Almost half a billion teenagers around the world will be obese by 2030, experts have warned.
Adolescent health has reached a "tipping point", according to the Lancet Commission on Adolescent Health and Wellbeing.
Experts warned the main drivers of ill health among teens has switched from cigarettes and alcohol to weight gain and mental health problems.
They projected 464 million teenagers around the world will be overweight or obese by 2030 - 143 million more than in 2015.
But they did acknowledge that substance abuse among teenagers had declined.
It comes as separate research claimed weight-loss jabs for obese children could be effective in reducing parent battles around mealtimes. But the UK's National Institute for Health and Care Excellence (Nice) is "unable to make a recommendation" about use of the jabs on teenagers because of a lack of evidence of their impacts.
Weight-loss jabs, also known as GLP-1 receptor agonists, work by reducing food cravings but are known to cause nausea, diarrhoea, stomach cramps and vision problems.
They have surged in popularity in recent years, and are available on the NHS for adults with a high body mass index (BMI) or through private providers.
Progress in teen health 'lagging well behind'
Today's Lancet report warned that progress in teenage health has "lagged well behind" the improvements that have been made in the health and development of young children, adding that these benefits could subsequently be "undermined" by increasing ill health as children enter their teenage years.
With a shift to an increasingly online world, the commission also argued against overly restricting access. Australia has recently approved a law to ban children under the age of 16 from using social media, in one of the strictest measures in the world.
The experts also said this generation is the first to grow up under harsher climate conditions making them the "first cohort of humans who will live their entire life experiencing the growing reverberations of climate change".
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Commission co-chairwoman Professor Sarah Baird, from George Washington University in the US, said: "The health and wellbeing of adolescents worldwide is at a tipping point, with mixed progress observed over the past three decades.
"Whilst tobacco and alcohol use has declined and participation in secondary and tertiary education has increased, overweight and obesity have risen by up to eight-fold in some countries in Africa and Asia over the past three decades, and there is a growing burden of poor adolescent mental health globally.
"Additionally, the challenges faced by the world's adolescents are at risk of being exacerbated by emerging global issues including climate change, world conflicts and a rapid transition to a more digital world.
"Investing in the health and wellbeing of young people is crucial for safeguarding our collective future."
Last year, a report by the Food Foundation said children in the UK were getting shorter, fatter and sicker due to "shocking levels" of poverty and food deprivation.
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