Podium: Is that first cup of coffee really a pick-me-up?

Extract from a speech given by Bristol University's senior lecturer in psychology to the British Association Festival of Science

CAFFEINE IS the most widely and frequently consumed drug in the world. Coffee and tea account for the vast majority of this intake, estimated at about a cup of coffee's worth of caffeine consumed by every person on Earth every day. Therefore, even if caffeine has only a small effect on the health and functioning of individuals, its overall impact on human well-being is likely to be significant.

This impact will occur owing to the effects of caffeine itself and through the effects of other constituents of caffeine-containing drinks. Currently, for example, there is considerable interest in the possible beneficial effects of antioxidant compounds consumed in tea. The evidence for the beneficial effects of caffeine is mixed. The view that caffeine is a useful psychostimulant is challenged by findings showing the negative effects of short-term abstinence in regular consumers, including fatigue, headache and impaired psychomotor performance.

Why do we consume caffeine-containing drinks? One factor influencing our consumption is the recognition of their potential psychostimulant properties. Thus the coffee-drinker may choose to consume coffee at breakfast for its expected alerting effects, but may then avoid coffee late in the evening because it could lead to difficulty in getting to sleep.

However, if people are asked why they drink coffee or caffeine-containing drinks, they are likely to say that this is because they like the "taste" of the drink - people typically do not consume coffee as if it were a medicine, prepared to tolerate its taste in the expectation of a benefit. At the same time, it is fairly certain that human beings are not born with a liking for the taste of coffee or tea, because these drinks contain bitter constituents and bitterness is innately aversive. This raises the question of how people come to acquire a liking for the sensory qualities of these drinks.

An important way in which liking is modified is through the association of the taste of foods and drinks with the after-effects of eating and drinking. The most dramatic example of this is the strong and specific aversion that can develop when consumption of a food is followed by stomach pain, or vomiting. Similarly, there is now good evidence that association of a taste or flavour paired with positive after-effects can result in increased liking for that specific taste or flavour. The potent pharmacological activity of certain food and drink constituents may be particularly important in this respect, so that, for example, a liking for the taste of coffee, tea, beer or wine is reinforced by the psychoactive effects of caffeine or alcohol.

We tested this idea in studies in which caffeine ingestion was paired with the consumption of novel-flavoured fruit juices. Caffeine was given either in the drink or in a capsule swallowed with the drink. A drink of a different flavour was given without caffeine, or with a placebo capsule. The results showed that caffeine acts most reliably as a reinforcer in the context of short-term caffeine withdrawal, and perhaps when relieving other negative states such as the feelings of fatigue experienced by some people after lunch. In other words, caffeine influenced participants' liking for the drink mainly because it relieved the negative effects arising from withdrawal from the caffeine they had consumed the previous day.

These findings led us to re-evaluate the research claiming to show that caffeine has useful psychostimulant properties. Such effects have been widely demonstrated in placebo-controlled studies, but in almost all of these studies participants had a history of regular caffeine consumption, and they were tested on caffeine and placebo after caffeine abstinence.

What this experimental protocol leaves open is the question of whether the results obtained are due to the beneficial effects of caffeine, or to the deleterious effects of caffeine deprivation. That is, lower alertness after ingesting a placebo may be due to the fatiguing effects of caffeine withdrawal, which are reversed by caffeine administration, thereby merely reinstating alertness to a "normal" level.

In other words, although regular caffeine consumers "feel" a benefit when they consume caffeine, this is much less than the real or "net" benefit they gain.

The writer of the Podium piece on 22 September was Dr Johann Wiechers, not Dr Chris Dederen as stated