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A Review on the Effects of Caffeine Supplementation on Physical and Mental Performance of Trained Subjects.

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Effects of caffeine on trained subjects.

A review on the effects of caffeine supplementation on physical and mental performance of trained subjects.

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Effects of caffeine on trained subjects.


Caffeine is a commonly used, legal ergogenic aid used among athletes. This is a short and concise summary of the main finding of the effect of caffeine on trained individuals.


Caffeine supplementation has shown to have an ergogonic effect on physical ability and has been linked to cognitive performance (article 4). Much controversy is involved in relation to the cognitive response to caffeine (article 3), in particular reaction time (article 1), concentration and memory (article 3). An increase in endurance may be due to caffeine’s ability to increase the gastrointestinal absorption of glucose for energy (article 3), This effect relies on the presence of carbohydrates (CHO) for oxidation (article 2). Caffeine is an antagonist toward adenosine receptors, which stimulates the central nervous system (CNS) (article 1). By stimulating the CNS this can potentially increase the activation of motor neurons and therefore increase power output (article 1). Therefore the subject’s performance on short repeated sprints is improved. The point of this review is to compact the results of the numerous studies that analyzed the effects of caffeine on endurance, utilization of CHO, power output, cognitive performance and reaction time of trained individuals.

Caffeine effect on cognition and reaction time.

Caffeine supplementation has been shown to improve physical performance (article 3) and physical activity alone has also been shown to improve cognitive ability (article 1). Article 3 and Article 1 both investigated the extent to which caffeine effects cognition and reaction time. Article 3 focused on complex cognitive abilities both during and after exercise. It was found that concentration and reaction time was improved but memory was not, these results were consistent for during and after exercise (article 3). In contrast to theses findings article 1 tested only reaction time and found no significant evidence to suggest that caffeine ingested would improve reaction time (article 2). Six out of 10 subjects in the article 1 study had a faster simple reaction time while on the caffeine as apposed to the placebo, but these effects were moderate and not considered significant enough to conclude that caffeine increases reaction time (article 1). This was also the case for choice reaction time; reaction time was shown to be faster post exercise rather than pre exercise (article 1).
This controversy in results may be attributed to a number of factors; the first being the method of experimentation used by each author. Article 1 subjects were asked to abstain from food two hours prior to being tested, the subjects were given only caffeine supplements and water and nothing else (article 1). While article 3 subjects were given one of three supplements, either a performance bar containing mainly CHO, some fats and proteins with no caffeine, or the same performance bar but with caffeine or a placebo, which was basically flavored water (article 3). The carbohydrates in the energy bar helped maintain glucose concentration in the blood while exercising, and the presence of caffeine increases exogenous oxidation of carbohydrates (article 3). These physiological processes do not directly explain the reason behind the improved cognitive performance, but it does explain the improvement in physical ability. The physiology behind cognition was not analyzed in article 1 or 3 .Differences in results may also be the cause of different exercise protocols, article 1 used repeated short sprints, and while article 3 used prolonged exhaustive exercises.

Caffeine and exercise endurance training.

Upon examination of the effects of low doses of caffeine along with carbohydrates, it has shown to improve exercise ability in relation to prolonged exercise trials (article 2)
Article 2 investigated the effects of both low (1.5 mg .kg-1) and moderate (3 mg .kg-1) Intakes of caffeine on endurance exercise (article 2). The results found that both increased performance, although there was no significant difference between the two doses (article 2). These results showed that caffeine’s ability to increase endurance performance is limited (article 2). The results obtained from article 2 were in contrast to other studies which showed that the higher the dose of caffeine, the higher the endurance performance (reference 10, article 2). A reason for this discrepancy may be attributed to the ingestion time of the caffeine. The alternative study administered caffeine upon the onset of fatigue, whilst article 2 administered caffeine at the beginning of the exercise trial. The physiology behind caffeine’s ability to increase performance lies in the gastrointestinal tract. The effect caffeine has on the gastrointestinal tract was investigated by article 2, article 3 and article 1 and all showed that caffeine increased the absorption of glucose from the gastrointestinal tract. Article 2 demonstrated that caffeine’s ability to improve the utilization of glucose is limited to the CHO levels during exercise (article 2). This conclusion is also reinforced in article 3, which states that CHO ingested with the caffeine increased the rate of carbohydrate oxidation for energy (article 3). Article 2 also demonstrated that when there are high amounts of glucose in the blood, it is sufficient enough to optimize the gastrointestinal transport of this glucose and caffeine ingested cannot further enhance CHO oxidation (article 2).

Caffeine and exercise power output.

The effect caffeine has on short bursts of activity has not been as widely investigated as endurance exercises (article 1), but has shown to improve power output in short sprint exercise trials (article 4). Article 1 and 4 examined the response of caffeine supplementation on repeated sprints. Both studies found that caffeine improved sprint time; article 1 found that eight out of ten participants that had ingested the caffeine had a faster sprint time (article 1). Article 4 had two exercise trials, both separated by 7 days, and found that on the first trial performance was increased by 8.5% and on the second trial it had increased by 7.6% (article 4). Article 4 found that there was no significant difference in the development of fatigue or the perceived effort (article 4). There are many physiological explanations for these results; the first is to do with caffeine’s molecular structure. Caffeine is structurally very similar to adenosine and can bind to its receptor and block its action (article 4). This antagonism of the adenosine receptors stimulates the CNS; this enables the additional recruitment of motor neurons or just more frequent motor neuron activation (article 4). It is assumed that this antagonism effect will increase the work and power output during the repeated sprints (article 4). The antagonism of adenosine receptors also has the potential to decrease the perception of effort (article 1). Both article 1 and 4 examine the role of potassium ions (K+) and the onset of fatigue and came to the same conclusion. It is thought that K+ accumulation in the muscle can lead to early fatigue (article 1), caffeine facilitates Na+/K+ ATPase activity which therefore causes a decrease in the plasma K+ levels, therefor delaying the onset of fatigue (article 4). Recovery time is linked to plasma K+ levels, as caffeine needs sufficient time between bouts of exercise to optimize its ergogonic potential (article 1). Article 1 found that subjects on caffeine had an increase concentration of hydrogen ions, which are a by-product of anaerobic glycolysis (article 1). This shows that there is an increase in the breakdown of glycogen; therefore caffeine has increased the utilization of glycogen to produce ATP for energy (article 1), therefore improving sprint performance.


In summary caffeine proved to enhance physical performance, although its effects on cognition are arguable. Article 3 tested complex cognitive performance and found caffeine to improve concentration and reaction time, but not memory (article 3). While in contradiction of these findings article 1 found that reaction time was not significantly improved (article 1).

Article 1 and 4 showed an increase in physical power output during sprint tests, via the antagonism of adenosine receptors (article 4), facilitation of Na+/K+ ATPase activity and an increase in anaerobic glycolysis (article 1).

Antagonism of the adenosine receptors, which stimulate the effects of the Central nervous system, this can potentially increase the amount of motor neuron activation, to increase power output (article 1), also caffeine can have a direct effect on the muscle by mobilizing the intracellular calcium, this will increase the efficiency of a muscle contraction and in turn increase power output (article 4). Hydrogen ions increase as a result of anaerobic glycolysis, so caffeine ingested showed a greater concentration of hydrogen ions so therefore greater use of glycogen to produce ATP (article 1). Facilitation of the CNS may have caused the perception of effort decreased in subjects while exercising (article 4, objected in article 3).

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