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Caffeine Overview

Caffeine is a powerful CNS stimulant that decreases fatigue and increases wakefulness. Perhaps the worlds most popular drug,  an estimated 90% of North Americans consume caffeine on a daily basis. Caffeine is naturally occurring and is found in high concentrations in tea leaves and coffee beans.

Inside the brain, caffeine works by blocking the action of adenosine which increases dopamine, serotonin and adrenaline levels. Numerous studies have found that caffeine can delay fatigue, improve exercise, increase reaction time,  and increase subjective feeling of motivation.



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Important Information

Sources of Caffeine

Caffeine is naturally found in the seeds and leaves of different plants included cocoa beans, coffee beans, guarana (Paullinia Cupana), kola nuts, tea leaves (Camellia sinensis) and yerba mate (46). Caffeine is also chemically synthesized  and is frequently added to sodas/carbonated beverages, drug products and energy drinks.

Other names

Alert-pep, Cafeina, Guaranine, Koffein, Mateina, Methylytheobromine, Theine, Thein

Caffeine Dosing

For a total caffeine newcomer, 100mg would be a reasonable place to start. Caffeine tolerant individuals typically ingest anywhere from 200-600mg per day.

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Background Information

Caffeine is considered, by far, the most commonly used drug in the world (1, 2, 3). Data in a 2005 study revealed that a staggering 87% of Americans consume caffeinated food and fluids with an average intake of 193 mgs per day (4). An earlier study also suggests that individuals from Denmark, Germany, Netherlands, Norway and Sweden have the highest caffeine intake from coffee with an average of 300 mgs per day (5).

Caffeine Structure


The chemical name of caffeine is 1,3,7-trimethylxanthine. Caffeine is a xanthine compound that is comprised of three methyl groups that are attached to the 1, 3 and 7 carbons of the xanthine backbone (8). Pure caffeine is naturally colorless and is moderately soluble in water at room temperature but very soluble in boiling water.

Caffeine Absorption, Distribution, Metabolism and Excretion

Caffeine is water soluble and is almost completely absorbed in the gastrointestinal tract after oral intake (25). Human studies have shown that peak plasma concentration after 30 to 60 minutes (23). The half life of caffeine is 2-5 hours (24).


Caffeine is metabolised in the liver by the cytochrome p450 enzyme system into paraxanthin, theobromine and theophylline. A variety factors can speed up or slow down caffeine metabolism, including liver function, certain medications. Of particular note:

  • The antidepressant fluvoxamine increases caffeine increase the half life 10 fold to 56 hours (26)
  • The contraceptive pill doubles caffeine’s half life (27)
  • Heavy smokers appear to have a decreased half life of up to 50% (27)
  • Pregnancy can triple caffeine’s half life in the third trimester (27)

Caffeine Benefits

Improved physical performance

One benefit associated with caffeine intake is improved physical performance.  Numerous studies also demonstrated that caffeine can increase muscle performance and high intensity short term as well as long term/prolonged exercise performance (3, 9, 10, 11, 12, 13, 14).

Improved Arousal and Motivation (Short Term)

Studies have also shown that caffeine has favourable effects on cognitive performance enhancer (9, 16, 17). Among these studies, one interesting subject has been identified – that is the effect of caffeine intake on motivation.

Research involving endurance cyclists demonstrated that pleasure ratings. perceived exertion and motivational states were improved with caffeine ingestion relative to placebo. This short term boost in motivation is probably related to caffeine’s effects on dopamine and other neurotransmitters. (17).

Improved Cognitive performance

A great deal of studies has been undertaken to assess the effects of caffeine on memory and cognitive performance. Caffeine can  increase reaction time (33) and help sustain attention on certain cognitive tasks. Some studies have also shown that it caffeine in motor skill performance, attention and subjective alertness (18, 19).

Caffeine’s Mechanism Of Action


Adenosine receptor antagonist

Caffeine is an adenosine receptor antagonist. Adenosine binding to the adenosine receptor is generally inhibitory in nature. In essence, caffeine binds to the adenosine receptors in place of actual adensoine thus blocking it’s effects. This process is also known as competitive inhibition.


In rodents, caffeine increases acetylcholine release in the prefrontal cortex (29). These effects are thought to be downstream of adenosine antagonism, in particular A1 and A2(a) receptors.

Dopamine release

With the competitive inhibition of adenosine,  dopamine release is  increased. Increased facilitation of dopamine would therefore help in improving memory function and could be responsible for some the motivating effects felt after consuming caffeine. In one study, it was revealed that increased dopamine from caffeine intake helps in improving working memory amongst extroverts (20, 21).


There appears to be a dose dependent increase in adrenaline following caffeine intake (31). In rodents, chronic caffeine intake is associated with a decrease of beta adrenergic receptors (30).

Increased Testosterone Levels

Taken pre-exercise, caffeine has been shown to modestly impact testosterone levels.

One study (32) aiming to identify dosing effects of caffeine on cortisol and testosterones revealed that caffeine increased both testosterone (15%) and cortisol (50%) in a dose dependent manner.

Another study (28) revealed that caffeine reduces accumulated fatigue which led to substantial physical performance amongst sprint cyclists. In this particular study, sprint cyclists ingested caffeinated chewing gum before undergoing four exercise sets. The decrease in fatigue experienced by the sprint cyclists was also associated with increase in testosterone levels and decrease in cortisol levels as compared to placebo intake (regular chewing gum).

Testosterone is known as a potent inductor of protein synthesis and in muscle hypertrophy – both of which are important in intense physical activities (11).

Caffeine: Safety and Side Effects

Caffeine safety

The US Food and Drug Administration (USFDA) limits beverages to contain no more than 0.02% caffeine. Previously, the World Anti-Doping Association (WADA) has banned its use; its removal on the banned list of drugs however gave the opportunity for athletes to exploit its ergogenic potential (2).

Caffeine toxicity

Men and women aged 19 and older are recommended to have an intake of not more than 400 mgs of caffeine per day. Pregnant and lactating mothers on the other hand are recommended to have an intake of not more than 300 mgs of caffeine per day. These caffeine amounts are considered safe (6).

Too much caffeine intake (more than 400 to 500 mgs at a time) may lead to toxicity characterized by anxiety, excitement, insomnia, irregular heartbeat, muscle twitching, slurred speech, muscle twitching and irritability. Larger overdose may lead to delusions, psychosis, disorientation, judgment lapses, depression and mania (5).

Caffeine side effects addiction and tolerance

Caffeine addiction or dependence occurs with repetitive and frequent caffeine intake. With frequent intake, its effects wane overtime hence, will lead to tolerance (5). Noted side effects of excessive caffeine intake are irritability, nervousness, palpitations, insomnia and rapid heart tate.

References used