https://www.scientificamerican.com/article/unexpected-clues-emerge-about-why-diets-fail/

The physiology of weight regain still baffles scientists, but surprising insights have emerged.

Why is it so hard to lose weight and keep it off? For a moment, several years ago, it looked like we had an answer. In May 2016, The New York Times ran a front-page story on the findings from a study out of the US National Institute of Diabetes and Digestive and Kidney Diseases: fourteen reality show contestants had been tracked for half a dozen years after appearing on the program The Biggest Loser. Through dieting and very intensive exercise, each had lost at least 50 pounds during their time on the television series—and a couple had shed more than 200—but the follow-up study found they’d regained about two-thirds of what they’d lost, on average. A handful ended up even heavier than when they first appeared on the television program.