Lewis Wolpert: The disturbing increase in cases of obesity

Losing weight can become obsessional. One evening when I was putting on my dinner-jacket trousers I found I could not do them up round the waist - hysteria! - but when I came home I found that they were not mine; my stepson had borrowed them and returned from a party with someone else's. Relief - but I am now on a diet and exercise like mad.

Obesity is a major worldwide problem and is listed by the World Health Organisation as one of the top 10 global health problems, claiming an increasing number of lives. It is a cause of life-threatening diseases such as Type II diabetes, heart attacks, strokes, and recent evidence shows that many cancers are associated with obesity. Current estimates put the number of overweight US adults at 60 per cent, and those obese at 30 per cent, a number that has doubled in the past 20 years.

Obesity is defined by a number - the weight in kilograms divided by the square of the height. I am OK as I weigh 83kg and my height is 1.8 metres, and so my number is 25 - just on the border of being overweight, which is 25; obese is 30 or above.

If only we had a good understanding of the mechanisms that regulate our weight and how it could be controlled. There is progress, but there is still some way to go.

The body has both short- and long-term regulators of appetite. Two hormones, one of which is gherlin, produced in the gut, have recently been identified as regulators of short-term appetite, while leptin regulates long-term weight by controlling appetite. Gherlin stimulates the appetite and its increase during the morning stimulates the desire for lunch.

Leptin evolved to increase appetite when food supply was low. When well fed, leptin is high and suppresses appetite but, beyond a quite low weight increased leptin does not act - the reason is not known. Leptin given to overweight people does not seem to slim them down. Insulin, too, is involved, and normal insulin sensitivity is associated with less eating. Clearly, all of these signals are targets for potential drugs. The current drugs on the market are regarded as not very effective.

The causes of obesity seem to be less in the genes than in the environment. The major cause is that energy is too easily available in a wide variety of tasty and inexpensive foods. There is also a reduction in jobs requiring physical labour, and an increase in couch-sitting non-activities.

The average eight-year weight gain in one US study was about 15 pounds in the 30-year-old age group. Calculations suggest that a reduction of energy gain of 50 kcal/day would offset weight gain in 90 per cent of the population. This could be achieved by exercise and a reduction of food intake of just 100 kcal/day.

All this is of particular relevance to children. Childhood obesity is increasing at a most disturbing rate. Parents are going to have to better manage the food intake and physical activities of their children.

Lewis Wolpert is professor of biology as applied to medicine at University College, London