For some people, no matter how much time they spend at the gym or on the couch, it may have little effect on their cholesterol levels.

A new study of identical twins suggests that genes play a major role on how sensitive our bodies are to fat in the diet, regardless of physical activity levels.

Researchers say the findings may help explain why some sedentary people can eat a high-fat diet without suffering the artery-clogging consequences while others who exercise regularly still suffer from high cholesterol levels.

The risk of heart disease increases with elevated low-density lipoprotein (LDL) cholesterol levels, and decreases with rising high-density lipoprotein (HDL) levels.

Eating a healthy, low-fat diet and regular exercise can help the vast majority of people maintain healthy cholesterol levels. But researchers say their results show that some people may be born with genes that make them naturally more sensitive or resistant to dietary cholesterol, whether they're a couch potato or a marathon runner.

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Genes May Determine Cholesterol Levels

In the study, researchers compared the effects of switching between a high-fat diet and a low-fat diet in a group of 28 male twin pairs in which one twin was an avid runner and the other was sedentary.

The running twin logged about 40 kilometers more per week than his brother, if the brother exercised at all.

Each twin followed a high-fat diet in which 40 percent of the calories came from fat for six weeks and then switched to a low-fat diet with 20 percent fat for another six weeks. Their cholesterol levels were measured after each six-week period.

The results showed that the twins' cholesterol levels responded very similarly to the different diets, despite the big difference in their exercise levels.

For example, if one sedentary twin experienced a rise in total or LDL "bad" cholesterol levels on the high-fat diet, so did his "running" brother. Similarly, if one active twin saw his LDL cholesterol decrease on the low-fat diet, so did the couch-potato twin.

But some twins seemed to be born with genes that made them insensitive to dietary fat and experienced little change in their cholesterol levels on either diet.

"Our results suggest there are genes that strongly influence the LDL-cholesterol response to diet, even in the presence of large differences in physical activity," write researcher Paul Williams, of Lawrence Berkeley National Laboratory, and colleagues in the July issue of the American Journal of Clinical Nutrition.

Read WebMD's "Newly Diagnosed with High Cholesterol? Start Here"

By Jennifer Warner, reviewed by Brunilda Nazario, MD

SOURCE: Williams, P.American Journal of Clinical Nutrition, July 2005; vol 82: pp 181-187.