We all know exercise is important. But a new McMaster University study shows that the physical effort makes a key difference for those with a so-called fat gene.

A large percentage of people have a gene called FTO that predisposes them to obesity. But new research shows that moderate exercise — even as little as one or two hours per week — substantially weakens the gene's influence.

The team looked at data from 17,400 people with FTO from six ethnic groups in 17 countries and followed them for more than three years.

The findings, published Monday in Scientific Reports, showed that being more active blunts the effect of the inherited obesity gene by as much as 75 per cent, said David Meyre, an associate professor of clinical epidemiology and biostatistics in McMaster's DeGroote School of Medicine.

"This provides a message of hope for people with obesity-predisposing genes that they can do something about it," said Meyre. "Our body weight destiny is not only written in our genetic blueprint."

The findings have broad implications. Some 43 per cent of Europeans have the FTO gene, Meyre said, while, 31 per cent of South Asians, 24 per cent of South Americans, 17 per cent of East Asians and six per cent of Africans carry it.

The cause of obesity is more complex than just the FTO gene's effect, Meyre said. And FTO's influence isn't enormous — it adds about three kilograms, on average, to someone's weight.

Hudson Reddon, a McMaster PhD student, is the study's first author. McMaster's Population Health Research Institute and Hamilton Health Sciences also contributed.