Weight loss significantly improves physical health but effects on mental health are less straightforward, researchers said.
In a study of 1,979 overweight and obese adults in the UK, people who lost 5 per cent or more of their initial body weight over four years showed significant changes in markers of physical health, but were more likely to report depressed mood than those who stayed within 5 per cent of their original weight.
The research by University College London (UCL) highlights the need to consider mental health alongside physical health when losing weight.
Clinical trials of weight loss have been shown to improve participants' mood, but this could be a result of the supportive environment rather than the weight loss itself, as the effects are seen very early on in treatment and are not related to the extent of weight loss.
It's important to note this new result does not mean that weight loss necessarily causes depression directly, as depression and weight loss may share a common cause.
However, it shows that weight loss outside the clinical trial setting cannot be assumed to improve mood and raises questions about the psychological impact of weight loss.
The data came from the English Longitudinal Study of Ageing, a UK study of adults aged 50 or older, and excluded participants with a diagnosis of clinical depression or a debilitating illness.
Depressed mood and overall wellbeing were assessed using standard questionnaires and weight was measured by trained nurses.
Of the 1,979 overweight and obese participants, 278 (14 per cent) lost at least 5 per cent of their initial body weight with a mean weight loss of 6.8kg per person.
Before adjusting for serious health issues and major life events such as bereavement, which can cause both weight loss and depressed mood, the people who lost weight were 78 per cent more likely to report depressed mood.
After controlling for these, the increased odds of depressed mood remained significant at 52 per cent.
"We do not want to discourage anyone from trying to lose weight, which has tremendous physical benefits, but people should not expect weight loss to instantly improve all aspects of life," said lead author Dr Sarah Jackson.
The study was published in the journal PLOS ONE.