Building Muscle with BCAAs and Leucine


In order to get the most from your time in the weights room, you might want to start thinking about adding more Branched-chain Amino Acids (BCAAs) and Leucine to your diet. But what are they? What will they do for you? Do you even need them? Here’s everything you need to know.

Protein and Essential Amino Acids Explained

Protein is a macronutrient – nutrients that are required in larger amounts – and plays numerous key roles in the body, one of which is in maintaining, repairing and building muscle.

Protein molecules vary in structure and length and are made up of ‘units’ known as amino acids. Several hundred different amino acids have been identified in nature, 20 of which are particularly important because they help protein synthesis.

Nine of these are essential for humans to consume in our diets as they can’t be created from other amino acids, and these are known as Essential Amino Acids (EAAs). Three of these are categorised as Branched-chain Amino Acids – so-called because of their structure – and these are leucine, isoleucine and valine.

Different protein-rich foods vary in their protein quality, and you can read more about this in our Guide to Protein Quality, Digestion and Absorption.

What is Protein Synthesis?

Protein synthesis is the mechanism by which certain key structures in the body, including muscle tissue, skin, enzymes, some hormones, neurotransmitters, and immune factors (to name but a few) are formed from amino acids.

Without consuming a good amount of protein in the diet, which includes sufficient amounts of each of the nine EAAs, other less-essential tissues are broken down in order to prioritise what the body needs to keep us alive.

What does protein malnutrition look like? Signs and symptoms may include fatigue – especially during physical activity – muscle wasting, and a compromised immune system.

The Role of BCAAs

BCAAs are present in high concentrations in muscle tissue, which helps explain their popularity with bodybuilders and strength athletes. BCAAs have other key roles – for example, isoleucine is involved in increasing glucose uptake into cells, and this process doesn’t involve the other BCAAs, so it may help to promote energy during exercise[1].

However, the most important function of BCAAs – in particular leucine, which is more potent than isoleucine and valine – is their role in controlling the onset of protein synthesis.

BCAAs stimulate the activity of an enzyme called mTOR[2-5]. mTOR (which stands for “mechanistic, or mammalian, target of rapamycin”) is an enzyme that is the key regulator of protein synthesis, as well as other processes like cell growth and breakdown[6].*

mTOR is a very important enzyme as it regulates all metabolic processes in most animal tissues, including muscle[7], and poor functioning of it is linked to diabetes, obesity, depression and certain cancers[8-10].

Here we’re more interested in mTOR’s role in regulating protein synthesis for muscle growth and recovery after exercise. If mTOR is inactive for long periods, this will result in muscle wasting, and this is part of the reason for the loss of muscle mass and tiredness in people with advanced wasting diseases – cancer cachexia, for example.

To put it simply, think of mTOR as a switch, which when turned to ‘on’ will initiate protein synthesis, and leucine is the pressure required in order to press the switch.

When the level of leucine is high enough, the ‘switch’ is turned ‘on’ and muscle tissue can be repaired. When leucine levels go down, the ‘switch’ is released and protein synthesis turns ‘off’.


How Much Leucine and BCAAs Do You Need?

Supplementary leucine may enhance strength and performance[11-14], and resistance exercise may further promote its effects[15-16]. However, the effect is related to how much leucine is present in a meal containing at least 20g of protein[17].

As part of a protein meal, a total BCAA level of at least 4.5g of which at least 2.2g is leucine[16], has been shown to be sufficient to stimulate protein synthesis. This could be in the form of a supplement, or by consuming a high-quality protein meal or snack.

Shop Huel

Leucine and BCAAs in Huel Products

Huel Product Serving Size Protein Calories BCAAs Leucine
Complete Protein (v1.0)* 29g 20.4g 105 4.8g 3.1g
Powder (v3.0)* 100g 29g 400 5.0g 2.3g
Black Edition (v1.1)* 90g 40g 400 7.4g 3.5g
Hot & Savoury (v1.0)** 95g 25g 400 4.3g 2.0g
Ready To Drink (v1.0)* 500ml 20g 400 3.5g 1.7g
Bar v3.1 49g 12g 200 1.9g 0.9g

Table showing the levels of BCAAs and leucine compared to the total protein and calories per serving:

* Vanilla flavours

** Korma flavour

*** Chocolate flavour

Key Takeaways

  • Protein is made up of amino acids
  • Nine amino acids are *essential*, and are known as EAAs
  • Three of these – leucine, isoleucine and valine – are BCAA BCAAs – in particular leucine – ‘switch on’ protein synthesis through the enzyme mTOR
  • A protein meal containing 4.5g BCAA, including 2.2g leucine, will stimulate protein synthesis

Recommended Reading

* For accuracy, mTOR is actually an enzyme complex of which there are two and these also involve other protein factors; for ease when referring to mTOR, we mean mTOR Complex 2.


  1. Doi M, et al. Isoleucine, a potent plasma glucose-lowering amino acid, stimulates glucose uptake in C2C12 myotubes. Biochem Biophys Res Commun. 2003; 312(4):1111-7.
  2. Anthony JC, et al. Orally administered leucine stimulates protein synthesis in skeletal muscle of postabsorptive rats in association with increased eIF4F formation. J Nutr. 2000; 130(2):139-45.
  3. Anthony JC, et al. Leucine stimulates translation initiation in skeletal muscle of postabsorptive rats via a rapamycin-sensitive pathway. J Nutr. 2000; 130(10):2413-9.
  4. Blomstrand E, et al. Branched-chain amino acids activate key enzymes in protein synthesis after physical exercise. J Nutr. 2006; 136(1 Suppl):269s-73s.
  5. Drummond MJ, et al. Rapamycin administration in humans blocks the contraction-induced increase in skeletal muscle protein synthesis. J Physiol. 2009; 587(Pt 7):1535-46.
  6. Kim DH, et al. mTOR interacts with raptor to form a nutrient-sensitive complex that signals to the cell growth machinery. Cell. 2002; 110(2):163-75.
  7. Wipperman MF, et al. Mammalian Target of Rapamycin: A Metabolic Rheostat for Regulating Adipose Tissue Function and Cardiovascular Health. Am J Pathol. 2019; 189(3):492-501.
  8. Kennedy BK, et al. The Mechanistic Target of Rapamycin: The Grand ConducTOR of Metabolism and Aging. Cell Metab. 2016; 23(6):990-1003.
  9. Perluigi M, et al. mTOR signalling in ageing and neurodegeneration: At the crossroad between metabolism dysfunction and impairment of autophagy. Neurobiol Dis. 2015; 84:39-49.
  10. Saxton RA, et al. mTOR Signaling in Growth, Metabolism, and Disease. Cell. 2017; 168(6):960-76.
  11. Ispoglou T, et al. Daily L-leucine supplementation in novice trainees during a 12-week weight training program. Int J Sports Physiol Perform. 2011; 6(1):38-50.
  12. Norton LE, et al. Leucine regulates translation initiation of protein synthesis in skeletal muscle after exercise. J Nutr. 2006; 136(2):533s-7s.
  13. Tipton KD, et al. Stimulation of net muscle protein synthesis by whey protein ingestion before and after exercise. Am J Physiol Endocrinol Metab. 2007; 292(1):E71-6.
  14. Tipton KD, et al. Timing of amino acid-carbohydrate ingestion alters anabolic response of muscle to resistance exercise. Am J Physiol Endocrinol Metab. 2001; 281(2):E197-206.
  15. Anthony JC, et al. Leucine Supplementation Enhances Skeletal Muscle Recovery in Rats Following Exercise. The Journal of Nutrition. 1999; 129(6):1102-6.
  16. Layman DK, et al. Defining meal requirements for protein to optimize metabolic roles of amino acids. Am J Clin Nutr. 2015; 101(6):1330S-8S.
  17. Bauer J, et al. Evidence-based recommendations for optimal dietary protein intake in older people: a position paper from the PROT-AGE Study Group. J Am Med Dir Assoc. 2013; 14(8):542-59.

    Please log in to your store account

    To share with your friends, log in is required so that we can verify your identity and reward you for successful referrals.

    Log in to your account If you don't have a store account, you can create on here

    Join over 100,000 Hueligans @huel

    Use #huel in your Huel photos for the chance to feature on our Instagram

    Join our VIP list

    Never miss out on new products, exclusive offers, and more when you join the Huel mailing list.

    This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply. You can unsubscribe at any time. Huel Privacy Policy