Oh, it's very unhealthy. That 19 lbs was probably mostly muscle you burned because you essentially went into starvation mode, Christopher McCandless style. There aren't enough calories in meat and (most) vegetables to survive on.

Safe weight loss is pretty simple and there's no other way around it. Cut out 500 calories per day, either by diet or exercise and you'll lose 1-2 pounds per week. Any more and you aren't burning the fat you have you are burning through your muscle.