After reading the thread upto this point (Dr.M's 1st post), looking for someone to argue FOR the correct response to "riding a bike to lose weight", I have to agree with Dr.M. It is the intensity and how long one maintains the ideal intensity that matters. I have been riding a MTB for with my heart rate monitor on average 45min - 90min rides and find that I can keep up heart rate up at 80% ride thru'. Becuase of the variations in terrain, having to navigate rocks, roots, mud, gravel, constant hills and small jumps, stairs etc., (the list is endless), my entire body is being exercised to stay on the bike and keep momentum. Personally, I would find it boring to ride a Road Bike for 2 hours to gain the same amount of calorie burn. The downside to MTB might be more maintenance/repairs to your bike and bodily injury, depending on your level of riding. FYI, a combination of circuit training at the gym (45min x 3days/week) plus 2-3 days of trail riding over the summer (3 months) have helped me drop my body fat percentage from 25% down to 17.5%. I can't wait for the next season, because it's too damn cold to ride out here in SouthWestern Ontario now.
cheers,