I get what you're saying about fad diets and I'm sure we'd agree there. However, your theory about the simple math of calories in calories out just doesn't agree with science, or at least the studies I've read. I'm not sure if a strict keto diet would prohibit an apple or not (probably depends on what else you're eating too), but I definitely wouldn't think twice about eating an apple. As long as I didn't plan on sitting still for a long period of time afterward. Apple juice however is an entirely different story. You're body has no idea what to do with a blast of that much sugar with no fiber.