Science: Girth of a nation

UNDER THE MICROSCOPE; WEIGHTY MATTERS TO CHEW OVER

FAT IS NOT just a feminist issue. It is, according to the World Health Organisation, a major health problem that is getting worse. Obesity, which is largely due to excessive fat, is a medically defined condition - it refers to anyone who is more than 30 per cent overweight for their height. A very reliable measure of obesity can be obtained by simply measuring the waistline.

In this country the number of people defined as being obese has doubled over the last 10 years. The current figure of 17 per cent of the population is a figure not very different to that found elsewhere in the European Community. In the USA it is even worse - the figure there being as high as 33 per cent - and they have had to widen the seats at the Yankees' Stadium.

Women in the lower social classes are particularly vulnerable. The health target in the UK has been set at 10 per cent so there is a long way to go yet. It is not just a problem that is present in Europe and the USA, it also has a high incidence in rural areas in South America. In India, while the incidence is less than 1 per cent, white collar workers in Bombay have a similar incidence to that found in Europe. In total, it is estimated that there are some 250 million obese people across the world.

What is the root of this epidemic? One major cause is that more energy is being taken in than spent. While there is evidence that people on the whole are eating less, they are using up even less energy, and a small imbalance between calories into the body and calories out can have, with time, major effect. Just an extra 200 calories a day if not used up will result in an increase in weight of about seven to eight pounds a year. One problem is that foods contain fats which the consumer just has not come to terms with. There is also a very strong genetic factor and any one with an obese parent should take extra care.

Take care? Does being obese really matter? Studies in the USA have found that being obese not only has serious negative implications for health, that include an association with a form of diabetes, back pain and breathlessness, but also an association with low income, and lower educational achievement. Even at school there is discrimination, and obesity at age 12 is a strong predictor of adult obesity.

Losing weight is really difficult - numerous studies show that there is a strong tendency to keep returning to the weight which one started dieting. It is as if there were some internal fixed point, like the setting of a thermostat that regulates ones appetite so as to keep the weight more or less constant. But hope is on the horizon.

By some chance, some laboratory mice were found - mice that were very fat; and their condition was genetic. The genes that are responsible for their obesity have been identified. One codes for a protein called leptin which is produced by fat cells and circulates in the blood. It enters the brain and acts on the region known as the hypothalamus to suppress appetite. In the mice that lack leptin there is no longer a brake on appetite and so they continue to eat much more than normal nice. Obesity in mice can also result from the absence of the receptor for leptin. Here then, is a possible mechanism for the control of appetite. What is more, there are examples of excessively obese humans whose condition is also due to the absence of the leptin control system.

These were exciting findings and suggested the possibility of a natural way of controlling weight - all one had to do was to make a drug which mimics the action of leptin. Alas the system is more complex and there is no simple relation between the amount of leptin in the blood and weight. But it has opened the way to further investigations as to how chemicals in the blood relate to food intake and fat cells can act on the brain to control appetite. It turns out to be complex but it would be disappointing if a way of controlling appetite were not discovered in the near future. Hold your tummy in and hope that I am right.