The study, published in The Archives of Internal Medicine, suggests that exercise can blunt the effects of the FTO gene. Carrying two copies of this gene significantly increases the chances of becoming obese.
The study was carried out on the US Amish community - where an active lifestyle was found to remove this risk. Several genetic variants have been linked to obesity, but none is wholly responsible for it. The most common of these is FTO, with half of all people in Europe carrying either one or two copies of it.
It is not clear how it influences weight gain, although some scientists have suggested it may play a role in an individual's appetite.
The researchers looked at 704 Amish men and women, chosen because of that community's relative genetic isolation, with members generally able to trace their ancestry back for 14 generations to early settlers from Europe.
Volunteers were fitted with "accelerometers", measuring their precise movements over a period of time.
They found that while the expected link between the number of copies of FTO carried and increased body mass index could be seen in less active volunteers, that link was broken once in those who recorded high levels of activity - equivalent to three to four hours of moderately intensive activity.
