Blogger's Note: The purpose of this post is not, obviously, to dissuade anyone from cycling. Rather, it's a heads-up for everyone to be aware of the issue before it becomes an issue for you.
David at the Fredcast Cycling Podcast brought this story to my attention in his episode #115, and references an article in the Los Angeles Times.
According to studies reported in the journal Medicine & Science in Sports & Exercise, avid or competitive cyclists are at increased risk for loss of bone density, making fractures due to falls more likely. This is because cycling is a low impact activity and doesn't put stress on bones that would make our bodies add density to them to compensate. Swimming is another sport that might make its participants suffer such a risk, but swimmers obviously don't have nearly the risk of a fall breaking a bone. The lower spine is particularly susceptible to this problem.
Athletes in sports like running and cross-country skiing aren't affected by this as they are forced to support their weight much more than cyclists are, making their bodies reinforce their bones with minerals.
Some other issues that might cause the problem are nutritional factors, as cyclists sweat out lots of important nutrients. There are possibly hormonal issues at play here, too.
The result could be an increased risk of osteopenia - lowered bone mineral density, or osteoporosis - basically osteopenia taken to the extreme. This is similar to the bone conditions encountered in elderly people but is happening in people much younger.
David (and I) would recommend that you mention this study to your doctor at your next visit, and see what sorts of solutions might be available. In the meantime, increased consumption of vitamin D and calcium can help.
People, not speed.