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Branched Chain Amino Acids (BCAAs): Are they worth the hype?

After whey protein, BCAAs are perhaps the most popular supplement for gym bunnies and bodybuilders – based on the claim that they enhance muscle recovery and hypertrophy, particularly when training in a fasted state. But is there the science to back up these gain claims, or are we throwing our hard earned cash down the drain when we pick these up off the shelf?

What are they?

The branched chain amino acids are leucine, isoleucine and valine. They ‘essential’ amino acids, which means we must obtain them in our diet as we cannot make them in our body.

What do they do?

There are two key areas we focus on when talking about BCAAs and exercise: Muscle and Fatigue.


As amino acids, BCAAs are of course a building block for protein and therefore muscle protein. In fact they are thought to comprise up to 35% of muscle protein (see Wolfe RR 2017 and references therein). Whilst they are critical components of muscle protein, the question is whether specific BCAA supplementation can support additional muscle hypertrophy. The strongest evidence for muscle hypertrophy in response to BCAAs are studies showing that an essential amino acid plus carbohydrate mix immediately post-resistance exercise spike amino acid uptake into the trained muscles (see Wolfe RR 2017 and references therein). However, amino acid uptake is not the same as muscle protein synthesis. Moreover, such studies were rarely restricted to BCAA supplementation alone; BCAAs were typically given with a carbohydrate (sugar) dose and sometimes even as part of a complete essential amino acid dose. My suspicion is that this indicates they did not see a response without these given in parallel – because if there was an effect without these they would not have been given, as it would have been cleaner experimental conditions.

Leucine is a direct activator of the signalling pathway required for muscle hypertrophy (mTOR) and appears required for maximal mTOR activation (See Ben-Sahra and Manning2017; see Sengupta et al 2010). As such it has been suggested that this is one reason supplementation with BCAAs will support maximal muscle hypertrophy, particularly for those in a fasted state where the pathway may not be otherwise maximally activated and some studies appear to support this. However a closer look indicates the evidence for this is not robust (see Dieter et al 2016). The catch here is that leucine alone is insufficient for maximal mTOR activation – nutrients and growth factors must also be present at sufficient concentrations (See Dieter et al 2016; see Sengupta et al 2010). This indicates that whilst important, leucine supplementation alone may not be sufficient to drive hypertrophy.


This is an interesting one! Tryptophan is the precursor for serotonin, which is thought to promote central nervous system fatigue in exercise. BCAAs and tryptophan are taken up into the brain via the same transporter. During exercise, the level of circulating BCAAs typically falls as they are taken up by muscles. In theory this could increase the tryptophan entering the brain and accelerate the feeling of fatigue (See Meeusen et al 2006). As such, it has been proposed that supplementing with BCAAs pre-exercise and therefore maintaining levels in circulation can reduce tryptophan uptake and therefore delay fatigue. Some studies in rodents support this hypothesis (see Cordeiro et al 2017; Falavigna et al 2012; see Meeusen et al 2006), however others do not (see Cordeiro et al 2017; see Meeusen et al 2006; Pitkanen et al 2003; see Davis et al 2000). It is worth noting that it has been suggested that there is a dose dependent effect whereby BCAAs enhance performance at low doses but above a threshold impair performance as a result of being used to produce glucose (gluconeogenesis) to sustain exercise and the resulting build up of toxic ammonia (Falavigna et al 2012). As such, it seems the jury is still out on this one!

How do we get BCAAs?

All three BCAAs are found at high levels in all animal protein sources. Whey protein is very high in each of these – in fact, BCAAs comprise 25% of the protein in whey, meaning that there are ~5g of them in 1 scoop of whey protein supplement! You can obtain a similar amount from 225g cottage cheese. Plant protein sources typically lack or are very low in at least one of the BCAAs, and so vegetarians must ensure they eat a variety of plant protein sources to consume a complete (all essential amino acids) source; combination of legumes and wholegrains will achieve this.

Should I supplement with BCAAs?

If you are eating sufficient complete protein, I would say don’t bother! And if you aren’t eating sufficient complete protein, then eat a complete protein source rather than paying money for yet another processed supplement!

The evidence indicating they drive muscle hypertrophy over and above a complete protein source is weak, and the evidence for a role in preventing fatigue is inconsistent. I would also worry that overloading with BCAAs such that the ratio of these to other essential amino acids in the body is ludicrously high could be detrimental to long term muscle hypertrophy, particularly for those eating in a heavy calorie deficit. I would worry that increasing BCAA concentration to over and above that of other amino acids that are taken up into muscles by the same transporter may mean you do not get the full complement of amino acids in the muscle and thus cannot synthesise complete proteins effectively and / or that the other amino acids become rate limiting for muscle protein synthesis. However I would highlight this is just my own musing and, as far as I am aware, has not been addressed in rigorous scientific studies. What is known, however, is that in the absence of sufficient nutrients the growth pathways are not fully activated, even in the presence of the BCAA leucine (See Ben-Sahra and Manning 2017; see Sengupta et al 2010).

There is robust scientific evidence to show that taking a complete protein source such as whey before or soon after resistance training optimises muscle hypertrophy, and so this is what I do (Tipton et al 2007 and other studies referenced therein). The theoretical time window where muscle protein synthesis is optimised post training, i.e. the so-called ‘window of gains’ is a topic for another time; there is convincing evidence it exists, but the duration of it is going to differ on physiological state, exercise type and gender etc etc etc.

As for fatigue, there is no simple quick fix – training, recovery, mindset, micronutrient availability and caloric availability all play into this. The successful athlete must address and optimise each of these and, in this case, I don’t think this supplement is a worthy support for any of these.


Ben-Sahra, I, Manning, BD. mTORC1 signaling and the metabolic control of cell growth. Current Opinion in Cell Biology. 46:72-82. 2017.

Cordeiro, LMS, Rabelo, PCR, Moraes, MM, Teixeira-Coelho, F, Coimbra, CC, Wanner, SP, Soares, DD. Physical exercise-induced fatigue: the role of serotonergic and dopaminergic systems. Brazilian Journal of Medical and Biological Research. 50(12):414-431. 2017.

Davis, JM, Alderson, NL, Welsh, RS. Serotonin and central nervous system fatigue: nutritional considerations. American Journal of Clinical Nutrition. 72:573S-8S. 2000.

Dieter, BP, Schoenfeld, BJ, Aragaon, A. The data do not seem to support a benefit to BCAA supplementation during periods of caloric restriction. Journal of the International Society of Sports Nutrition. 13:21-25. 2016.

Falavigna, G, de Araujo Junior, AA, Rogero, MM, de Oliveira Pires, IS, Pedrosa, RG, Martins Junior, E, de Castro, IA, Tirapegui, J. Effects of diets supplemented with branched-chain amino acids on the performance and fatigue mechanisms of rats submitted to prolonged exercise. Nutrients. 4:1767-1780. 2012.

Meeusen, R, Watson, P, Dvorak, J. The brain and fatigue: New opportunities for nutritional intervention. Journal of Sports Sciences. 24(7):773-782. 2006.

Pitkänen HT, Oja SS, Rusko H, Nummela A, Komi PV, Saransaari P, Takala T, Mero AA. Leucine supplementation does not enhance acute strength or running performance but affects serum amino acid concentration. Amino Acids. 25(1):85-94. 2003.

Sengupta, S, Peterson, TR, Sabatini, DM. Regulation of the mTOR Complex 1 pathway by nutrients, growth factors, and stress. Molecular Cell. 40:310-322. 2010.

Tipton, KD, Elliott, TA, Cree, MG, Aarsland, AA, Sanford, AP, Wolfe RR. Stimulation of net muscle protein synthesis by whey protein ingestion before and after exercise. American Journal of Physiology Endocrinology and Metabolism. 292:E71-E76. 2007.

Wolfe, RR. Branched-chain amino acids and muscle protein synthesis in humans: myth or reality? Journal of the International Society of Sports Nutrition. 14:30-36. 2017.

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