More than half of those trying to lose weight risk being left miserable, despite reaping the health benefits of slimming down, a University College London (UCL) study found.
The “mental toll” of cutting out favourite high calorie foods and trading unhealthy activities for gruelling exercise could be behind the effect on emotions.
Dieters may also be setting unrealistic goals, spurred on by adverts for diet aids and foods which misrepresent how tough losing weight can be, researchers said.
A study of 2,000 overweight and obese adults found they were more likely to become depressed after losing five per cent or more of their body weight.
Researchers said previous studies, which linked happiness to healthiness among dieters, could be flawed because participants were given emotional support. Public health expert Dr Sarah Jackson said: “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.
“Advertising may give people unrealistic expectations about weight loss. They often promise instant life improvements, which may not be borne out in reality for many people.”
The study analysed 1,979 dieters over a four-year period, 278 of whom lost at least five per cent of their bodyweight.
All the participants were aged 50 or older but excluded those with a diagnosis of clinical depression or a debilitating illness.
The results, published in the journal PLOS ONE, showed those who shed over five per cent of their bodyweight were 52 per cent more likely to feel depressed.
The experts pointed out their findings did not show a link to clinical depression, as depression and weight loss may share a “common cause”. Dr Jackson said: “Resisting the temptations of unhealthy food in modern society takes a mental toll, as it requires considerable willpower and may involve missing out on enjoyable activities.
“However, mood may improve once target weight is reached and the focus is on weight maintenance.”
She added: “Healthcare professionals should monitor patients’ mental as well as physical health when recommending or responding to weight loss.”
Senior author Professor Jane Wardle, director of the Cancer Research UK Health Behaviour Centre at UCL, added: “People who lost weight achieved a reduction in blood pressure, significantly reducing the risk of heart disease. However, patients and doctors should be aware there is no immediate psychological benefit.”