Is 500 calories a day a healthy diet?

500 calories a day is never healthy.  I know some of these so called diets out there even have you on this type of calorie deficit in the beginning but it is never a good idea.  What you have to remember is that your body needs a certain amount of calories per day just to do its basic functions.  So when you eat too few calories your body goes into starvation mode and slows down your metabolism.  Sure you might lose some weight in the beginning but long term you can actually gain more weight back then when you started.  For starters eat at least 1200 calories per to giver your body what it needs so you have a good balance and have good energy throughout the day.  If you really don't know if something is safe always consult your fitness professional first before trying anything extreme like this.  Sometimes more is better in this case.