Debate over the obesity epidemic sweeping parts of the world has focussed on whether lifestyle -- too much junk food and couch-potato living -- is the big culprit or whether genes are also to blame.
A new study may help tip the balance in favour of those who claim that fat runs in their family and there is little they can do about it.
People who are morbidly obese lack a tiny stretch of DNA containing around 30 genes, according to the investigation released on Wednesday by the British journal Nature.
Obesity means having a body mass index of 30 or more, while morbid obesity is classified as having a BMI of at least 40. BMI is determined by one's weight in kilos divided by one's height, in metres, squared.
The probe by a consortium of European scientists found that 0.7 percent, or seven in every thousand, of morbidly obese people have a "micro-deletion" of genetic code, located on Chromosome 16.
The telltale sign was initially found among 31 individuals and confirmed among 19 other cases in a trawl through the genomes of 16,053 other people who were either obese or of normal weight.
Those with the deletion tended to be of normal weight as toddlers, became overweight in childhood and then became severely obese in adulthood.
The researchers also took a look at parental DNA where samples were available.
Eleven people inherited the deletion from their mother and four from their father, while 10 deletions apparently occurred by chance, they found. All parents with the deletion were also obese.
What the missing genes do is unclear. Previous research has suggested some of them may be associated with autism, schizophrenia and delayed development.
The study is the first to confirm that severe obesity in otherwise physically health individuals can be caused by a rare genetic variation involving deletion of DNA, say the authors.
Until now, genes linked to weight gain have had a relatively modest effect on fat accumulation, of just a kilo (two pounds) or less.
Other genetic gaps or variations involved in chronic obesity may yet be uncovered, lead author Philippe Froguel, a professor at Imperial College London, said in a press release.
"Although the recent rise in obesity in the developed world is down to an unhealthy environment, with an abundance of unhealthy food and many people taking very little exercise, the difference in the way people respond to this environment is often genetic," he said.
"It is becoming increasingly clear that for some morbidly obese people, their weight gain has an underlying cause.
"If we can identify these individuals through genetic testing, we can then offer them appropriate support and medical interventions, such as the option of weight-loss surgery, to improve their long-term health."