|
|
Caffeine - a wonder drug?
 |
|
|
Although it is common knowledge that caffeine is a stimulant, what are the specific effects of caffeine on mind and body? Perhaps for convenience we should separate the actions of caffeine into psychological and physiological actions.
Caffeine stimulates the brain and spinal cord, and at ‘moderate' doses enhances alertness, energy, motivation and concentration. Caffeine is a powerful psychostimulant, significantly increasing cognitive (understanding & reasoning) performance, and has also been found to improve the perception of well-being in individuals deprived of sleep for two days. Coffee is used relieve both normal and tension headaches, which are characterised by the dilation (widening) of blood vessels around the skull, often in combination with a painkiller like aspirin or ibuprofen.
Caffeine is also widely used in the treatment of migraine in combination with an ergot alkaloid. Its effectiveness is largely due to its actions in antagonising (countering) the effects of the brain transmitter adenosine. Adenosine is an important modulator of brain activity, possessing sedative (calming), anxiolytic (anxiety dissolving), and anti-convulsant properties. Adenosine also dilates blood vessels in cerebral and coronary circulatory networks, a cause of many headaches, whilst caffeine produces the opposite effect, constricting the blood vessels. Indeed chronic caffeine exposure has been shown to increase the number of adenosine receptors in the brain, and explains why adenosine activity increases with coffee consumption, and why abrupt withdrawal from coffee consumption causes tension headaches. Another possible explanation for the potent psychostimulant effects of caffeine is because it enhances the release of the brain's natural excitatory transmitters such as glutamate and aspartate. Indeed, in our hands a physiological dose of caffeine stimulated an almost 200% increase in the release of glutamate from nerve terminals in the hippocampus, a region of the brain involved in learning and memory.
Brain imaging studies confirm that a caffeine dose of 200 milligrams decreases blood flow through the brain by a third within an hour of administration, most likely by affecting adenosine receptor activity. Given that the average daily dose of caffeine in the United States (slightly more than two cups of coffee per person) is around 238 milligrams, this is likely a typical response. Indeed so profound are the effects of caffeine on adenosine receptors that habitual users show altered patterns of blood perfusion in their brains, so "the more caffeine you drink on a regular basis, the higher your cerebral blood flow will be when you do not consume caffeine" according to researchers.
Caffeine reduces brain blood flow in the grey matter by about 26% in heavy coffee drinkers and by 19% in lighter drinkers. The image inset shows an FMRI image demonstrating activation of a structure associated with fear and anxiety called the amygdala in response to the infusion of caffeine.
Researchers have found that consuming coffee and caffeine has little or no harmful effect on exercise performance in healthy people, or in those with cardiovascular disease. Although caffeine has a mild diuretic action (promoting water loss through the kidneys through urine), there is no evidence that caffeine consumption causes a fluid-electrolyte imbalance that is harmful to health or to exercise performance. Recent studies on caffeine and dehydration concluded that drinking between one and four cups of coffee a day does not cause significant dehydration, although consuming greater quantities (5-6 cups of coffee a day) substantially increases urine production which may lead dehydration, especially during heavy exercise or in hot weather.
Although caffeine is not an appetite suppressant, it has profound effects upon the metabolism and may even aid weight loss. Caffeine increases the level of circulating fatty acids, which in turn increases the oxidation of these fuels to release energy, thereby promoting fat loss. This is probably why caffeine has been used by runners and endurance athletes for years to enhance fatty acid metabolism and endurance, as fat stores provide an abundance of energy, although caffeine appears more effective in those who are not habitual coffee drinkers. Many studies have suggested that caffeine has a positive effect on exercise performance, by increasing both energy and endurance, so much so that it is on the current list of drugs banned by the International Olympic Committee.
There are additional mechanisms by which caffeine enhances performance. It increases circulating adrenaline (epinephrine) levels, increasing both the rate and output of the heart, and also increases the resistance of blood vessels to the flow of blood by constricting them (increasing blood pressure and therefore blood supply to the muscles). Caffeine also increases the force of muscular contraction during low-frequency stimulation by enhancing the release of calcium from its cellular stores, calcium being the trigger for muscle contractions. Caffeine also increases the metabolism of energy rich fatty acids and thus spares precious muscle glycogen stories, explaining its so-called ‘ergogenic' (energy producing) properties and increased endurance associated with caffeine intake. So perhaps the Ethiopian hunters were right in eating coffee beans wrapped in animal fat all those years ago.
But it's not all good news for coffee drinkers. Caffeine has many side effects, especially at high concentrations. Caffeine may inhibit the protective activity of common anti-epileptic drugs and has been shown to raise the level of lactate, a substance known to produce panic attacks. Some of coffee's components have a mild oestrogen-like effect on the body, worsening the oestrogen-dependent symptoms of premenstrual syndrome, which include tension, irritability, anxiety, fatigue and sleep disturbance. Indeed caffeine may be the single biggest cause of anxiety within our society.
Although the American Institute of Cancer Research and the World Cancer Research Fund have concluded that "Most evidence suggests that regular consumption of coffee and/or tea has no significant relationship with the risk of cancer of any site". Caffeine is but one of many recognised carcinogens (cancer causing compounds) found in coffee, many of which are produced by the roasting process itself. As might be expected, studies linking coffee consumption with a higher incidence in cancers of the pancreas, ovaries, bladder, and kidneys in coffee drinkers are hotly debated.
There is great dispute as to whether caffeine or coffee causes or worsens the symptoms of heart disease and hypertension. One study suggested that there is an increased risk of developing hypertension associated with drinking five or more cups of coffee per day over a long period of time (over 33 years, 2002). However the prestigious Framingham Heart and Harvard University studies found that caffeine intake does not "appreciably increase the risk of coronary heart disease or stroke". Although it is generally accepted that caffeine does cause a short-lived increase in blood pressure, this only lasts for a few hours and is generally less than that normally experienced when climbing a set of stairs. However even a single dose of caffeinated coffee can increase intraocular pressure in people with glaucoma, a vascular disease of the eye associated with diabetes.
Caffeine raises levels of both adrenaline and cortisol, two of the body's primary stress hormones from the adrenal gland. Indeed heavy coffee consumption can cause adrenal gland exhaustion, leading to adrenal "insufficiency", a condition where the adrenal glands are unable to release enough adrenaline in response to stress. Adrenal insufficiency may lead to a weakened immune response and anxiety or panic attacks. In combination with increases in cortisol, which increases blood pressure by causing blood vessels to constrict and by increasing the contractile force of the heart's pumping mechanism, large amounts of coffee may lead to a worsening of the symptoms of people with high blood pressure. Caffeine may not be good news for fertility either. Researchers suggest that women who drink more than one cup a day have a reduced likelihood of conceiving by 50%, and men who drink two or more cups of coffee a day have an increased incidence of abnormal sperm, and those who consume larger amounts often have sperm with reduced motility.
It is perhaps common knowledge that coffee decreases the quality of sleep and is one of the major causes of sleep disturbance. People suffering from insomnia often have elevated caffeine levels when compared to those of normal sleepers, and often as much as 40% of the caffeine they consumed in the morning is still in their system at bedtime due to its slow metabolism within the body. Foetuses and newborn babies are unable to metabolize caffeine in their livers, so it remains in their bodies for up to four days, stimulating their nervous system for the entire period, promoting irritability and difficulty in sleeping.
Of course at the other end of the scale a dose of more than 250 milligrammes (around three cups at once) at one time is necessary to cause symptoms of caffeine intoxication syndrome, whose features include restlessness, nervousness, excitement, insomnia, flushing of the cheeks, diuresis, gastrointestinal disturbance, muscle twitching, disordered thought and speech, tachycardia (increased heart rate), cardiac arrhythmia and periods of inexhaustibility.
Given our love affair with caffeine and its presence in many foods and beverages including tea, soft drinks, chocolate and its availability in tablet form, perhaps one or two of us might feel a little uncomfortable when presented with this list.
|
|
