Blood levels of "bad" low-density-lipoprotein (LDL) cholesterol vary widely between individuals, and the blame for at least part of that variance may lie in genetics, researchers report.

"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," researcher Paul Williams, of the U.S. Department of Energy's Lawrence Berkeley National Laboratory, said in a prepared statement.

His team studied the response of 28 pairs of identical male twins to diets either high or low in fat. In each pair of twins, one brother was an avid exerciser while the other was a "couch potato."

"Although identical twins share exactly the same genes, we chose these twins because they had very different lifestyles," Williams said.

For the first six weeks, the twins ate either a low-fat diet (20 percent of calories from fat) or a high-fat diet (40 percent of calories from fat). They then switched diets for the final six weeks of the study. The twins' blood cholesterol levels were checked after each six-week period.

The study found a strong similarity in the way each pair of twins responded to the diets, regardless of whether they exercised or not.

"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," Williams said.

The study wasn't able to identify exactly what genes are responsible for a good or poor LDL response to specific diets, the researchers pointed out. Instead, "this type of experiment allows us to test whether genes are important without having to identify the specific genes involved," Williams said.

The study appears in the July 8 issue of the American Journal of Clinical Nutrition.