Anti-obesity campaigns may be having unintended mental health consequences for teenagers, with a study showing six in 10 have exercised to lose weight.

Significantly higher numbers of Generation Z boys and girls are dieting or exercising to shed pounds and are likely to overestimate their weight, according to a study led by University College London (UCL).

And girls trying to lose weight are more likely to experience depressive symptoms than in previous years, the findings suggest.

The researchers said efforts to reduce obesity across the nation may be having “unintended consequences” on weight-control behaviours and mental health.

Mounting societal pressures, the rise of the fitness industry and social media may also be causing harm, they said.

Media portrayals of thinness, the rise of the fitness industry and the advent of social media may all partly explain our results, and public health messaging around calorie restriction and exercise might also be causing unintended harm.

The research team reviewed data from 22,503 adolescents in the UK collected over three decades – in 1986, 2005 and 2015.

They found that 42% of 14-year-olds said they were trying to lose weight in 2015 – up from 30% in 2005.

Some 8.5% said they were trying to gain weight, a rise from 5.2% in 2005.

Some 44% said they had dieted and 60% had exercised to lose weight, compared to 38% and 7% of those in 1986.

The researchers said this is concerning because studies have shown that dieting is ineffective in reducing weight in young people, and is linked to depression and eating disorders.

Senior author Dr Praveetha Patalay, from UCL’s MRC unit for lifelong health and ageing, said: “It seems that young people are exercising for different reasons than they did before – more adolescents seem to be thinking of exercise predominantly as a means to lose weight rather than exercising for fun, socialising and feeling healthy.

“We suspect that recent controversial calls to add ‘exercise-equivalent’ labels on food packaging may exacerbate this.”

The research shows girls have consistently been more likely than boys to diet to lose weight, but there has been a greater increase among boys over the years.

Boys are also becoming more likely to try to gain weight, the researchers said, which could be due to a shift in media representation of male ideals, with “lean muscular bodies” increasingly normalised.

Both sexes also became more likely to overestimate their weight, which adds to the researchers’ concerns that increased efforts to lose weight are not necessarily due to increased obesity rates.

These behaviours were linked to depressive symptoms, and for girls this relationship strengthened over the three decades of the study.

Lead author Dr Francesca Solmi, from UCL’s Division of Psychiatry, said: “Media portrayals of thinness, the rise of the fitness industry and the advent of social media may all partly explain our results, and public health messaging around calorie restriction and exercise might also be causing unintended harm.

“Public health campaigns around obesity should consider adverse mental health effects, and ensure they avoid weight stigma. By promoting health and wellbeing, as opposed to focusing on ‘healthy weight’, they could have positive effects on both mental and physical health.”

Campaigns should not foster feelings of guilt or shame but highlight positive aspects of exercise, such as improving strength, learning new skills, and socialising, the researchers said.

The study, by researchers at UCL and the Universities of Edinburgh and Liverpool, and supported by other UK organisations, is published in JAMA Pediatrics.