All theory. In real life, I watched a neighbor literally starve herself to be “fit” ... she actually once cut a blueberry in two to share with her husband...and it was the only blueberry (dessert). She died weighing 98 pounds, ... she was “fit!” but she died.
I can see the “intermittent fasting” craze as great cover for aspiring anorexics. They are just purists at it.