It used to be a stereotype that being fat meant you had a happy personality.  Then culture went out of its way to vilify fat people and make them miserable - when they weren't vilifying culture or food companies for making people fat.

Now researchers claim new genetic evidence about why some people are happier than others - and it involves a gene implicated in obesity. The gene FTO, which is correlated to obesity by the 'being fat is exculpatory' segment of science, has now been similarly associated with an eight percent reduction in the risk of depression. In other words, it's not just an obesity gene but a "happy gene" as well, if your correlation and causation errors roll that way.

 The paper was produced by senior author David Meyre, associate professor in clinical epidemiology and biostatistics at the Michael G. DeGroote School of Medicine and a Canada Research Chair in genetic epidemiology; first author Dr. Zena Samaan, assistant professor, Department of Psychiatry and Behavioural Neurosciences, and members of the Population Health Research Institute of McMaster University and Hamilton Health Sciences.

"The difference of eight per cent is modest and it won't make a big difference in the day-to-day care of patients,"  said senior author David Meyre, associate professor in clinical epidemiology and biostatistics at McMaster University. "But, we have discovered a novel molecular basis for depression." 

In the past, some family studies on twins, and brothers and sisters, showed a 40 percent genetic component in depression. However, scientific studies attempting to associate genes with depression have been unsuccessful and produced no convincing evidence so far - similar to obesity genes.

The McMaster discovery seeks to challenge the modern perception of a reciprocal link between depression and obesity - that obese people become depressed because of their appearance and social and economic discrimination which lead to less active lifestyles and changes in eating habits to cope with depression that causes them to become obese. 

 "We set out to follow a different path, starting from the hypothesis that both depression and obesity deal with brain activity. We hypothesized that obesity genes may be linked to depression," Meyre said. The researchers investigated the genetic and psychiatric status of patients enrolled in the EpiDREAM study led by the Population Health Research Institute, which analyzed 17,200 DNA samples from participants in 21 countries.

In these patients, they found the previously identified obesity predisposing genetic variant in FTO was associated with an eight per cent reduction in the risk of depression. They confirmed this finding by analyzing the genetic status of patients in three additional large international studies.

Meyre said the fact the obesity gene's same protective trend on depression was found in four different studies supports their conclusion. It is the "first evidence" that an FTO obesity gene is associated with protection against major depression, independent of its effect on body mass index, he said.

 Published in Molecular Psychiatry