The search for genes predisposing people to depression has taken an unexpected twist, according to Canadian researchers who found a clue in an obesity gene.

Studies on families and twins suggest depression has a genetic component, but for 15 years, scientists haven't been able to find genes associated with the illness.

Scientists have been looking for genes associated with depression but they were unsuccessful for more than a decade. (iStock)

Researchers at McMaster University in Hamilton, Ont., took a different approach by testing how obesity genes may be linked with depression.

"We found the first gene predisposing to depression with consistent results," said David Meyre, an associate professor in clinical epidemiology and biostatistics at McMaster and a Canada Research Chair in genetic epidemiology.

In Monday's issue of the journal Molecular Psychiatry, Meyre and his co-authors reported that a variant of the FTO gene may be associated with a lower risk of depression independent of the gene's effect on obesity.

The common perception is that obese people become depressed because of their appearance and poor self-esteem or discrimination. Another common thought is that those who are depressed are less likely to be physically active or follow healthy eating habits. Taking antidepressants can also lead to weight gain.

But the genetic findings challenge that thinking, Meyre said, since those with the genetic mutation predisposing to obesity were protected from depression.

"This suggests that the FTO gene may have a broader role than initially thought with an effect on depression and other common psychiatric disorders," the researchers wrote.

The protective effect showed up not just once or twice but in four different studies of people of various ethnicities around the world.

In total, the researchers analyzed FTO gene mutations in 6,591 people with depression in standard screening and more than 21,000 others who weren’t diagnosed with depression.

Meyre cautioned there are no clinical applications for the discovery since the genetic effect was "relatively modest" — an eight per cent reduction in depression risk for those with one copy and a 16 per cent reduction for those who inherit a copy from each parent. The gene is also present to varying degrees depending on ethnicity.

The main impact is on knowledge and countering the classic view that obesity leads to depression, he said.

The researchers said the idea makes biological sense since the gene is active in the brain. They said more biological analyses are needed to support the link between FTO and major depressive disorder.

Their next step is to look at the 59 other known genes associated with obesity and to use the same approach for other diseases like Type 2 diabetes, which is associated with obesity.

With files from CBC's Amina Zafar