500 cals a day for an active male is downright dangerous. I hate to burst your bubble but you are not losing fat alone..you are actually losing muscle mass which is much easier for the body to metabolize when in 'starvation mode' Starvation mode for your height/weight is about 1200 cals. You should also remember that the 'heart' is a muscle.Muscle once lost cannot be replaced. The remaining muscle can be bulked up, but not replaced..if you regain weight in the future it will be even more fat. There is no easy way to permanent healthy weight loss...slow is best. As a health professional I am used to seeing sick people who are unable to take nutrition 'waste' and organ failure is a common side effect of wasting. Wise up..get a sensible diet plan going here before its too late.

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