Being slim may not always lead to a lower risk of heart disease and diabetes, scientists said on June 26 after they identified a gene linked both to having a lean body and to a higher risk of metabolic diseases.
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 - illnesses normally associated with being overweight.
"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 in Nature Genetics journal.
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 levels of cholesterol and glucose in blood - key markers for so-called metabolic diseases like heart disease and diabetes.
They found the gene was only linked to lower levels of fat under the skin, called subcutaneous fat, but not to the more harmful fat that surrounds the organs, called 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."
Heart disease is the world's biggest killer, claiming 17.1 million lives a year, according to the World Health Organisation. Experts say a global epidemic of obesity is threatening to cause a wave of heart disease and type 2 diabetes.
A study found that the number of adults around the world with diabetes has more than doubled since 1980 to 347 million - a far larger number than previously thought.
Loos said that while this study pointed to genes as one factor in determining the risk of developing these conditions, it was important to remember that lifestyle factors like diet, exercise, cutting out smoking and maintaining a healthy weight also play a vital role in reducing the risk.
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