Low fat diets may be useless or even damaging for up to two thirds of the population, according to Dr. Ronald Krauss of the Lawrence Berkeley National Laboratory in California, US. It all depends on your genes. There is a 'bad' type of cholesterol, called low density lipoprotein (LDL), which sticks to the artery lining, and a 'good' type of cholesterol, high density lipoprotein (HDL), which protects against disease.

Dr. Krauss has discovered a further factor. A third of adult men and one in five to six postmenopausal women have a particular type of LDL called pattern B, which gives them raised risks of both heart disease and diabetes. It is these people who are most likely to benefit from low fat diets. For the rest, with pattern A, the opposite may be true. Most experience no benefit from a low fat diet and, worse, in roughly a third of men they flipped over to pattern B LDL. Dr. Krauss agrees that the standard advice to reduce the total amount of calories taken as fat to 30% can do little harm but cautions against lowering fat levels further.

Genetic susceptibility to certain illnesses is beyond doubt. People with a genetic trait called apoE4 tend to have higher blood cholesterol concentrations and higher risks of heart disease and Alzheimer's. People with another genetically-influenced condition, low density lipoprotein (LDL) subclass pattern B, run a higher risk of developing diabetes mellitus and three times the risk of heart disease. Only these people, Krauss suggests, should consider an extremely low fat diet.