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People who are obese are more likely to have depression than people who are not, but it’s been unclear how one might cause the other. A study of the relationship between depression and genetic variants linked to higher body mass index published today (November 13) in the International Journal of Epidemiology suggests that obesity causes depression, and that it is the untoward psychological effects related to obesity that drive the mood disorder.

“These new findings are perhaps the strongest so far to suggest higher weights may actually contribute to depression,” Naveed Sattar, a professor of cardiovascular and medical sciences at the University of Glasgow who did not participate in the work, tells 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.”

To determine the direction of causality, researchers at the University of Exeter and the University of South Australia examined genetic data from the UK Biobank including 48,791 individuals with depression and 291,995 without. They used genetic predisposition to a higher body mass index (BMI) as a proxy for actual BMI (which they also had data on) to disentangle BMI and its relationship to depression from other factors that could confound the results.

The authors looked at two types of genetic variants and their relationship to depression: those that are associated with higher BMI and also linked to metabolic problems, such as diabetes, and those that are linked to higher BMI but are also tied to a lower risk of metabolic problems. The researchers reasoned that if it were health issues attendant with obesity that were making people depressed, rather than obesity itself, the genetic variants that didn’t carry any metabolic baggage would not be associated with depression. However, what they found was that the genetic variants linked with a lower metabolic risk also correlated with depression.

“This analysis was important in that it suggests a psychosocial effect of higher BMI as well as, or instead of, a physiological effect driven by adverse metabolic health,” the authors write in their study.