The genetic cause of extreme thinness was discovered by British and Swiss researchers, who hope that the breakthrough may lead to new treatments for obesity.
Researchers at Imperial College London and Switzerland's University of Lausanne said Thursday that people with duplications of chromosome 16 were more likely to be skinny.
They studied the DNA of over 95,000 people and found that the genetic duplication affected one in 2,000 -- making men 23 times and women five times more likely to be underweight.
"So far, we have discovered a large number of genetic changes that lead to obesity ... This is the first genetic cause of extreme thinness that has been identified," according to study leader Professor Philippe Froguel, from Imperial College London.
He added, "It's also the first example of a deletion and a duplication of one part of the genome having opposite effects."
Froguel said the research was also significant because it showed that half of the children found to possess an extra part of chromosome 16 in the study previously were diagnosed with a "failure to thrive."
"If a child is not eating, it's not necessarily the parents' fault," he added.
The researchers said that they hoped the study, published in the journal Nature, "might throw up new potential treatments for obesity and appetite disorders."