The research, published in JAMA Pediatrics, found that girls who are trying to lose weight are also more likely to experience depressive symptoms than in previous years.

In 2015, 42% of 14-year-old girls and boys said they currently were trying to lose weight, compared to 30% in 2005.

Lead author Dr Francesca Solmi (UCL Psychiatry) said: "Our findings show how the way we talk about weight, health and appearance can have profound impacts on young people's mental health, and efforts to tackle rising obesity rates may have unintended consequences.

"An increase in dieting among young people is concerning because experimental studies have found that dieting is generally ineffective in the long term at reducing body weight in adolescents, but can instead have greater impacts on mental health. We know, for instance, that dieting is a strong risk factor in the development of eating disorders."

The research team reviewed data from 22,503 adolescents in the UK, in three different decades, who are part of different cohort studies: the British Cohort Study (of people born in 1970; data was collected in 1986), the Children of the 90s study (born 1991-92, data collected in 2005), and Millennium Cohort Study (born 2000-02, data collected in 2015).

The adolescents were all asked questions about whether they were, or had been, trying to lose weight, whether they had dieted or exercised to lose weight, whether they perceived themselves to be underweight, about the right weight or overweight (which was compared to their actual height and weight measurements), and they filled out questionnaires that gauged depressive symptoms. The researchers found that in 2015, 44% and 60% of all participants had dieted or exercised to lose weight, respectively, compared to 38% and 7% in 1986.

The researchers say other evidence suggests that engagement in vigorous physical activity has remained relatively stable among adolescents over the past few decades.

Senior author Dr Praveetha Patalay (Centre for Longitudinal Studies and MRC Unit for Lifelong Health & Ageing, UCL) 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."

While girls have consistently been more likely to diet to lose weight, the researchers found a greater increase over the years among boys, who were also becoming more likely to be trying to gain weight.

Dr Patalay said: "Societal pressures for girls to be thin have been around for decades, but body image pressures on boys may be a more recent trend. Our findings underscore the impact that societal pressures and public health messaging around weight can have on children's health behaviours, body image and mental health."

Both girls and boys also became more likely to over-estimate their weight from 1986 to 2005, and even more so by 2015, which the researchers say adds to their concerns that increased efforts to lose weight are not necessarily due to increased obesity rates.

The reported weight-related behaviours and weight misperception were associated with depressive symptoms, and among girls, this relationship was becoming even stronger over the three decades examined in this study. The findings could possibly be part of the explanation for increases in adolescent depressive symptoms that have been observed in recent decades.

Dr Solmi 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."

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Materials provided by University College London. Note: Content may be edited for style and length.

Journal Reference:

1. Francesca Solmi, Helen Sharpe, Suzanne H. Gage et al. Changes in the Prevalence and Correlates of Weight-Control Behaviors and Weight Perception in Adolescents in the UK, 1986-2015. JAMA Pediatrics, 2020 DOI: 10.1001/jamapediatrics.2020.4746