I went on an 800 calorie diet once for about 6 weeks and got sick. Yeah, I lost weight but I felt like my body was eating muscle.
I am on an 800 cal/day diet right now. That initial period where you feel like crap goes away. I am fine and losing rapidly about 10# per month. Sometimes more.
Enough healthy fats and protein that you should not be losing muscle.
The website mercola.com is one I have found to be very reliable - he talks a lot about intermittent fasting and a ketogenic diet. It is meant to cycle, not do for weeks on end.
Very low calorie diets alone are not the answer. Avoiding simple carbs (sugars, high glycemic fruits, starchy vegetables, white flours) and decreasing low glycemic fruits and vegetables, and stopping grains altogether, will result in improved body function, large amounts of weight loss, and normalized blood sugars, IF you also eat mostly fats and proteins. Ketogenic diet. My plan allows 21 grams of carbohydrates, 88 grams of protein, and 112 grams of fat/day. You calculate the amount based on your height, weight, and amount of body fat. Guaranteed, you will NOT get hungry. Fats cause satiety, a feeling of appetite satisfaction. It takes a lot of time and energy to break down proteins into glucose, the nutrient your body usually uses for fuel. Fat doesn’t break down into glucose. Most carbohydrates break down quickly into glucose. Fats will break down into nutrients your body can use, other than glucose. It’s complicated, and I’m not explaining it very well, but the ketogenic diet is healthy for most people, and it works! Getting off grains is the hardest part for me, but you can make bread from nut meals and seeds.