For the past five years I’ve been keeping an eye on two trendlines—one creeping up and the other down—expecting imminent crossover. In September Unicef reported that it had finally occurred: obese children now outnumber those who are underweight.

The crossover is symbolic of a bigger, worrying story. The rising obesity capitals of the world are no longer just rich countries like America. Junk food is now crowding out healthy, nutritious foods in some of the poorest countries, too. Why ultra-processed foods lead to rapid weight gain is something that scientists are starting to unpack.

My colleagues and I have also been covering the things that could turn the tide, such as Britain’s trailblazing ban on junk-food ads online. The recent arrival of weight-loss drugs like Ozempic and Wegovy holds promise, and not just for obesity. But they are pricey, so governments must decide who needs them the most. A new medical definition of when obesity counts as a disease could help.