Keto dieters lose weight rapidly because the carb-free program induces ketosis. In that state, the body is burning fat, and it can burn a lot of fat quickly. That's keto's appeal.
Dr. Williams, a past president of the American College of Cardiology, believes short-term keto is fine. Speaking with Plant Based News (yes, there's news based on plants), Dr. Williams said he likes the idea, "the basic concept: you change your dietary habits and you change something. "However, he said, "the science of it is wrong."
https://www.maxim.com/food-drink/top-cardiologist-no-one-should-do-keto-2018-9