In the excellent book "Bike For Life" by Roy Wallach and somebody else, they have a huge chapter on osteoporosis and cycling. Cyclists are definitely at risk for osteoporosis. While it would make some sense that cyclists would be more likely to get osteoporosis than runners or weightlifters, studies have shown that cyclists get osteoporosis more often than couch potatoes. The Bike for Life authors found this shocking. There is no doubt that when doing a lot of biking you are really working your leg muscles. You'd think that would stimulate bone growth the way weight lifting does.

The authors interviewed a number of experts on osteoporosis and came to the conclusion (not backed up by any studies though), that cyclists are at risk for calcium and other vitamin deficiencies due to the amount of sweating. On a typical 3 hour ride you can sweat 100 oz or more of water. THe high amount of fluid consumption and sweating dilutes calcium and vitamin D, two of the main factors in bone density. Their recommendation (as I recall) is to take at least 1500mg of calcium supplements a day and vitamin D - even if you are getting a decent amount of calcium from other sources.

While their conclusions are not backed up by research, they do make sense.

BTW- the book "Bike for Life" is fantastic. I wouldn't have bought it based on the description but received it for Christmas from my brother. It covers a lot of things that other books don't hit on much or at all - stretching and yoga, effect of biking on relationships and how to harmonize the two, osteoporosis, common cycling overuse injuires and how to prevent them, cycling and impotence, etc.