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Then don’t do it? It is not that complicated.
What health complication are you seeing that leads you to believe it is not sustainable?
[EDIT]
Adding to my answer: fasting works on through a different mechanism than calorie restriction. Under caloric restriction your body still tries to use all the sugars it can find. As you feed it more things to digest it will try to use those and slow down your metabolism if it can’t find any. With fasting it literally switches over to use ketones, which can only find by breaking down fats. Loosing half a pound under caloric restriction is indeed unsustainable, loosing half a pound because your liver metabolizes the fat you want to get rid of is a whole different ball game altogether.
Not to mention that if your diet is very low in carbs that you will see a lot of water losses. That’s what happen when you get rid of Adenosine triphosphate (ATP), the molecule that stores sugars in your system. That locks a lot of water in our system.