or tired, then you probably don't have to worry too much about being overweight. If, on the other hand, you have an absolute need to run two miles or more as fast as you can in order to burn off as many calories as possible, then you may well have a problem at the anorexic end of the scale. If your body weight varies from year to year by over seven pounds up or down - or both up and down at different times - then you may well have an eating disorder, particularly where there is no other obvious medical or social reason for that change. You may also have an eating disorder if you control your body weight by alternately bingeing and starving, by exercising excessively or by vomiting or purging.

Similarly, if your exercise level changes dramatically from year to year other than in response to a specific medical or social change, then again an eating disorder (in the form of the commonly associated exercise addiction) may be a possible cause. That being said, the vast majority of people who are on diets at any one time do not have eating disorders and do not have any significant medical condition. They are simply trying to change the way they feel by changing the way they look to themselves and, by inference, to other people.

Telling a classroom of sixteen-year-old girls that putting on four or five pounds in weight will probably not be of any medical importance whatever would be likely to be greeted with derision. They would ask what planet you came from. For them, an increase in body weight of four or five pounds would be considered to be an utter disaster and it is unthinkable that it could be considered to be medically irrelevant. These are normal perceptions in normal sixteen-year-old girls: a great deal of their self-image is centred upon their physical appearance, as interpreted by themselves and within their socio-cultural group. Young men might have equally fixed impressions on the importance of muscularity. They may play sports in order to demonstrate not only their physical fitness but also their attractiveness. If only human relationships were so simple!

It would be nice to believe that we would grow out of these false perceptions when we are no longer teenagers - but we don't. Many women retain an admiration for the "androgynous" body shape, resembling that of men. They may hate their breasts, bellies and thighs precisely because they look like the breasts, bellies and thighs of women. The distribution of their body fat is under direct control of ovarian hormones (or, to be scientifically precise, to the absence of male hormones). Correspondingly, men tend to go haring off to the gymnasium as soon as they suffer an emotional crisis such as discovering that their wives have been unfaithful to them. In each case our self-perception is very much tied up with our body weight and shape.

It is this self-perception, so intricately bound up with our emotional lives, that results in the diet industry being so massive. The very fact that slimming magazines can survive beyond their first and second issues is manifest proof that