Genes that result in a slender figure have been linked to heart disease and type-two diabetes, conditions normally associated with being overweight.
A study suggests variants of the IRS1 gene reduce fat under the skin, but not the more dangerous visceral fat around organs such as the heart and liver.
The Medical Research Council study,published in Nature Genetics, examined the genes of more than 76,000 people.
The link between the genetic variants and the conditions was stronger in men.Skinny risk
The lead researcher, Dr Ruth Loos, said: "We've uncovered a truly fascinating genetic story and when we found the effect of this gene we were very intrigued.
"It is not only overweight individuals who can be predisposed for these metabolic diseases, and lean individuals shouldn't make assumptions that they are healthy based on their appearance."
Dr Iain Frame, director of research at the charity Diabetes UK, said the study could "shed light on why 20% of people with type-two diabetes have the condition despite being a healthy weight".
"It is also a clear message that people who appear slim shouldn't be complacent about their health," he added.
British Heart Foundation associate medical director Prof Jeremy Pearson said: "These results reinforce the idea that it is not just how fat you are, but where you lay down fat that's particularly important for heart risk.
"Fat stored internally is worse for you than fat stored under the skin.
"However, they don't detract from the fact that being overweight is bad for your heart health, so we should still try to stay lean and fit."