The research carried out in the US on 28 pairs of identical twins found that even if one twin was a regular exerciser and the other a couch potato, both had a very similar cholesterol response when put on a high fat diet.
As identical twins have identical genes, the authors suggest that genetics play a much bigger role in fat metabolism than lifestyle.
However until there is further understanding of the interaction between genes and cholesterol, a UK expert cautioned that the small study offers limited relevance for advice to the whole population.
Annemarie Minehaine, reader in integrated nutrition at the University of Reading in the UK, says that it is very difficult to determine how much influence comes from each of the various factors that can alter cholesterol levels, including weight, sex, and diet.
"There are very rare cases where people have defects in one gene, and for them it is very difficult to not have high cholesterol," she told NutraIngredients.com.
"But genetic variations are common in the general population. You can't say for example that genes account for a certain percentage of cholesterol and diet and weight another amount."
The new trial, published in this month's issue of the American Journal of Clinical Nutrition (vol 82, no 1, pp181-187), does nevertheless show how important genes are in fat metabolism.
The findings could in the future be relevant to the growing number of companies making foods to lower cholesterol levels.
Scientists from the Lawrence Berkeley National Laboratory and the Children's Hospital Oakland Research Institute in California selected twins where one of the pair was a regular long-distance runner and the other "clocked 40 kilometers a week less, at least, if he exercised at all," according to author Paul Williams.
The researchers gave the twins either a high-fat diet (40 per cent of its calories from fat) or a low-fat diet (only 20 per cent of its calories from fat) for six weeks. Then the pairs switched diets for another six weeks. Blood cholesterol levels were tested after each period.
The researchers found a 0.7 correlation in responses to the change in diet, a very strong similarity in the way each pair of twins responded (a correlation of zero between the two would mean that their responses to the diets had no relation to each other, while a correlation of 1.0 would mean that their responses were identical).
"If one of the twins could eat a high-fat diet without increasing his bad cholesterol, then so could his brother," said Williams. "But if one of the twins' LDL cholesterol shot up when they went on the high-fat diet, his brother's did too."
The correlations showed that the twins had very similar changes in LDL cholesterol because they had the same genes. 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.
"Our experiment shows how important our genes are," said Williams. "Some people have to be careful about their diets, while others have much more freedom in their dietary choices."
Although several specific genes have been associated with cholesterol changes in response to changes in diet, these cannot account for the large correlations seen in this study, said Williams.