Fighting the flab: it ain't what you eat - or is it?

Fat people are more energetic than thin people, but they lie about what they eat. Uncovering this deception has led scientists to a new understanding of the role of physical activity in controlling weight.

As every dieter knows, climbing on an exercise bike or making daily visits to the swimming pool is an inefficient way of shedding flab. Exercise can make you fit but it cannot make you thin.

Yet the population is getting fatter when, as figures show, average calorie intake has barely changed. Obesity rates have doubled since 1980 to 16 per cent in men and 17.3 per cent in women - despite the fact that, as a nation, we are eating no more. The reason, researchers say, is that physical activity has declined.

Explaining this paradox has resulted in a reassessment of the role of exercise in controlling weight - and caused some red faces among the scientists involved. At a conference in London yesterday, "Appetite, Obesity and Disorders of Over and Under-Eating", organised by the Royal College of Physicians, researchers explained how exercise cannot reduce weight but it may be the key to holding weight down.

For years, scientists were misled about the role of physical activity because fat people tend to deceive themselves and others about what they eat. Studies around the world suggested that fat people ate no more than thin ones. Since they needed more energy to carry out the same activities (because of their greater bulk) scientists believed they were victims of their own "energy efficiency". Fat people must, they believed, burn calories more slowly than thin ones.

"That deception cost hundreds of millions of pounds in research funding because we were all barking up the wrong tree," said Dr Andrew Prentice of the Medical Research Council Dunn Clinical Nutrition Research Unit.

The deception was uncovered with the development of "w-labelled water" - water with an added radio-isotope which could be measured in the urine up to 14 days after it had been drunk. That gave an accurate measure of energy expenditure and revealed that fat people were expending more energy than thin ones. It meant they must be consuming more calories than they admitted. They were fat not because of a disorder of energy expenditure but because they ate too much. "For the first time we could say that with certainty," said Dr Prentice.

However, other evidence shows that obesity is linked with inactivity. One in five couch potatoes who watches TV for more than 21 hours a week is obese, compared with one in 20 of those who watch for less than six hours a week.

Dr Prentice says satiety exerts a weaker control on over-eating than hunger does on under-eating. Physical activity may be the essential extra element that controls weight. "The social change in energy expenditure - increasing car use, more sedentary work - puts our physiological systems in a different environment. High fat, high energy foods are a major part of the problem, but what makes us vulnerable to obesity is our low level of physical activity."