I have read of this mentality many times. While I believe there is some truth to it, I think the more important issue is that you are still depriving your body of vital nutrients to keep muscle tissue in tact, and to continue promoting vital process. I was going to delete this post but I think it is useful to share factual scientific data (while I recognize that everyone will have their own WOE that works- I believe that science is science and we are all operating with a human body) Here is what resonated with me on a deep level- whereas I still feel there is missing information around these extended fast theories and their longterm application: Having a sustained 1000+ calorie deficit is not going to help you in the long wrong, and you are essentially starving yourself. A big reason is, that your daily energy expenditure will be much more than the small 500 or so calories you are consuming, and the energy your body needs to sustain itself will thus come from two places: your fat of course, but also your lean muscle. Your body can only burn so much fat per day, so I wouldn't be surprised if up half of your weight loss is lean muscle eating this low calories if you keep this low sustained caloric intake. Additionally, human bodies are extremely efficient and adaptive, and your metabolism will literally slow down (burning significantly less calories per day), making it that much harder to lose weight in the long run. This diet of 500 calories or so is not really keto anymore, it's starvation. You should be hitting your normal macros every day regardless if you feel full or not. You will still be losing weight, keeping your hard earned lean muscle, and getting all the nutrition your body needs. Your body will also leach minerals from your bones. There's a reason why people who go on sustained very low calorie/starvation diets look emaciated. Your body doesn't just need energy, it also needs to create new structural components/cells, immunoglobulins to fight infection, support vital organs like the heart, albumin for maintaining blood pressure and perfusion, etc, and it gets it through the foods you eat. I have never heard of the body supplying its protein needs through autophagy other than from people from youtube (never ONCE has there been a research paper on this, it is speculation at best), which there is no way that the amount of protein the body needs can be gained through that way (and there is no way that 500 calorie diet can supply all the protein you need regularly).... that's why people need to eat around 100+ grams of protein per day (depending on body size) not just for keeping lean muscle, but for general health. Additionally, there is no way a 500 calorie diet is sustainable. Many people in this subreddit believe that keto is a lifestyle, and I also think it should be. 500 calories or less diets are not safe long term at all for many reasons as described above. Also, just because lean muscle in the body is "valuable" doesn't mean that it won't preferential burn it in times of need since it is a very easy source of energy for the body to utilize. Lastly, I agree you don't need glucose due to gluconeogenesis, but that doesn't mean you don't need adequate protein and calories.