Escrito por: Karen Fick
Drinking a cup of coffee makes most of us feel good, even very good - it tastes great (if well prepared), energises, can improve mood, increase concentration and short term memory and even make us feel more sociable. But how does it do this?
Coffee contains caffeine which is probably the world's most widely consumed psychoactive drug. Inevitably, such wide consumption has led to a vast array of studies into the complex effects of caffeine on the body.
Here are a few of these psychological and physical effects, which will vary from one person to the next, depending on the amount of caffeine consumed and its source.
Caffeine: a most enjoyable white powder
Used globally as a stimulant for many centuries, caffeine was first isolated as a chemical compound in 1819 by the German chemist Friedlieb Ferdinand Runge. In its pure state, it is white and odourless and has a bitter taste not dissimilar to cocaine which is in the same molecular school.
Caffeine is soluble in water and can be extracted from the coffee bean at low water temperatures and more rapidly at higher temperatures. It's possible to extract nearly all the caffeine from the bean thus leaving decaffeinated coffee.
About 1.5% of a coffee bean is caffeine and this caffeine accounts for around 10% of the bitter taste in your cup. The amount of caffeine per cup varies according to several factors including temperature of extraction, type of coffee bean, size of the grind and the pressure at which the water is put through the grind. On average, a 200 ml cup of filter coffee contains around 0.12g of caffeine.
A natural poison
Along with cocaine, atropine, quinine and the active ingredient in the deadly nightshade plant, caffeine is one of many naturally occurring organic chemical compounds known as alkaloids. All alkaloids have a physiological effect on the human body ranging from medicinal to poisonous.
Alkaloids have a complex chemical structure that always includes at least one nitrogen atom, usually alongside oxygen, carbon, and hydrogen. The chemical name for caffeine is 1,3,7-trimethylxanthine.
The caffeine alkaloid is found in over 60 other plants besides coffee, including the opium poppy, tea, cacao and cola nuts but it can also be made synthetically. Growing conditions, the variety of coffee bean and processing methods all have an impact on the caffeine content of each coffee bean.
The coffee plant is found in the tropics where lush growing conditions mean that survival is quite a competitive issue and plants need good defence mechanisms against pests, disease and fungus. Because caffeine is a natural deterrent against insects it helps the coffee plant to thrive in this challenging environment.
Robusta beans contain a good deal more caffeine (anything from 40 to 80% more) than Arabica beans and so have a much more bitter taste. Because of its higher caffeine content the Robusta plant exhibits better disease resistance than Arabica so it can grow at lower altitudes and at higher humidity levels than Arabica which would be destroyed by fungus, disease and pests in equivalent situations.
Alkaloids are also found in a handful of animals including the poison-dart frog which gets its supply of the deadly chemical lipophilic alkaloid by consuming poisonous alkaloid bearing rainforest plants. The frogs secrete the alkaloid through their skin as a defence mechanism against predators. Natives of the Amazon rainforest use poison from the poison-dart frog to tip their hunting weapons - but we're getting off the point here.
And in the human body?
Caffeine, along with tobacco, alcohol, cannabis, ecstasy, cocaine and heroin is a psychoactive drug, meaning it can be a mood-changer. It acts as a stimulant to the nervous, circulatory and respiratory systems, dilates blood vessels and sends extra blood to the muscles, resulting in bursts of energy, and it is a mild diuretic.
The effects of caffeine on the body can be felt almost immediately after consumption and its impact peaks about 30 minutes later. About three hours after consumption effects are half what they were at 30 minutes and the caffeine can remain in the body for over twelve hours until the last traces are removed in the urine.
Caffeine in its whole molecular form, i.e. as caffeine itself, has one set of effects on the body, and because the liver is very good at metabolising (breaking down) caffeine into smaller molecular compounds, these have another series of effects.
The brain tricker
Caffeine absorption starts on the tongue and in the throat. Because it is both fat and water soluble it passes quickly and easily into the blood stream and beats a rapid path to the brain where one of its principal effects is to reduce drowsiness. Caffeine can do this because it has a molecular formation that is almost the same as adenosine, a hypnogenic substance that acts to inhibit neurotransmitters that form part of the central nervous system. In other words, adenosine allows the body to talk to the brain to tell the brain when it's doing too much. It is a by-product of neural activity (brain busyness) produced in the brain throughout the waking day with levels increasing by the hour. Adenosine binds itself to the neurotransmitters that generate brain activity, slows them and eventually stops them from firing at all, resulting in drowsiness and eventually sleep. The more adenosine in the brain the drowsier you feel. Stress produces high levels of adenosine which is why stress can cause fatigue. During sleep adenosine levels fall to a level that is favourable to alertness and awakening.
What has this got to do with coffee and caffeine?
Because it has a similar molecular structure to adenosine, caffeine acts as a competitive inhibitor to adenosine, binding to the same neurotransmitters that adenosine binds to, thus blocking up adenosines docking stations. As it isn't detecting any tiring adenosine, the brain doesn't feel drowsy - on the contrary, not only does caffeine prevent the nervous systems adenosine receptors from shutting down, it actually encourages the neurons to fire faster.
As the body is now in a state of mild uproar the pituitary gland stimulates the fight or flight branch of the sympathetic nervous system by releasing epinephrine (also known as adrenaline). Now all set and ready to deal with a crisis the pupils are dilated, heart rate has increased and sugar is being released by the liver to supply extra energy. This is the caffeine buzz and you can see why it's probably not a great idea to keep the body in this prolonged state of emergency by consuming too much caffeine.
At the same time the pituitary gland increases levels of dopamine, serotonin and acetylcholine (the levels of which some classes of antidepressant drugs are designed to keep high) which is why caffeine really does have a positive effect on mood, concentration and memory.
What about the caffeine that makes it to the liver?
Caffeine that makes it down to the liver is metabolised into paraxanthine, theobromine and theophylline. Paraxanthine is the chief metabolite of caffeine and has the same adenosine antagonistic effect in the brain as caffeine. It also promotes lipolysis (the breakdown of fats) and these fats are released into the bloodstream to fuel muscles and even enhance sports performance - think sports drinks.
Theobromine opens up blood vessels, increasing the flow of oxygen and nutrients to the brain. The opening of blood vessels also speeds up the filtering process in the kidneys, i.e. theobromine is a diuretic which is why you often need to go to the loo soon after drinking a cup of coffee.
Theophylline relaxes smooth muscle tissue, the kind found in the respiratory and digestive tract.
Is caffeine addictive or bad for you?
Research suggests that the body can develop a mild physical dependence on caffeine with withdrawal symptoms including headaches, nausea, fatigue, irritability and anxiety, though these seem to disappear within a few days.
Studies also suggest that high caffeine intake can lead to the development of additional adenosine receptors so that when there is no caffeine available and adenosine is free to bind to all the extra receptors the brain can feel even drowsier than normal. It's likely that these additional receptors soon disappear if caffeine intake is reduced.
Because caffeine can lead to a rise in levels of dopamine, serotonin and acetylcholine (the mood controlling neurotransmitters), its withdrawal can lead to irritability, anxiety and lack of concentration. To alleviate these symptoms the user might consume more caffeine for a temporary fix and in severe cases this cycle can lead to depression.
As caffeine has a diuretic effect high consumption can lead to frequent urination which in turn can irritate the bladder, leading to bladder spasms and eventually urinary incontinence. This symptom is particularly prevalent in women.
And never forget this
Caffeine does not reduce the effects, or cancel out the effects of alcohol, it simply masks intoxication by energising so don't be fooled into thinking that you are sobering up if you have a large cup of coffee before you drive home.