People with extra copies of certain genes are much more likely to be very skinny, scientists said Wednesday in the first finding of a genetic cause for extreme thinness.
In a study in the journal Nature, researchers from Britain's Imperial College London and the University of Lausanne in Switzerland found that a duplication of a part of chromosome 16 is associated with being underweight.
Previous research has found that people with a missing copy of these genes are 43 times more likely to be morbidly obese.
"This is the first genetic cause of extreme thinness that has been identified," said Philippe Froguel from Imperial's school of public health, who led the study. "It's also the first example of a deletion and a duplication of one part of the genome having opposite effects."
He said one reason this latest finding was important is that it shows that failure to thrive in childhood can be genetically driven. "If a child is not eating, it's not necessarily the parents' fault," he said.
Normally, each person has a copy of each chromosome from each parent, giving them two copies of each gene. But sometimes sections of a chromosome can be duplicated or deleted, resulting in an abnormal "dosage" of genes, the researchers explained in their study.
But in around one in 2,000 people, part of chromosome 16 is duplicated, making men 23 times and women five times more likely to be underweight.
Being underweight is defined as having a body mass index (BMI) below 18.5 kg per meter squared.
Froguel's team examined the DNA of more than 95,000 people for their study. They found that half of all children with the duplication in the study had previously been diagnosed with a "failure to thrive" -- meaning that their rate of weight gain is significantly lower than normal.
A quarter of people with the duplication had microcephaly, a condition in which the head and brain are abnormally small and which is linked to neurological defects and shorter life expectancy.
Froguel said scientists still have much work to do to find out more about the genes in this region, but their discovery could eventually lead to new potential treatments for obesity and appetite disorders.
"We now plan to sequence these genes and find out what they do, so we can get an idea of which ones are involved in regulating appetite," he said.