lose less lean mass than the carbohydrate group. Given the dubious reporting of the differences in fat loss, this might be the result of losing almost no body fat rather than the BCAA supplement itself. The loss of fat free mass reported in the carbohydrate group could be the result of a faster weight loss, not the lack of BCAA supplementation.
Other scientists also pointed these discrepancies out in a review article the year following the publication of the original study.22
The participants in the BCAA group reduced their resting metabolic rate by an average of 400 kcal per day. The carbohydrate group not exhibit any such effects. This astounding and unexplainable drop in resting metabolic rate only adds to the uncertainties surrounding this study.
An earlier study showed that elite wrestlers lost more body fat when they supplemented a hypocaloric, high-protein diet with large amounts of BCAAs compared to the same amount of soy protein.23
BCAAs and Delayed Onset Muscle Soreness (DOMS)
The mechanisms of delayed onset muscle soreness are not fully understood, although there are a number of proposed theories. The most prevalent of these is that the pain originates from microscopic injures to the muscle fibers, leading to inflammation and mobilization of neutrophils following mainly eccentric training.
Several studies have demonstrated that supplementation with branched-chain amino acids can reduce DOMS. A recent meta-analysis lends support to the results of these individual studies and concludes that BCAA supplements significantly reduce delayed onset muscle soreness following exercise.24
Exactly how BCAAs reduce DOMS is unknown, but the activation of mTOR mediated by leucine resulting in enhanced recovery and reduced muscle pain following training is a potential mechanism. Other possible mechanisms include reduced damage to the sarcolemma, the cell membrane of a muscle fiber cell, hormonal effects, and a decreased inflammatory response to training.
Even if BCAA ingestion reduces the perceived levels of muscular pain, actual physical performance does not seem to be affected, as long as an adequate amount of dietary protein is consumed.25
BCAAs, Mental Focus, and Fatigue
High-intensity or long-duration training sessions increase brain serotonin activity and decrease dopamine activity, causing the central nervous system to activate protective mechanisms leading to fatigue, preventing extreme physical exertion to cause bodily harm.26
During exercise, branched-chain amino acids oxidize in skeletal muscle, decreasing plasma BCAA concentrations. When plasma BCAAs decrease, brain tryptophan levels increase, leading to increased brain serotonin synthesis.27 This contributes to feelings of fatigue during training.
BCAAs and Immune Function
Training sessions of long duration or high intensity lower plasma glutamine levels, which is associated with decreased immune function. Branched-chain amino acid supplementation can prevent this decrease in plasma glutamine, even in elite athletes competing in Olympic triathlons, although the exact mechanisms are still not fully understood.32 This protective effect on plasma glutamine levels following exercise after the intake of BCAAs do not seem to occur to the same extent by a glutamine supplement.
A decrease of up to 30% in upper respiratory infection frequency has been observed in athletes receiving BCAAs the week following an exhausting competition. However, it is unknown if the same amount of extra BCAAs from an increase in dietary protein would have resulted in the same effect.
Another study demonstrated that BCAA supplementation eliminated the expected decrease in plasma glutamine levels in elite runners following a race. This also affected cytokine production, leading to more efficient immune function.33
It is unknown if this immunomodulation by BCAA supplementation also leads to a lower risk of being infected by a virus or developing an upper respiratory infection, compared to a diet providing the same total amount of BCAAs. However, available evidence indicates that BCAAs have a positive effect on immune function.
Rodent studies have not demonstrated any toxic effects from long-term BCAA administration, either from 2.5 grams per kilogram of bodyweight per day over the course of three months or from 1.25 grams per kilogram of bodyweight per day over the course of an entire year.34
One rat study showed that treatment with leucine in amounts far larger than humans are normally exposed to do not affect the outcome of pregnancy or cause fetotoxicity in the offspring.35
A human study showed that a prolonged intake of more than 550 mg of leucine per kilogram of bodyweight per day increased ammonia levels in the blood, possibly indicating potential negative health effects.36
Some recent research has shown an association between high intakes of BCAAs and negative health effects. A new rat study showed increased rates of obesity, metabolic disturbances and mortality in rats administered feed containing twice as much BCAAs as normal.37 These rat studies cannot be determined to be relevant to human health without further research.
There is little doubt that both dietary branched-chain amino acids and BCAAs in supplement form activate signaling pathways for muscle protein synthesis, primarily through elevated plasma leucine concentrations. For this activation to produce results in the form of actual muscle protein, to incorporate amino acids into new muscle protein, an abundant availability of all essential amino acids is required. Otherwise, not much of anything happens. Even the activation of the signaling pathways for protein synthesis are halved if only the branched-chain amino acids are provided compared to an intact protein like whey protein.
BCAA ingestion effectively reduces muscle protein breakdown, at least during resting conditions. No studies have demonstrated such a reduction of muscle protein breakdown following exercise. Suppression of muscle protein breakdown following resistance training might not even be desirable, if the goal of the training is muscular hypertrophy. Such interventions might instead reduce the available building blocks for new muscle tissue, thus reducing the hypertrophic potential of the training session.
BCAA supplementation enhances mental focus during prolonged exercise and seem to reduce the perceived sense of effort during exertion. These effects seem to be restricted to the feeling of effort, not actually improving physical performance.
To optimize the hypertrophic potential of resistance training, available evidence indicate that it is more effective to satisfy protein requirements, and therefore also BCAA requirements, through a protein-rich diet rather than through BCAA supplementation.
Branched-chain amino acids are essential for life, health, performance and training adaptations. However, it is likely superior to supply them through dietary means rather than consuming them in isolated form.
Summary and Take-home Message
The hypertrophic potential of BCAA can be summarized in the following three points:
- BCAA supplementation in the fasted state stimulates muscle protein synthesis, but actual increases in muscle mass cannot happen without the provision of the other essential amino acids.
- BCAA supplementation in combination with a protein source providing enough leucine to stimulate muscle protein synthesis does not offer any advantages.
- BCAA supplementation in combination with a protein source that does not provide enough leucine does not offer any advantages. Leucine supplementation without the other two BCAA, however, does.
BCAAs can be compared with LEGO. If you buy a LEGO set with nine pieces, using only three of those pieces during the construction will make the finished model look very incomplete compared to the model on the box.
That’s it! You’ve reached the end of our guide on BCAA.
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