Scientists say being skinny is associated with depression.
Experts at the Seoul National University of Medicine believe they've found the first evidence of a significant link between being underweight and being depressed.
But they were unable to say whether mental health issues lead to thinness or vice versa.
The team analysed date from 183 separate studies and also found that obesity increased the risk of depression in a weight-response manner, in that the fatter people were the more depressed they felt.
The experts said the findings are important as people with eating disorders often assume that losing weight will improve their happiness.
This study shows that the opposite is true and malnutrition has a detrimental effect on people's mood.