Some severely obese children lack key segments of DNA, which causes them to overeat, researchers in the UK have found.
The study, led by Dr Sadaf Farooqi from the University of Cambridge and Dr Matt Hurles from the Wellcome Trust Sanger Institute, checked the DNA of 300 obese children who weighed up to 100kg by age 10, looking for deletions or extra copies of DNA segments, called copy number variants (CNVs). These were compared with more than 7000 people in a control group.
They discovered that several rare deletions may promote obesity, including one kind, on chromosome 16, found in less than one percent of about 1200 obese children, Associated Press reported.
"We found that part of chromosome 16 can be deleted in some families, and that people with this deletion have severe obesity from a young age," Dr Farooqi said in a media statement.
"Our results suggest that one particular gene on chromosome 16 called SH2B1 plays a key role in regulating weight and also in handling blood sugar levels. People with deletions involving this gene had a strong drive to eat and gained weight very easily."
This study is the first to show that obesity can be caused by a genetic alteration.
The research has already made an impact. Four children involved in the study were on Britain child welfare authorities' "at risk" register on the assumption that they were being overfed. They have since been removed.
"This study shows that severe obesity is a serious medical issue that deserves scientific investigation," Dr Farooqi said.
"It adds to the growing weight of evidence that a wide range of genetic variants can produce a strong drive to eat. We hope that this will alter attitudes and practices amongst those with professional responsibility for the health and wellbeing of children."
The study was published in the journal Nature.
The federal government's 2007-08 National Health Survey found a quarter of Australian children are overweight or obese. NSW Health says environmental changes, which have reduced physical activity and passive energy consumption, are to blame.
Learn more about childhood obesity.