There has been an observed limit on how much body fat can be lost in a day, namely, 31 calories per pound of body fat per day
Even if you get sufficient dietary protein (very difficult on a 500 cal/day diet) AND you engage in resistance training, if your daily deficit is larger than the maximum rate of fat transfer, you're almost certainly going to lose muscle (never mind the part where the claims about HCG being muscle sparing are without any evidence).
Let's use an example. Let's say you weigh 160 pounds, and you have 25% body fat. This is 40 pounds of fat. At 31 calories per day, per pound of fat, the most you can lose is roughly 1240 calories worth of fat every day...about one third of a pound.
At 160 lbs, 25% body fat, your resting metabolic rate is around 1550 calories per day. Even with a sedentary day where you have essentially no activity, you'd probably burn about 1860 calories.
Now if you are eating 500 calories, your deficit is 1360.
This means that even under ideal conditions - eating enough protein for a zero nitrogen balance, and engaging in resistance training, you still can't ensure no muscle loss with a deficit of 1360 calories. But on 500 calories, you're probably not getting enough protein, and we've already assumed no exercise (otherwise the deficit would be even higher).
So you're looking at an absolute minimum
of 120 calories in energy that can't be met with body fat. And catabolizing muscle mass yields about 600 calories per pound (unlike 3500 per pound of fat), so you can pretty much assume that you're losing at least
a pound of muscle every 5 days, and probably more. If you do ANY sort of activity at all, including light walking, you're increasing the calorie deficit, and increasing the lean mass loss (which isn't just muscle, it also comes from SKIN, collagen, and your internal organs
So, you can waste money on an hcg placebo and lose "weight" faster, but you'll end up being poorer and looking (and feeling) worse than someone who ate more sensibly and took just a little bit more time. (This is just an example though. The numbers and results can change dramatically depending on whether you have more or less body fat, and are more or less active. And the composition of your diet matters too).
Seems like a no-brainer to me. Why race to the goal for what are likely worse results, in terms of both health AND appearance?