People with the most "fat genes" are around 25 times more likely to become severely obese than people whose DNA has the greatest protective effect, a study has found.
While diet, exercise, ill health and hundreds of other factors are major determiners of obesity, US researchers found genetics can have a "profound effect".
While just 10% of a person's obesity risk may be affected by DNA, this adds up to a difference of two stone (13kg), on average, between the most affected and protected groups at middle age.
The researchers, led by MIT and Harvard Medical School, developed their findings into a genetic "risk score" that can potentially be used from birth to take protective action.
"We have always had a hunch that some people may have been born with a genetic profile that predisposes them to obesity, and we now confirm that this is both true and quantifiable," said Dr Amit Khera, one of the authors and a cardiologist at Massachusetts General Hospital.