I am about 30-40 lbs overweight myself. I would regularily do a 5 day fast to detox my body where I would only intake water, and I would exercise for about 3 hours a day. Every single time I would loose 14 lbs in 5 days.....if that helps you. After the 5 days I would then eat only natural foods very high in fiber at about600 cals per day. The first week of eating I platueaed...from my fasting loss of 14 lbs I didn't gain anything back. THe second week up to 8 weeks I lost about 5 lbs per week. So in 8 weeks with only 5 days fasting I lost about 40 lbs going from a size 12 to almost a 4. I exercised like you wouldn't believe and stuck with the 600 calories. I could onyl take it for so long and ended up gaining all teh weight back.
So my point to you is yes its possible just make sure you going into it with the mindset of a lifestyle change and nost just a way to loose it fast. I had it right infrom of me to make the lifestyle change and I didn't....The longest I have fasted was 10 days....after 7 days really the worst sideaffect would be not sleeping and being bored. Its very hard ot keep up with exercising 3 hours a day while eating nothing. I am someone who loves to exercise so after the first 5 days I cut back to only walking 2 miles a day...or a light pilates video tape.