if you talk to a doctor they will tell you the same. the reason being is that most overweight people are already low in much needed nutrients etc. when you sweat and work out, (mild stretch and yoga totally fine)you lose minerals you body needs to keep functioning. someone on this board. (i dont know if ther are still here) ended up in the ER because her potassium was dangerously low and she was having heart palps. she was exercising during water fasting. after heart palps is heart failure.
not something i would want to chance. and its not something a doctor would tell you chance either.