The psychological effects of being obese can cause depression even in the absence of other weight-related health problems, according to new research by scientists from the University of South Australia and the University of Exeter in the UK.
There has long been a link between obesity and depression but, as the study authors note in their paper, “the causal relationship between obesity and depression is complex and uncertain.” Now, publishing their work in the International Journal of Epidemiology, the team has found evidence that a higher body mass index (BMI) can cause – at least in part – depression.
Taking a genetic approach to test the relationship between the two, researchers pulled data from the UK Biobank, a database encompassing a total of 500,000 people, for more than 48,000 people who met the criteria for being depressed as characterized by hospital records or self-reports. They then looked at 73 genetic variants linked to high BMI but also associated with other diseases, like heart disease and diabetes. For these genetic traits, depression risk increased and could be explained by both biological or psychological mechanisms.
Additionally, the team looked at 14 other genetic variants that are linked to a high percentage of body fat but are not associated with other health issues. They found that these variants were also related to depression, and that this depression could only be explained by psychological mechanisms.
"We separated the psychological component of obesity from the impact of obesity-related health problems using genes associated with higher body mass index (BMI), but with lower risk of diseases like diabetes," study author Elina Hypponen said in a statement.
Essentially, the researchers were able to “uncouple” the psychological aspects of BMI-depression relationships from physiological ones.
"These genes were just as strongly associated with depression as those genes associated with higher BMI and diabetes,” explained Hypponen, adding that being overweight causes depression both with and without related health issues, particularly in women. Very thin men are also more prone to depression than men of normal weight or very thin women.
But it’s not that simple. The data is taken from people born in the UK between 1938 and 1971 and it’s unknown whether the same results follow for younger people or those born elsewhere. Furthermore, the authors note that the definition of depression was not “the gold standard" as some cases were self-reported. Nonetheless, the work adds to our understanding of whether obesity causes depression so that we may address public health and medical intervention planning.
"The current global obesity epidemic is very concerning," said Hypponen. "Alongside depression, the two are estimated to cost the global community trillions of dollars each year."