Losing those extra kilos by dieting can make you miserable and more at risk of depression, a study claims.
Researchers at University College London found losing weight did not make people happy. Instead, those who successfully slimmed down were almost twice as likely to feel sad, lonely and lethargic as those who stayed the same weight or got fatter.
The large-scale study also found the advertising industry tells dieters their lives will be transformed when they are thinner, but they feel disappointed when they discover little has changed other than their weight.
And dieting itself can be hard, which can cause people to feel down.
Almost 2000 overweight men and women aged 50 and over were weighed at the start of the study and answered questions about how often they felt sad, lonely and listless.
Four years later, 14 per cent of the volunteers had lost at least 5 per cent of their body weight - an amount known to improve health. Tests showed lower blood pressure and harmful blood fats but their mood was also lower.
Those who had lost weight were 78 per cent more likely to have symptoms of depression than those who had not.
And importantly, the link could not be explained away by participants' having fallen ill or suffered a bereavement or divorce during the intervening years, the journal PLOS ONE reported.
Researcher Sarah Jackson said: "We don't 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.
"Aspirational advertising by diet brands may give people unrealistic expectations about weight loss," Dr Jackson said.
"Dieting requires considerable willpower and it might involve missing out on special meals and eating in restaurants. It's not necessarily the most pleasant experience for people.
"Lots of people want to lose weight thinking it will fix all their problems. But while it will go some way towards fixing their health, it won't necessarily make them happier in the short term."
She added that it was possible slimmers became happier eventually, especially if they felt a sense of achievement from reaching their target weight and maintaining it.
The researchers said that whatever the reason, with 60 per cent of overweight Britons trying to lose excess kilos, doctors and dieters needed to be aware of the link with depression.