A 500-calorie diet is an extreme form of a very low-calorie diet (VLCD). It requires you to drastically reduce the amount of food you eat, usually to a maximum of 800 calories per day.
VLCDs use meal replacements like drinks, shakes, and prepared food bars instead of meals for at least two meals a day. VLCDs are meant for people who are very overweight and have been unable to lose weight after trying many diet plans. Depending on how much weight you have to lose, it’s important that you monitor your caloric intake. Typically a low calorie diet consists of 1500-1800 calories per day. For many, it's as simple as shaving 500 calories off of what they're currently consuming.
Within the first week or so of trying a low calorie diet, you might find that you lose a little more weight as your body adjusts to the new routine. Another very important aspect of a low calorie diet is drinking plenty of water.