Genetics could explain why some morbidly obese people gain weight, new research suggests.
A study at Imperial College London found that a minority of obese people are missing part of their DNA, in some cases around 30 genes, which could impact upon their weight.
It is the first time genetics can be attributed to why obesity appears in otherwise physically healthy people.
Study leader Professor Philippe Froguel said: "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.
"It is becoming increasingly clear that for some morbidly obese people, their weight gain has an underlying genetic 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."
The abnormality is thought to affect around seven in every 1,000 obese people whose weight is great enough to pose a risk to health.
In England there are around 700,000 such "morbidly" obese individuals.