By Peter Rogers, University of Bristol
Caffeine is humankind's favourite drug. It's estimated to be consumed by 80% of people worldwide, and most of us consume several doses a day. This popularity would appear to be explained in large part by caffeine's psychostimulant effect - starting the day with tea or coffee helps us wake up, and later on rescues us when our energy begins to decline.
The alerting, and other less noticeable effects of caffeine, include enhanced physical performance, less hand steadiness, increased blood pressure and increased anxiety - "jitteriness".
When exposed to a drug, our physiology mounts varying levels of responses to counter these effects and maintain normal functioning. Our bodies' tolerance to caffeine is low when it comes to physical performance and high to anxiety - which is a good thing. But tolerance is also high to wakefulness.
And unfortunately, the tolerance to the wakefulness effect means that what we most value in repeated caffeine use is really illusory.
Caffeine cold turkey
The elimination of caffeine, as it is metabolised in the liver and excreted in the urine, leaves our bodies in a physiological state adapted to the presence of caffeine. So the sense of alertness we get from regular, frequent consumption of caffeine - our daily cups of coffee - is only reversing the drop in alertness we get when caffeine is eliminated from our body. This essentially means that we use caffeine to counter the effects it caused in the first place.
Decreased alertness due to caffeine withdrawal, as well as the related falls in performance in tasks that require sustained attention, is easily noticed overnight when we don't get our caffeine fix and has been observed in various studies measuring alertness and vigilance.
Also well documented is the effect of caffeine on motor speed and endurance. Although the physiological mechanism isn't fully understood, caffeine provides a small net advantage to athletic performance. So caffeine can make us faster, but not smarter.
This is illustrated in a single, simple test. In a choice/reaction time task, caffeine doesn't reduce the number of errors made by frequent consumers compared with non-consumers receiving placebo but it does slightly reduce the component of reaction time.
Our pattern of caffeine consumption usually allows us to avoid significant adverse effects of caffeine withdrawal. But an intermittent pattern - for example drinking significant amounts of coffee during the working week but not at weekends - risks a sleepy and headachy Sunday.
Caffeine causes vasoconstriction (a narrowing of the blood vessels caused by contracting vessel walls, which is how it raises our blood pressure), but when we go through a withdrawal, a re-widening of the vessels increases blood flow to the brain, leading to "caffeine withdrawal headache".
There are lots of reports on the positive and negative consequences of consuming caffeine - even a claim that it could cut the risk of suicide. The latest includes a study suggesting a positive association between coffee intake and deaths in younger people (aged under 55).
The reason for this is unclear. It's quite likely this isn't caused directly by consuming coffee, and therefore not a reason to cut down, but from other associated, though unknown, behaviour in people who do drink a lot of coffee.
In its entirety, current evidence indicates that tea and coffee consumption isn't associated with significant adverse health effects. In view of caffeine's blood pressure raising effect, it's surprising that coffee and tea don't increase risk of cardiovascular disease, stroke and cognitive decline. Some studies have even found consumption to be associated with a reduction in cardiovascular and/or cerebrovascular disease. This suggests that other effects of tea and coffee must outweigh the negative consequences.
These benefits include polyphenols, which have various good effects on our vascular health, and theanine, an amino acid present in tea that lowers blood pressure. Another repeated observation is that coffee intake is associated with reduced risk of Type 2 diabetes.
Unfortunately, consumption of cola (both diet and sugared) is associated with a raising of blood pressure, which suggests it lacks the protective substances found in tea and coffee that counter this effect. There's also a higher risk associated with Type 2 diabetes - perhaps because of its sugar content. So the "vehicle" in which we consume caffeine matters.
While caffeine appears to be increasingly added to a variety of products, tea and coffee remain by far the main sources of caffeine worldwide. In the UK around only 3% of adults consume no caffeine (if chocolate, which naturally contains small amounts, is included). And coffee, which has the highest amount of caffeine, is overtaking tea and as our number one source.
From a public heath perspective, most caffeine consumption is probably quite safe. But if you decide to give up, how hard is it? Actually, rather easy. Everyone who consumes the equivalent of more than three cups of tea or two cups of coffee, and possibly much less, is caffeine dependent (in that continued caffeine consumption is necessary for the body to maintain "normal" functioning). But adverse withdrawal effects do begin to abate within a few days with normal functioning resumed around a week or so.
Withdrawal also doesn't appear to be accompanied by a craving or compulsion to consume caffeine, which would point to addiction. Caffeine doesn't have very strong positive psychoactive effects - affecting mood, thinking, behaviour - and higher amounts have unpleasant effects, such as anxiety. Dependency might be high, but the risk of an addictive relationship is low. But even eating carrots poses a small risk of addiction.
And tea and coffee can still be enjoyed without caffeine, as done by this decaffeinated author, while still giving you the health benefits.
Peter Rogers has received public money and funding from industry to support research into the effects of caffeine on human behaviour