The primary reason we have gained weight at a pace unprecedented in human history is that our diets have radically changed in ways that have deeply undermined our ability to feel sated. My father grew up in a village in the Swiss mountains, where he ate fresh, whole foods that had been cooked from scratch and prepared on the day they were eaten. But in the 30 years between his childhood and mine, in the suburbs of London, the nature of food transformed across the Western world. He was horrified to see that almost everything I ate was reheated and heavily processed. The evidence is clear that the kind of food my father grew up eating quickly makes you feel full. But the kind of food I grew up eating, much of which is made in factories, often with artificial chemicals, left me feeling empty and as if I had a hole in my stomach. In a recent study of what American children eat, ultraprocessed food was found to make up 67 percent of their daily diet. This kind of food makes you want to eat more and more. Satiety comes late, if at all.

One scientific experiment — which I have nicknamed Cheesecake Park — seemed to me to crystallize this effect. Paul Kenny, a neuroscientist at Mount Sinai Hospital in New York, grew up in Ireland. After he moved in 2000 to the United States in his 20s, he gained 30 pounds in two years. He began to wonder if the American diet has some kind of strange effect on our brains and our cravings, so he designed an experiment to test it. He and his colleague Paul Johnson raised a group of rats in a cage and gave them an abundant supply of healthy, balanced rat chow made out of the kind of food rats had been eating for a very long time. The rats would eat it when they were hungry, and then they seemed to feel sated and stopped. They did not become fat.

But then Dr. Kenny and his colleague exposed the rats to an American diet: fried bacon, Snickers bars, cheesecake and other treats. They went crazy for it. The rats would hurl themselves into the cheesecake, gorge themselves and emerge with their faces and whiskers totally slicked with it. They quickly lost almost all interest in the healthy food, and the restraint they used to show around healthy food disappeared. Within six weeks, their obesity rates soared.

After this change, Dr. Kenny and his colleague tweaked the experiment again (in a way that seems cruel to me, a former KFC addict). They took all the processed food away and gave the rats their old healthy diet. Dr. Kenny was confident that they would eat more of it, proving that processed food had expanded their appetites. But something stranger happened. It was as though the rats no longer recognized healthy food as food at all, and they barely ate it. Only when they were starving did they reluctantly start to consume it again.

Though Dr. Kenny’s study was in rats, we can see forms of this behavior everywhere. We are all living in Cheesecake Park — and the satiety-stealing effect of industrially assembled food is evidently what has created the need for these medications. Drugs like Ozempic work precisely by making us feel full. Carel le Roux, a scientist whose research was important to the development of these drugs, says they boost what he and others once called “satiety hormones.”