What happens when you go on a very low calorie diet?

Tissue breakdown: Very low calorie diets puts a person in ketosis. The body does not have the proper amounts of protein, carbohydrates and fat to carry out all the functions it has to do. You will lose fat, for sure, but you will do this at the expense of losing protein, thus muscle breakdown. A very low calorie diet shold not be undertaken unless under the care of a doctor well versed in the diet and close f/u.

Many things: A very low calorie is defined as less than 800 calorie a day. The first thing will be a slowing of your metabolism. Secondly, if you restrict your carbs to 40 or 50 grams a day and get enough protein you will start to use body fat (ketones) for you energy needs. Only do this under a doctors supervision, before you start.