Amino acid supplementation: performance enhancer or placebo effect?

Can pre-exercise single amino acid supplementation improve athletic performance? Andrew Hamilton looks at the evidence

Amino acids are the building blocks of all proteins, including muscle protein. Unsurprisingly therefore, amino-acid nutrition has been extensively studied in relation to muscle recovery/repair following exercise, and of course muscle mass and strength gains. There are nine ‘essential’ amino acids, which the body needs to construct proteins. There are also a number of other ‘non-essential’ amino acids, but these can all be synthesised in the body from essential amino acids. The key point here is that whenever we consume foods containing amino acids – ie protein foods such as meat, fish, milk, cheese, eggs, nuts, beans etc – we always consume a wide mix of all the amino acids.

Single vs. mixed amino acids

However, using sophisticated manufacturing techniques, it’s possible to isolate single amino acids – and that’s where things get rather interesting. That’s because some of the key neurotransmitters in the brain are very closely related chemically to certain amino acids – ie these neurotransmitters are synthesised from amino acids in the brain. An example is the neurotransmitter serotonin (low serotonin levels are known to be a prime cause of depression), which is synthesised in a 2-step process from the amino acid tryptophan (see figure 1).

Figure 1: Synthesis of neurotransmitter serotonin from the amino acid tryptophan

This means that taking a dose of a single amino acid can affect brain chemistry in a way that doesn’t occur when consuming a mix of amino acids (ie eating protein). This altered brain chemistry can also affect hormone balance in the body. Why do single amino acids have this effect while mixtures of amino acids don’t? The reason is that many of these amino acids ‘compete’ to cross the blood brain barrier. When there are no other amino acids to compete for absorption, consuming a single amino acid means that it can cross this barrier unimpeded, which significantly raises the level of that amino acid in the brain, potentially affecting brain chemistry.

Single amino acids and sport performance

If single amino acid consumption can alter brain chemistry/function and affect hormone balance, are there some amino acids that could help alter this in a way, which is favourable for sport performance?


One amino acid that has received particular attention is tyrosine. Tyrosine is the precursor to the ‘catecholamine’ neurotransmitters dopamine and norepinephrine. These catecholamines are produced by sympathetic nervous system activation, which results in activity autonomic arousal – (more commonly known as the fight-or-flight stress response) – exactly what athletes need during flat-out exercise! In theory, increasing tyrosine uptake should positively influence catecholamine-related psychological functioning. But what does the science say?

Unfortunately, the results are rather mixed. One study on cycling performance in very hot conditions found that consuming 150mgs of tyrosine per kilo of bodyweight one hour beforehand improved the time to exhaustion of the cyclists(1). The researchers surmised that the tyrosine helped the cyclists combat the physical stress induced by the heat/exercise combination. It has also been shown that depleting tyrosine in the brain (by administering a tyrosine-free amino-acid mix) worsens exercise performance in the heat(2).

In contrast however, a 2015 study concluded that there were no significant effects of tyrosine on exercise performance(3). However, it did find that tyrosine supplementation counteracted the decrements in working memory and information processing induced by demanding situational conditions such as extreme weather or cognitive load. In other words, for athletes whose events involve information processing in demanding conditions, tyrosine may help cognition thanks to its ability to combat depleted brain catecholamine levels.

Meanwhile, a study on military recruits found no benefits – either in terms of physical performance or cognition – when consuming pre-exercise tyrosine(4). But just to confuse matters further, a review study (pooling the results from a number of previous studies) found that tyrosine supplementation can boost cognition but has little effect on exercise performance(5). These findings (for cognitive but not exercise benefits) were replicated by another large review study also carried out that year(6).


As a single amino-acid, phenylalanine has been little researched in the context of exercise. However, a recent study found that while there were no cognitive benefits, a pre-exercise dose of three grams of phenylalanine significantly enhanced fat burning during sub-maximal exercise(7). Likewise, another study found that phenylalanine (in combination with arginine and alanine) stimulated fat burning during exercise(8)

This fat-burning effect is most probably because phenylalanine ingestion increases carbohydrate storage hormone glucagon, forcing the body to derive more fat from fuel. But while this approach might be good for fat-burning during early-season training, anything that reduces carbohydrate availability is NOT recommended for athletes engaging in competition, where maximum performance matters!


Taurine is another amino acid of interest, and one which finds its way into a number of pre-exercise drinks where it is claimed to enhance focus and performance. Once again however, the evidence for its benefits is far from clear. A study on runners found that six grams of taurine taken pre-exercise did not improve high-intensity running performance, although there was a non-significant increase in the ability of runners to sustain an oxygen deficit (9). Meanwhile, an earlier study on cyclists found that 1.7 grams of pre-exercise taurine didn’t enhance time trial performance, but did increase fat oxidation(10).

In its favour though, a review study carried out last year pooled the data from seven previous studies on taurine and exercise performance. It found overall that there was a small but modest performance benefit for endurance performance. However, there was no evidence that taurine boosted sprint/high-intensity performance or mood or cognitive performance.


A final mention is for the less well known amino acid called theanine. Theanine is a naturally occurring amino acid found in small amounts in tea. There’s particular interest in theanine as it appears to favourably affect cognitive function, especially when combined with caffeine. In particular, doses of 100-200mgs of theanine combined with 50-150mgs of caffeine have been found to:

  • Enhance cognitive function and mood(11).
  • lmprove target-specific attention to visual stimuli by decreasing mind wandering(12).
  • Improve and prolong vigilance during a sustained attention task(13).

Although more research is needed, there’s good evidence accumulating that pre-exercise theanine combined with caffeine could be especially useful for athletes whose events require skill, focus and concentration in addition to physical effort. Good news then for athletes who enjoy a cup of tea or two!


The evidence for the pre-exercise use of single amino acids is at best patchy. Athletes competing in high-stress environments and whose sports require cognitive processing (eg football players) might benefit from tyrosine supplementation. Phenylalanine might help to increase fat burning during sub-maximal endurance exercise, which could be useful for weight management. However, is unlikely to benefit (and could harm) performance during high-intensity exercise. Endurance athletes may receive a small benefit from taurine supplementation but the evidence overall is weak. Theanine in combination with caffeine on the other hand could be worth trying for athletes whose sports require visual processing, motor skills and concentration for example when learning or practising new techniques and/or skills.


  1. Eur J Appl Physiol. 2011 Dec;111(12):2941-50
  2. Eur J Appl Physiol. 2013 Jun;113(6):1511-22
  3. Pharmacol Biochem Behav. 2015 Jun;133:1-6
  4. Med Sci Sports Exerc. 2016 Feb;48(2):277-80
  5. Mil Med. 2015 Jul;180(7):754-65
  6. J Psychiatr Res. 2015 Nov;70:50-7
  7. J Int Soc Sports Nutr. 2017 Sep 12;14:34
  8. Int J Sport Nutr Exerc Metab. 2016 Feb;26(1):46-54.
  9. Appl Physiol Nutr Metab. 2016 May;41(5):498-503
  10. Int J Sport Nutr Exerc Metab. 2010 Aug;20(4):322-9
  11. Nutrients. 2017 Dec 7;9(12). pii: E1332
  12. Nutr Res. 2018 Jan;49:67-7
  13. Neuropharmacology. 2012 Jun;62(7):2320-7

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