>>9579
I think this would be a good story, and there are plenty of routes to take. Perhaps it's an airborne virus, or the result of some environmental disaster, or an unforeseen consequence of a technological innovation. Maybe it's just an act of God.
Here's an idea. Maybe nobody actually knows why?
Keep a justification in your head, but it's not important. We are already in an obesity epidemic, and the reasoning at present in real life is still a point of contention. What if it just started exponentially escalating? An average weight gain of a pound a year becomes a pound a month, becomes a pound a day, becomes a pound an hour. Think the Wildfire Virus in The Walking Dead. One day the world is getting fatter, and nobody knows why. People don't even notice it at first, that they're hungrier, that they aren't the only people gaining weight. Eventually, it's impossible to ignore. Data showing not a single person on Earth has lost weight for weeks, months. Scientists study it and come up flummoxed, in between fighting their new hunger cravings. Initially, doctors and governments try public health initiatives, but not only are people healthier than ever before, their metabolisms and bodies are actively working against them. People don't immediately accept it, and many struggle to come to terms with this change. Slowly, it becomes normal. People adapt.