Why does it seem like some people can eat all the ice cream they want without increasing their cholesterol or gaining much weight, while others with high cholesterol have to watch their diets like a hawk? Because no matter what their lifestyle, people's genes play an overriding role in their cholesterol response.
So says a new study by researchers at the Department of Energy's Lawrence Berkeley National Laboratory and the Children's Hospital Oakland Research Institute (CHORI), conducted by Paul Williams of Berkeley Lab's Life Sciences Division in collaboration with Robin Rawlings and Patricia Blanche of CHORI and Ronald M. Krauss of CHORI and Berkeley Lab's Genomics Division. They report their findings in the July 8, 2005, issue of the American Journal of Clinical Nutrition.
The investigators analyzed how "bad" cholesterol (low-density lipoprotein, or LDL, cholesterol) responded to diets that were either high or low in fat in 28 pairs of identical male twins -- one twin a vigorous exerciser, the other a comparative couch potato.
"Although identical twins share exactly the same genes, we chose these twins because they had very different lifestyles," says Williams. "One member of each pair was a regular long-distance runner, someone we contacted through Runner's World magazine or at races around the country. His brother clocked 40 kilometers a week less, at least, if he exercised at all."
For six weeks the twins ate either a high-fat diet (40 percent of its calories from fat) or a low-fat diet (only 20 percent of its calories from fat); then the pairs switched diets for another six weeks. After each six-week period the twins' blood cholesterol levels were tested.
The researchers were interested in learning if blood cholesterol changes due to the different diets would be the same or different in each pair of genetically identical twins, even though their lifestyles were very different. 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.
The researchers found an astounding 0.7 correlation in responses to the change in diet, an incredibly strong similarity in the way each pair of twins responded -- even though the responses themselves among different pairs of twins differed considerably.
"If one of the twins could eat a high-fat diet without increasing his bad cholesterol, then so could his brother," says Williams. "But if one of the twins' LDL cholesterol shot up when they went on the high-fat diet, his brother's did too."