The genetic cause of extreme thinness has been identified by scientists. Skinniness can lead to a syndrome in children called "failure to thrive".
The research, to be published Thursday in Nature, shows that people with surplus copies of certain genes are much more likely to be very skinny.
In one in 2,000 people part of chromosome 16 is duplicated, making men 23 times and women five times more likely to be seriously underweight, they found.
Normally, an individual inherits a duplicate of every chromosome from each parent, giving us two of each gene.
But sometimes sections of a chromosome are copied or deleted, resulting in too many or too few segments of genetic coding.
"In many cases, duplications and deletions have no effect, but occasionally they can lead to disease," said Philippe Froguel, a professor at the School of Public Health at Imperial College London.
In the study, Froguel and colleagues examined the DNA of over 95,000 people looking for patterns linked to extreme thinness.
They found that duplication of a part of chromosome number 16 containing more than two-dozen genes is strongly associated to being underweight, defined as a body-mass index (BMI) below 18.5.
The normal range is 18.5 to 25, with 25-to-30 corresponding to "overweight," and above 30 to "obese."
Half of all children in the study with this genetic glitch had been diagnosed with "failure to thrive," meaning that they did not gain weight at a normal rate as they grew older.
A quarter of the individuals with the extra genes had microcephaly, a condition in which the head and brain are abnormally small, and which is associated with neurological defects and shorter life expectancy.
Last year, the same team of researchers discovered that people missing a copy of these genes are 43 times more likely to be morbidly obese.
"One reason (the new finding) is 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," Froguel said in a statement.
In some children, especially boys, four-year-olds weighed no more than a normal child of a year-and-half, he told AFP by phone.
Froguel said he suspected that some parents with severely underweight children have been wrongly accused of negligence or abuse when in fact the condition was largely due to the genetic flaw uncovered.
Previous research has identified a large number of genetic changes that lead to obesity, but this is the first to pinpoint a genetic cause of thinness, he said.
"It's also the first example of a deletion and a duplication of one part of the genome having opposite effects," Froguel added.
"At the moment we don't know anything about the genes in this region. If we can work out why gene duplication in this region causes thinness, it might throw up new potential treatments for obesity and appetite disorders."
The next step, he said, is to sequence the genes to find out which ones are involved in regulating appetite.
The part of chromosome 16 identified in the study contains 28 genes. Duplications in this region have previously been linked with schizophrenia, and deletions with autism.