The largest study of its kind has given evidence to suggest that being overweight can cause depression, primarily due to psychological factors.
There is a well-known link between obesity and depression, but the causal relationship has been difficult to pin down. Now researchers at the University of South Australia and the University of Exeter have published data indicating that being overweight can directly cause depression, especially in women.
'The strength of our study design is its ability to focus on the effects of high BMI [body mass index], rather than any dietary or societal influences which would typically be associated with obesity,' Professor Elina Hypponen of the University of South Australia told the Sydney Morning Herald.
The researchers used genetic and medical data from more than 48,000 people with depression from the UK Biobank, comparing it to 290,000 people without depression. The team looked at 73 genetic variants that were linked to higher BMI and also to various health conditions such as type II diabetes and heart disease.
They also investigated 14 gene variants that were linked to higher BMI but did not carry the risk of these medical conditions. These variants were considered not to have a physiological effect that could cause depression.
The study found that for every 4.7 point rise in BMI the risk of being depressed increased by 18 percent. When focusing on the risk only to women, this rose to 23 percent more likely to be depressed for every 4.7 point rise.
Both groups of genetic variants showed this link, those that had physiological effects and those that didn't. Genetic variants without a physiological influence were thought to have a secondary effect on depression, through psychological factors such as poor body image due to higher BMI.
'It suggests the psychological component is just as strong as any physiological component, if [the latter] is there at all,' Professor Tim Frayling of the University of Exeter, also an author of the study, told the Guardian.
Researchers have welcomed the study as an effort to untangle cause and effect in the link between obesity and depression.
'These new findings are perhaps the strongest so far to suggest higher weights may actually contribute to depression,' Professor Naveed Sattar of the University of Glasgow, who was not an author of this study, told the Guardian. 'Of course, many other factors can cause depression, but, even so, weight loss might be helpful to improve mental health in some individuals, whereas keeping leaner in general should help lessen chances of depression.'
The research was published in the International Journal of Epidemiology.