1998 2ND PLACE

"Would you like a cup of coffee?"

By Paul Ganter

Can this seemingly ordinary question lead us to the understanding of our eating behaviour or maybe even to problems like obesity? Most of us will know from our own experience the responses elicited by this offer: attraction, pleasure, desire.

A cup of hot, freshly made coffee is appealing, especially in the mornings. Imagine, however, you had already drunk a couple of cups of the delicious drink and someone came again and asked you - after a certain coffee-intake your reaction will change to dislike, aversion or even active rejection. Why is that? Is it the taste which changes? Certainly not; it is still the same coffee and you will know from your own experience that you can still taste it perfectly well, though you will no longer regard it as pleasant.

The scientific term for this phenomenon is "sensory specific satiety". It is tightly related to the emotional aspect of taste perceptions. Another well known fact is, that being "fed up" with one food, a different one can still be extremely appetising. Just think of a neatly prepared pudding after a five course meal. This shows that it is indeed a response specific to a certain taste.

Asking about the neural basis of this feeling of having enough, one will naturally start by investigating the taste system. The taste-receptors, responding to a variety of basic stimuli (sweet, salty, sour, bitter and, as it was recently found, "protein" tastes, maybe also others), send signals about the food to the brain. In one area (the so-called "primary taste cortex") the entire information is about what the food tastes like; the same food will always produce the same response there. But then there is a different region (the "secondary taste cortex") situated in the more frontal part of our brain, where the incoming signals are analysed about how nice the food tastes. And this is exactly the region where one can find the explanation for the phenomenon we are looking at. The nerve cells in this region change their activity in response to the incoming signals. For the first cup of coffee the cells get vividly excited reflecting the attraction of the pleasant taste. Seventeen cups later however, not only will your coffee pot be empty but the cells will also have decreased their activity and finally stopped it, thereby telling you that you had enough.

Experiments have been done which showed that this decline in activity is independent of actual food intake! In these experiments, volunteers were asked to chew the food only but not to swallow it, which led to the same results.

The implications of this mechanism are as stunning as they are simple: give someone a bigger choice of food and he/she will eat more.1 In other words: variety can lead to obesity!

On the other hand an unbalanced supply of food will lead to a non-illusive decrease in appetite, so one can appreciate the role of spices.

Furthermore, we have here one of the reasons why people prefer to change the room after a dinner in order to return for coffee: taste is not completely independent of other sensations, as for example smell! These interactions count for the fact that a change in odour increases the pleasantness after a meal. Some people might take that as an excuse for the next after dinner cigar.

Finally, it gives us an explanation for the sporadic loss of the emotional response to food after head injuries 2 affecting the forehead; taste of food can still be differentiated but it simply doesn't matter any more for those people.

So the simple question "Would you like a cup of coffee?" can lead us to explore brain mechanisms which are as essential in our daily life as for some of us the coffee itself.

1. These experiments have been done!

2. Particularly common after motor-bike accidents.