Researchers from Britain's Medical Research Council Epidemiology Unit said that while a so-called "lean gene" was linked to having less body fat, it was also linked to an increased risk of heart disease and type 2 diabetes.
"We've uncovered a truly fascinating genetic story, and when we found the effect of this gene, we were very intrigued," said Ruth Loos, whose study was published by Nature Genetics.
Loos' team examined the genetic code of more than 75,000 people to look for the genes that determine body fat percentage, and found strong evidence that a gene called IRS1 is linked with having less body fat.
When they investigated further, they found IRS1 also leads to having unhealthy blood levels of cholesterol and glucose.
They found the gene was only linked to lower levels of subcutaneous fat, but not to visceral fat.
Loos said the findings suggest that people with the IRS1 gene are less able to store subcutaneous fat, and may therefore store fat in other parts of the body where it might pose more risk to organ function.
She added that the study results did not change the general message for most people. "People who are lean are generally healthier than people who are overweight or obese," she said in a telephone interview.
"But we all know some people who are lean and also may have high cholesterol or have a heart attack before the age of 50 - so maybe this gene is one factor in looking healthy but still being at risk for cardiovascular disease and type 2 diabetes." (Reuters Health, June 2011)