World’s favourite drug
Well known for its effects on reducing tiredness and improving alertness, caffeine has emerged in the 21st century as the most popular psychoactive drug in the world. In this day and age, caffeine can be found in a variety of food products including coffee, tea, soft drinks, energy drinks, chocolates and others. Furthermore, since the isolation of caffeine from coffee beans in 1820, the therapeutic use of caffeine both as an additive to other drugs or as a stand alone tablet has gained attractiveness in various pharmacological applications. In North America, the most common source of caffeine comes from drinking coffee beverages. In fact, more than 75% of the population reported drinking coffee on a regular basis and about 50% reported drinking at least 1 cup of coffee a day. Many consumers drink coffee for its invigorating flavour, many others drink coffee for its social benefits; however, most coffee drinkers rely on the caffeine in coffee to wake up in the morning, to get through a tough day at work or to stay awake enough to finish an assignment. The attractiveness of caffeine comes from its strong psychostimulating properties with minimal side effects. As such, caffeine is often regarded as the miracle drug that allows individuals to function beyond their normal physiological and psychological limits. With such a growing popularity, many urban myths regarding caffeine’s beneficial as well as toxic effects arise. This report will discuss the current understanding on the effects of caffeine in the human body.
Natural sleep and wake cycles in humans depend largely on the activation of cell membrane adenosine receptors in the striatum of the brain. Adenosine receptors are ubiquitously distributed in the human body with a variety of functions. The natural ligand for these receptors, adenosine, is a metabolic end product of most cells. Throughout the day, adenosine builds up in the circulation. The build up of adenosine in the brain binds and activates the adenosine receptors, which then causes decreased activity of the neurotransmitter dopamine. Dopamine in the brain has a stimulating effect causing improved cognition, motivation, increased movement and awareness. Therefore, by decreasing dopamine activity, the body falls into a state of fatigue indicating the need to sleep. During sleep, circulating adenosine is metabolised and cleared thereby restoring dopamine activity causing the brain to wake up.
Within 45 minutes following caffeine consumption, it is completely absorbed and distributed throughout the body. Circulating caffeine can readily cross the blood-brain barrier. Due to its structural similarity with adenosine, caffeine in the brain can act as an antagonist such that it binds adenosine receptors but do not activate them. Therefore, binding of caffeine reduces adenosine activity. This increases dopamine activity, which is responsible for most of caffeine’s psychostimulating effects. The clearance of caffeine varies among individuals depending on age, health conditions and current medications. In healthy adults the half-life of caffeine is approximately 3-4 hours. The half-life is much longer in patients with liver diseases, in young children and in women who are pregnant or taking oral contraceptives. Since caffeine blocks the binding of adenosine to its receptors but do not induce adenosine clearance, once the circulating caffeine level drops, the individual will feel fatigue and tired again. In other words, caffeine does not eliminate the need to sleep, but simply temporarily mask the sensation of sleepiness. Furthermore, while functioning on caffeine, the adenosine builds up even more in the brain such that when the caffeine effect wears off, the fatigue felt is much stronger than before caffeine consumption. Basically, caffeine can allow the individual to stay focused for a few hours, but the recovery sleep required is often more than just a few hours.
Adenosine build up in the brain also acts on the blood vessels to induce vasodilatation and increase blood flow, which are necessary for deep sleep stages. Needless to say, caffeine will antagonize these effects too preventing deep sleep stages from occurring. This explains why it is difficult to fall asleep or feel refreshed after sleeping while under the influence of caffeine.
Alcohol, like caffeine, is easily absorbed into the circulation and can pass through the blood-brain barrier with ease. Once in the brain, alcohol acts by increasing the turnover of dopamine thereby acting as a depressant and induces fatigue. Since caffeine is so effective at temporarily restoring alertness and keeping the individual awake, many people believe that drinking coffee after consuming large amounts of alcohol will speed up the sobering process. This is, however, false. Caffeine consumption, as described earlier, can increase dopamine activity. This will counter the alcohol effect and increase the individual’s alertness and awareness. However, alcohol also acts on various other parts of the brain including the cerebellum, which is important in muscle coordination for movement and balance. Caffeine cannot fully restore these detrimental effects. Therefore, drinking coffee after large amounts of alcohol consumption is actually more dangerous because it will give the individual a false sense of soberness. The individual would feel more awake and alert but his/her motor and coordination skills are still impaired.
The hangover experienced the morning after heavy alcohol consumption is due to vasodilatation of blood vessels in the brain as a result of dehydration. Caffeine can help reduce the symptoms of hangovers by again antagonizing adenosine receptors causing vasoconstriction. For this reason, caffeine is often included in many prescriptions and off the counter drugs for treatments of headaches.
Many commercially available diet pills have caffeine as an active ingredient. Surely caffeine can help increase motivation and activeness, but does caffeine have any direct effect on weight loss? Body weight depends on two factors: food intake and energy expenditure. Behavioural studies have shown reduced food intake after caffeine consumption but the exact mechanism remains unclear. However, contradicting argument exist stating that caffeine stimulates release of the stress hormone cortisol, which is associated with increased appetite. Therefore, the effect of caffeine on appetite remains highly controversial.
Circulating caffeine can stimulate activity in the adrenal gland increasing production and release of epinephrine (adrenalin) and norepinephrine (noradrenalin). These two hormones are the main activators of the sympathetic nervous system to induce a “fight-or-flight” response. On top of increasing heart rate and blood pressure, they also induce breakdown of stored energy into blood glucose. In this manner, caffeine can act to reduce body fat stores to feed the increased energy expenditure. However, these effects only last in the few hours before circulating caffeine is cleared. Repeated use of caffeine would build up tolerance to its effects and has not been shown to be feasible in causing long term weight loss when used alone.
Starting in the mid 1990’s, rumours arise stating that certain soft drinks would cause decrease in sperm count in men as well as other genital defects. These were attributed to the introduction of soft drinks that had caffeine contents higher than the existing brands at the time. Some even believed that caffeine is a spermacide and can act as a contraceptive. Studies have found no correlation between caffeine consumption and these claims. However, a recent study published in 2007 showed that men with substantial daily caffeine consumption (> 3 cups of coffee a day) have significantly increased sperm DNA damage associated with double-stranded DNA breaks. Following successful fertilization, these defective sperm DNA can be converted to chromosomal aberrations and genetic mutations. These will greatly increase the risk for developmental defects in the offspring. Ironically, caffeine has been shown to increase sex drive in female mice. The aphrodisiac effects of caffeine on humans still remain unclear though many believe it to be true. The mechanisms of this action are still being studied.
Miracle tonic or devil’s drink
Caffeine has also been shown in some preliminary studies to reduce risk and alleviate symptoms of Parkinson’s disease and heart diseases. It has been shown to improve short-term memory formation and thereby improve learning. But contradictory results exist as well. Very high doses of caffeine, equivalent of 60 cups a day, have been shown to prevent damage on hair follicles. This prompted cosmetic corporations to develop caffeine-rich ointments for topical application to bolding heads. Caffeine has also documented ergogenic properties to increase muscle work capacity to improve strength, speed and endurance. This has become a valuable marketing tool for caffeinated sports and energy drinks. With all these positive advertisements of caffeine consumptions, it almost seems like caffeine really is a miracle drug with little side effects. However, one must be careful when digesting these types of information. Many of these studies conclude based on preliminary results extrapolated from animal studies and their exact effects on humans are not well documented. Furthermore, in often cases the studies were done with much higher doses of caffeine than a cup of drip coffee. It is true that when taken in moderation, caffeine is relatively harmless with no long-term side effects. However, overuse of caffeine can have many detrimental effects ranging from muscle twitching to peptic ulcers. Extreme high doses of caffeine from ingestion of high quantity caffeine tablets can actually result in instant fatality due to overworking of the body.
Many who drink coffee or consume caffeine tablets are still chasing urban myths hoping to lose a few pounds, improve their memory, build up muscle or grow a head full of hair. Until these effects have been confirmed with more scientific studies, one must not expect coffee drinking to perform such miracles. If anything, love coffee for its flavour, drink it for its social benefits, use it for momentary focus for that is all it can do.