• Science Fueling Fitness

BCAAs and Muscle Protein Synthesis: A Myth or Reality? – The Evidence


Recently, branched chained amino acid (BCAA) supplements have become extremely fashionable within the gym community. Gym goers aiming to reap the claimed benefits associated with muscle protein synthesis (MPS) and post exercise recovery are carrying around whey protein shakes with added BCAAs or consuming them dissolved in water. However, is there even scientific evidence to support these claims suggesting improved MPS and enhanced muscle recovery? Or are they simply a waste of money?

So, how does muscle growth occur?

MPS occurs when there is a positive shift in net protein balance, favouring increased MPS and decreased muscle protein breakdown (MPB). In states of fasting there is increased MPB and decreased MPS. Increases in MPS (positive net protein balance) are achieved when there is a surge in blood amino acids. Additionally, exercise is a potent stimulant for MPS when amino acids are in circulation.


Currently, in cell culture models, as well as in rodents, BCAAs (particularly leucine) stimulate the mTOR pathway. mTOR is a major regulator in cell growth – therefore a master regulator in MPS. mTOR activation (by IGF-1, insulin or leucine) results in the activation of downstream 4E binding Protein 1 (4E-BP1) and p70 ribosomal protein S6 kinase (p70s6k) which are pre-cursors to protein initiation and elongation, and therefore muscle protein synthesis (all very sciencey stuff!). This right here is the mechanism of action for BCAA supplements – they can simply turn on MPS. This is what we are all interested in – maximizing our gainz! However, mTOR activation alone is unfortunately not the holy grail in muscle anabolism. Sometimes, what we see in controlled cell culture models and in our mice, is not able to be translated into humans. This is due to humans being much more complex than a basic cell model, with much more variable factors at play – e.g. inter-individual variation, genetic factors, environmental factors etc. In saying this, cells and mice are good models for hypothesis testing and understanding the BASIC fundamentals of molecular nutrition – so these are great places to start! All another story, for another day!



The growth of new muscle is limited by the lack of availability of essential amino acids (EAAs). After a meal rich in protein, all the EAAs are present. The issue with BCAA supplements is that skeletal muscle needs ALL the EAAs to make new muscle protein, and with only three amino acids present (as is the case with BCAAs), the muscle doesn’t have enough of the building blocks. However, since leucine is a MAJOR trigger of MPS, meaning MPS may still occur, minimal new protein is made as the body needs to increase MPB to make available more amino acids. In fact, studies where BCAAs are infused into humans have failed to significantly increase MPS, and have even decreased the rate of MPS and muscle protein turnover – meaning the body remained in a state of catabolism or break down.


But what about when we consume BCAAs with carbs or protein? There has been no evidence to show an improvement in MPS when carbohydrates were co-ingested with BCAAs after a bout of resistance exercise. However, there is evidence to suggest an enhancement of the anabolic effect of a protein meal when BCAAs are combined with the protein meal. A study which provided subjects with a sub optimal dose of whey (6.25g whey) alongside 5g of BCAAs saw comparable increases in MPS to when subjects consumed 25g of whey protein. This makes sense though, right? The protein is providing the building blocks, whilst the BCAAs are turning on MPS.


Don’t forget that BCAAs are comprised of three amino acids – leucine, isoleucine and valine. Much scientific evidence supports the role that leucine plays in augmenting MPS, whereas the evidence to support the role of isoleucine and valine in MPS is lacking/non existent. In muscle cells, the transport of all BCAAs is through the same transport system. Therefore, when consumed together, the BCAAs compete with one another to enter the cell. The addition of isoleucine and valine may hinder the entry of leucine (the most potent stimulator of MPS) into the cell – limiting the rates of MPS.


To conclude, the muscle needs all the EAAs to be present to synthesize new proteins – meaning BCAAs alone will not elicit an effective MPS response, and may in fact result in a negative net protein balance, as the muscle breaks itself down for the generation of the amino acids necessary for the MPS response. Therefore, instead of reaching for a BCAA supplement, we suggest you opt for a leucine supplement - alongside a submaximal protein dose - in order to optimise protein synthesis. This is particularly important if the protein source is not coming from whey (whey is already rich in leucine) but instead coming from soy or other plant based proteins which are lower in leucine quantity!


Of course, you could always just go for an adequate protein portion (25g) as this will always be the main suggestion before you turn to supplementation!


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