Researchers looked at UK Biobank data from more than 48,000 people with depression, comparing them with a control group of more than 290,000 people born between 1938 and 1971, who provided medical and genetic information.
"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," Prof Hypponen says.
"These genes were just as strongly associated with depression as those genes associated with higher BMI and diabetes. This suggests that being overweight causes depression both with and without related health issues -- particularly in women"
At the other ends of the BMI spectrum, very thin men are more prone to depression that either men of normal weight or very thin women.
"The current global obesity epidemic is very concerning," Prof Hypponen says. "Alongside depression, the two are estimated to cost the global community trillions of dollars each year.
"Our research shows that being overweight doesn't just increase the risks of chronic diseases such as cancer and cardiovascular disease; it can also lead to depression," Prof Hypponen says.