So my understanding is that there are positive changes associated with spending time in a state of ketosis, which is brought on by fasting as well as extremely

So my understanding is that there are positive changes associated with spending time in a state of ketosis, which is brought on by fasting as well as extremely low-carb diets: especially in the case of people who are already suffering insulin-related metabolic damage (like diabetes, prediabetes, or insulin insensitivity), there's evidence nutritional ketosis can help to halt or reverse that damage. What the talk I linked in the other comment talks about is something different: there's evidence that prolonged periods of severe calorie restriction can make your metabolism's bias move more toward storing calories and away from expending energy.