Of the twenty-eight pairs of twins who took part in the study, one member of each pair was a long distance runner, recruited through Runner's World magazine or at race meetings across the US.

Thus each pair had one keen athlete and one couch potato, and were allocated either a high or a low fat diet.

Each participant's blood cholesterol was measured, and for six weeks each twin ate either a high-fat diet, getting 40% of their calories from fat, or a low-fat diet, with only 20% of their calories coming from fat.

They then switched diets for another six weeks before their cholesterol was measured again.

The researchers found that each pair of twins responded in a very similar way to each diet, although there were significant differences between pairs of twins.

Some twins had one or more genes that made them very sensitive to the amount of fat in their diets.

Other twins had genes that made them insensitive to dietary fat, no matter how much they exercised.

The scientists said genes, not exercise, decided the effect.

Dr Paul Williams, the study leader, said that if one of the twins could eat a high-fat diet without increasing his bad cholesterol, then so could his brother.

But if one of the twins' LDL cholesterol shot up when they went on the high-fat diet, his brother's did too.

He says the experiment shows just how important our genes are.

While some people have to be careful about their diets, others are able to have much more freedom.

Professor Steve Humphries, a British Heart Foundation Professor of Cardiovascular Genetics at University College London, says the work is very exciting.

As he points out we all know people who put on weight by just looking at a cake and others who seem to able to eat anything and always stay thin.

Professor Humphries says doctors are already aware that some people respond well to a low fat diet while in others the amount of cholesterol in their blood stays high, how ever hard they diet, and they have in the past suspected that these people must be cheating on their diet.

This study he says shows that some people are unlucky enough to have genes that mean they just don't respond well to diet.

Professor Humprhies says future research might show which genes make a difference, and how, and lead the way to the development of special diets for those affected.

Until then lipid-lowering drugs, such as statins, will need to be used to get their cholesterol levels down.

The study is published in the American Journal of Clinical Nutrition.