I just read an article talking about the dangers of osteoporosis in cyclists and swimmers because of the low impact thing. Also, it's a double whammy because there is a lot of calcium lost in sweat. If you're going to use swimming and cycling as main workouts, do remember to lift weights or run, even a little. This guy that the article talked about had bones that were in horrible shape, even though everything else was great and fit--because he only cycled and never did anything else. Bones are something we never think about, but neglecting it will hurt down the road. Oh, also take at least 1200 mg of calcium supplements to replenish the calcium lost. Hope this will be valuable for happy bones!