US scientists say they have pinpointed why some people can eat all the chocolate and chips they want and not increase their cholesterol levels.
Genes determine how often you can eat these
A study by researchers at the Berkeley National Laboratory of identical twins show it really is all in the genes.
Each pairing - of a keen athlete and a couch potato - were allocated either a high or a low fat diet.
In the American Journal of Clinical Nutrition, the scientists said genes, not exercise, decided the effect.
Twenty-eight pairs of twins took part in the study.
One member of each pair was a long distance runner, located through Runner's World magazine or at race meetings across the US.
Each had a brother who ran around 40 kilometres a week less, if they exercised at all.
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 calories coming from fat.
Each participant's blood cholesterol was measured.
They then switched diets for another six weeks before their cholesterol was measured again.
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.
Dr Paul Williams, who led the study, said: "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 added: "Our experiment shows how important our genes are.
"Some people have to be careful about their diets, while others have much more freedom in their dietary choices."
Steve Humphries, British Heart Foundation Professor of Cardiovascular Genetics at University College London, said: "This work is very exciting.
"We all know people who put on weight as soon as they look at a cream cake and others who can seem to eat anything and always stay thin.
"Doctors also know 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."
He added: "Doctors have always suspected that these people must be cheating on their diet, but this study shows that some people are unlucky enough to have genes that mean they just don't respond well to diet."
Professor Humprhies said further research could show which genes make a difference, and how, which may enable special diets to be devised for those affected.
"Until we can do that, these people may need lipid-lowering drugs, such as statins, to get their cholesterol levels down."