Effects of Leucine & Alanine on Endurance Training & Racing

Sports Dietitian and Nutritionist, Ashley Thomas explores the scientific evidence on the effects the Amino Acids, Leucine and Alanine have on endurance training and racing. These are the two Amino’s found in Fixx Nutrition’s FUEL X PRO Endurance Fuel.

‘Sports nutrition products are abundant in cycling stores, supermarkets, petrol stations, supplements stores, and sporting expos. Every brand has its own unique marketing difference, but how do you know what’s based on real science and what has been taken out of context to try and sell you something? Let’s delve into whether the addition of leucine and alanine in sports nutrition products could actually enhance your performance.

Leucine and alanine are both amino acids, the smallest form of proteins. Leucine has been the most thoroughly investigated amino acid in relation to its effects on physiology. In relation to sports performance, leucine has a higher oxidation rate, much higher than the other amino acids. A high enough overall leucine content can enhance post-exercise muscle protein synthesis, activation of energy balance such as glucose homeostasis, food intake, and adiposity (1, 2, 3).

Leucine is also a branched-chain amino acid. Branched-chain amino acids have been thoroughly studied to reduce the rate of perceived exertion and mental fatigue when exercising (4). Reducing central nervous system fatigue can be very beneficial to females especially those who are pre-menstrual or post-menopausal due to our changes in hormones. Increases in progesterone levels can actually indirectly contribute to a breakdown in
muscle tissue. BCAA’S and in particular leucine can stop muscle breakdown and initiate muscle synthesis. They can also cross the blood-brain barrier which contributes to a reduction in central nervous system fatigue. Central nervous system fatigue is often heightened during high hormone phases.

Beta-alanine is a non-essential amino acid. Your body uses it to produce carnosine, which helps improve exercise performance. Supplementing with beta-alanine has been shown to elevate carnosine levels in muscles by up to 80% (5, 6, 7, 8 ). Carnosine serves as a buffer against lactic acid, reducing the acidity in muscles during high-intensity exercise. Lower acid levels in the muscles reduce your time to fatigue and therefore increase
performance potential (9).

The addition of these amino acids in sports nutrition products is a strategic sports performance tool that can help athletes increase their overall VO2 Max, remove lactic acid, time to fatigue, and blood sugar regulation. There are few sports nutrition products that combine both of these amino acids. The Fixx nutrition Fuel X Pro range contains both leucine and alanine to assist performance and endurance. Fuel X also uses the carbohydrate of dextrose instead of maltodextrin or fructose, the former two of which can cause many gut issues in athletes. Calcium carbonate is also included which is a mineral people often don’t consider important for sports performance, however, it can help line the gut and reduce the onset of cramps as we lose calcium throughout sweat.


If you struggle with concentration, fatigue, lactic acid production, gut issues or cramps and are looking to change up your nutrition, the new Fixx nutrition Fuel X Pro range may be something you need to add to your training and race day nutrition strategy.

Check out the Fixx nutrition products or contact Ash Thomo Nutrition for further information.’

Ashley Thomas – Instagram: ashthomo_nutrition

Bachelor of Physical and Health Education – Dietitian and Nutritionist (Masters)

Reference list


(1) Mero A. Leucine supplementation and intensive training. Sports Med. 1999 Jun;27(6):347-58. DOI: 10.2165/00007256-199927060-00001. PMID: 10418071.
(2) Waskiw-Ford M, Hannaian S, Duncan J, Kato H, Abou Sawan S, Locke M, Kumbhare D, Moore D. Leucine-Enriched Essential Amino Acids Improve Recovery from Post-Exercise Muscle Damage Independent of Increases in Integrated Myofibrillar Protein Synthesis in Young Men. Nutrients. 2020 Apr 11;12(4):1061. DOI: 10.3390/nu12041061. PMID: 32290521; PMCID: PMC7231404.
(3) Schwartz GJ. Central leucine sensing in the control of energy homeostasis. Endocrinol Metab Clin North Am. 2013 Mar;42(1):81-7. DOI: 10.1016/j.ecl.2012.12.001. Epub 2012 Dec 28. PMID: 23391241; PMCID: PMC3568262.
(4) Blomstrand E. A role for branched-chain amino acids in reducing central fatigue. J Nutr. 2006 Feb;136(2):544S-547S. DOI: 10.1093/jn/136.2.544S. PMID: 16424144.
(5) Derave W, Ozdemir MS, Harris RC, Pottier A, Reyngoudt H, Koppo K, Wise JA, Achten E. beta-Alanine supplementation augments muscle carnosine content and attenuates fatigue during repeated isokinetic contraction bouts in trained sprinters. J Appl Physiol (1985). 2007 Nov;103(5):1736-43. DOI: 10.1152/japplphysiol.00397.2007. Epub 2007 Aug 9. PMID: 17690198.
(6) Hill CA, Harris RC, Kim HJ, Harris BD, Sale C, Boobis LH, Kim CK, Wise JA. Influence of beta-alanine supplementation on skeletal muscle carnosine concentrations and high-intensity cycling capacity. Amino Acids. 2007 Feb;32(2):225-33. DOI: 10.1007/s00726-006-0364-4. Epub 2006 Jul 28. PMID: 16868650.
(7) Harris RC, Tallon MJ, Dunnett M, Boobis L, Coakley J, Kim HJ, Fallowfield JL, Hill CA, Sale C, Wise JA. The absorption of orally supplied beta-alanine and its effect on muscle carnosine synthesis in human vastus lateralis. Amino Acids. 2006 May;30(3):279-89. DOI: 10.1007/s00726-006-0299-9. Epub 2006 Mar 24. PMID:
16554972.
(8) Sale C, Saunders B, Harris RC. Effect of beta-alanine supplementation on muscle carnosine concentrations and exercise performance. Amino Acids. 2010 Jul;39(2):321-33. DOI: 10.1007/s00726-009-0443-4. Epub 2009 Dec 20. PMID:20091069.
(9) Hobson RM, Saunders B, Ball G, Harris RC, Sale C. Effects of β-alanine supplementation on exercise performance: a meta-analysis. Amino Acids. 2012;43(1):25-37. doi:10.1007/s00726-011-1200-z