BCAAs Before Exercise : Do They Build Or Do They Fuel?

BCAAs is the acronym for Branched Chain Amino Acids, and refers to Leucine, Isoleucine and Valine. BCAAs are mainly ingested through the consumption of meat, fish, egg, and peanuts, with the best sources being chicken, 95% lean beef mince, and canned tuna delivering approx 6.6g, 6.2g and 5.6g BCAAs respectively. Interestingly the trusty egg delivers the most BCAAs per gram of protein at 0.23g per gram compared to 0.18g per gram in chicken, making eggs the most concentrated source of BCAAs.

Why consume BCAAs?

BCAAs are key to muscular development because of their protein synthesising and anti-catabolic properties, and this is the reason why they have been harnessed and concentrated down to make the highly popular BCAA supplement! BCAA supplements are arguably one of the fastest growing supplements on the market in part because of growing literature supporting its effects, as well as the cumulative benefit of combining it with whey protein. BCAAs are best combined with whey protein by ingesting BCAAs pre and intra exercise, followed by whey protein after the gym session (a good whey protein will usually contain some BCAAs too).

Amino acids can be used for energy too

A common nutrition misconception is that glucose is the only energy source available to man, however both fatty acids and amino acids (including BCAAs) also serve as key substrates for Adenosine Triphosphate (ATP) synthesis, the key intermediate for energy production. Although amino acids can be used to produce energy, glucose is always the bodies preferred energy source meaning amino acids and fatty acids will be spared for emergency use, or ideally growth!

Protein, Carbs and Fat as energy substrates

The amount of energy each macronutrient releases is given an Atwater figure, this quantifies the energy released per gram. Fat leads the way at 9kcal per gram, whilst protein and carbs both deliver 4kcal per gram, the difference is that carbohydrate is broken down far more readily than both protein or fat. When broken down into their sub-units, fat forms fatty acids, carbohydrate forms glucose, and protein forms amino acids. Despite this, both amino acids and fatty acids can be converted into glucose via metabolic pathways such as Gluconeogenesis, but it’s the bodies preference to oxidise glucose for energy above all else.

So what happens when you consume BCAAs when exercising?

Assuming you’re in a fed state, the ingestion of BCAAs before exercise will result in Leucine, isoleucine and valine being deaminated (broken down) in the liver and channelled to your muscles for muscle protein synthesis. Conversely amino acids such as glutamine are primarily used to replenish the lining of the intestine, whilst other non-essential and essential amino’s are used to fuel the liver. Once in the liver their –NH2 groups are removed by a process called transamination, and the alpha-keto acid that is left over is then used as fuel, or for biosynthesis e.g. muscle protein synthesis!

What if you consume BCAAs in a relatively fasted state?

So you may get up in the morning, leave for the gym ready to commence your session on an empty stomach, and true to form you get your BCAAs down you about 30-40mins before. Granted you may have just observed an 8 hour fast over night, but your liver will likely have some glycogen stores in reserve ready to be broken down via glycogenolysis to yield glucose for energy (muscle cannot liberate glucose from it’s glycogen stores as it lacks glucose 6-phosphatase). Consequently your body will metabolise as much of the remaining glycogen it has to fuel your morning session. If glycogen was low before commencing then your body may start to revert to fatty acids and amino acids for energy. BCAAs are mainly transaminated (converted to non-essential amino acids) in the muscle forming alanine, particularly in the fasted state. Alanine and branched chain oxo-acids may be used as energy substrates or sent back to the liver where they’re converted back to branched chain amino acids. However, BCAAs are preserved and protect the muscle when in a fasted state in order to preserve skeletal muscle integrity, whilst muscle also contains a substantial pool of glutamine to help preserve muscle in a fasted state.

BCAAs preserve muscle in fasted state

During a starved or fasted state, the BCAAs that you consume are stored in the muscle, whilst all other amino acids (except for leucine, isoleucine and valine). The muscles have an abundant pool of glutamine, in fact, approx 80% of glutamine is stored intra-muscularly! When energy is needed in the fasted state, the BCAAs are preserved whilst glutamine is taken up by the kidney and small intestine and used for energy via gluconeogenesis. This process is known as the inter-organ glucose alanine cycle, and is a way in which amino acids are used to provide energy in the fasted state. Since muscle glycogen cannot be used to liberate glucose because it lacks glucose 6-phosphatase, it has no choice but to yield glucose 1-phosphate,  which is used in glycolysis (a metabolic pathway) to synthesise pyruvate, which is then used to produce alanine which will be at the expense of BCAAs, or if these aren’t present…your MUSCLE! Therefore it is integral that BCAAs are topped up when training, particularly in the fasted state, in order to limit muscle catabolism.

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Bender, D, A. (2012). Amino Acid Metabolism. Inter-Organ metabolism of amino acids. 3rd Ed. John Wiley & Sons, Ltd.

Da Poian, T., El Bacha, T., Luz, M, R, M, P. (2010). Nature Education. Nutrient Utilization in Humans: Metabolism Pathways.


About the Author

Job Role Qualified Dietitian and Sports Nutritionist Qualifications BSc (Hons) Sports Science | BSc (Hons) Dietetics Tom has always participated in sport both recreationally and competitively which led to an unquenchable thirst for information on anything health, nutrition and fitness. After leaving school Tom went on to play for a football academy during which time he studied Sport and Exercise Science. From here he went on to study a BSc (Hons) Sport Science at UEA followed by his second BSc (Hons) degree, this time at the University of Hertfordshire studying Dietetics. Tom has worked in the fitness, educational and clinical nutrition industry starting out at David Lloyd Health and Leisure Clubs. He then went on to work as a Dietitian (RD) in the NHS, during which time he conducted clinics for healthy eating, weight loss and weight gain, as well as specialised consultations on Diabetes, IBS and Coeliac disease to name a few. He has vast amounts of experience at devising diet plans and supplement regimens, as well as working in the community with schools and competitive athletes. As Head Nutritionist and Supplement expert at Discount Supplements Tom is here to provide current and evidence based health and nutrition information to help you reach your health and fitness goals!
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