I laughed my ass off the first time I heard of this. "Lose weight fast using HCG...and a 500kcal/day diet"
It would be nigh-on-impossible not to lose weight eating 500 calories a day, or 1200 for that matter, unless you are a bedridden 90-lb old lady. The HCG likely doesn't have a thing to do with it. You're putting your body into starvation mode...muscle is catabolized and the protein turned into fuel--this is not only quite taxing on your liver, but of course muscle tissue burns calories even while resting.
The less of it you have, the harder it is to keep fat at bay. Lean mass is inextricably linked to good health in so many ways...being "skinny fat" is not OK any more than being fat is. Lean mass is both a result and a cause of good overall health.
Metabolism slows to a crawl, which makes it certain that when you stop the diet, you WILL put all the weight back on, and then some. Endocrine system circles the drain, and with it your future ability to maintain lean mass and keep fat off. You can permanently damage your hormones with extreme calorie restriction. Someone who's obese and weak invariably has a lousy hormone balance to begin with. In short, every crash diet ever conceived only makes it MORE likely that you will retain fat in the future. Your body keeps fat for evolutionary reasons, as insurance against periods of food shortage. When you starve it, guess what, you are reinforcing that mechanism! The second you start eating any semblance of a normal diet, your body does its best to pack on fat in preparation for the next time you starve it.
I can't believe some of the foolish crap people buy into. We all know in our hearts that you generally get out of something exactly what you put into it. Fitness is no exception. There are NO shortcuts. NONE. If you aren't willing or able to make permanent lifestyle changes, you are going to stay the way you are. End rant.