Did your doctor put you on this diet? If not; see your Doctor as soon as possible.
A 500 calorie diet is a starvation diet and is not safe. Your body can not live on 500 calories/day.
Do not eat only 500 calories a day, that is extremley unhealthy. Look for a new diet. Do not starve yourself. Eating 1200 is what I would recomend. It's high enough that you can still eat three meals a day, but not low enough that you're starving yourself from nutrients that you need.
I'm starting the 500 calorie diet today! Keep me posted on your results and I will do the same!
500 calories a day on the hcg diet protocol isn't bad, because the hcg is releasing stored fats and you are living of the stored fats as well as the 500 calories. I lost over 20 pounds on it over a two years ago, but had gained 5 pounds back do to food intolerance. I have just finished a three day juice fast and lost 7 pounds. I plan to eat two meals and day and juice at night to keep my system flushed from toxins that was causing my weight gain.
There's no scientific evidence that the HCG does anything to help you lose weight -- it's the starvation diet that does it.