People are apples or pears depending on levels of a protein that helps distribute body fat, research has shown.
Scientists believe understanding how the protein works may lead to the development of new obesity drugs.
The protein, known as 11BetaHSD1, is found at higher levels in the presence of unhealthy fat stored around the torso.
This kind of fat is typically seen in people with "apple-shaped" bodies.
Healthier fat, linked to lower levels of the protein, tends to be stored around the hips and is a hallmark of "pear-shaped" individuals.
Researchers looked at genetically engineered mice to determine the effects of the protein.
They discovered that mice lacking the protein were less likely to build up unhealthy fat tissue after four weeks on a fatty diet.
The scientists, whose work is published in the journal Diabetes, are already investigating ways to develop medicines that inhibit the "apple-shape" protein.
Study leader Dr Nik Morton, from the University of Edinburgh's Centre for Cardiovascular Science, said: "This study opens up new avenues for research, and gives us a much better idea of why some fat in the body becomes unhealthy while other fat is safely stored for energy.
"Inflammation of the unhealthy fat leads to reactions that can cause harm locally to tissues and affect the whole organism, promoting diabetes. Limiting the presence of this protein could help combat this."