Previous studies have noted that depression often appears in individuals who are overweight or obese.
However, observational studies have not been able to demonstrate whether obesity causes depression, as there are many competing factors to consider.
For instance, obesity is a risk factor for a number of conditions, and so it might be that dealing with other health issues increases the likelihood of becoming depressed, rather than the obesity being the cause.
Some researchers have argued that the relationship might be the other way around: depression is a risk for obesity.
Others believe that depression and obesity exacerbate each other. For instance, obesity might make depression more likely to occur initially, but once depressive symptoms arise, they might compound obesity by making it harder for the individual to exercise.
Obesity and depression revisited
To gain a better understanding of this complicated relationship, researchers from the University of Exeter in the United Kingdom joined forces with scientists from the University of South Australia. They published their results in the International Journal of Epidemiology this week.
"Obesity and depression are both global health problems that have a major impact on lives and are costly to health services," explains lead author Dr. Jess Tyrrell. "We've long known there's a link between the two, yet it's unclear whether obesity causes depression or vice versa, and also whether it's being overweight in itself or the associated health problems that can cause depression."
The study used genetic data to inspect the causal relationship between obesity and depression. The team wanted to understand whether a higher body mass index (BMI) was related to an increased risk of depression without the presence of other health conditions.
The researchers used genetic and medical data from 48,000 people with depression and compared it with in excess of 290,000 controls, making it the largest study to address this question to date.
Psychological impact to blame?
Overall, as expected, a higher BMI was associated with higher odds of depression. This association was stronger in women than men, confirming earlier findings. Women with a high BMI had a 21 percent increase in risk, compared with 8 percent in men.
By investigating individuals with genes predisposing them to obesity but without ones that predispose them to metabolic conditions, such as diabetes — referred to as a "favorable metabolic profile" — the researchers could separate out the psychological component of obesity.
In their analysis, they accounted for a range of variables that could influence the results, including socioeconomic position, alcohol consumption, smoking, and physical activity.
They found that individuals with a favorable metabolic profile were just as likely to develop depression as individuals with obesity that carried genes predisposing them to develop metabolic conditions. This effect was most pronounced in women.
To double-check their findings, they also took data from the Psychiatric Genomics Consortium. And their second analysis returned similar results, adding further weight to their conclusions.
"Our robust genetic analysis concludes that the psychological impact of being obese is likely to cause depression."
Dr. Jess Tyrrell
These results provide vital insight, as Dr. Tyrrell explains, "This is important to help target efforts to reduce depression, which makes it much harder for people to adopt [healthful] lifestyle habits."
However, the relationship between obesity and depression is convoluted, and questions remain. As the authors write, "we have not ruled [out] a possible bidirectional causal relationship between higher BMI and depression [...] Further research is required to explore the causal role of depression on body mass index and obesity."
Because depression and obesity can have profound impacts on individuals and society at large, scientific attention is likely to continue to look at their links.