In his study, the bone density of 32 male, competitive bike riders, most in their late 20s and early 30s, was compared to that of age-matched controls, men who were active but not competitive athletes. Bone scans showed that almost all of the cyclists had significantly less bone density in the spine than the control group. Some of the racers, young men in their 20s, had osteopenia in their spines, a medical condition only one step below full-blown osteoporosis. “To find guys in their twenties with osteopenia was surprising and pretty disturbing,” Smathers says.

Important to point out that "Competitive Cycling" could be bad for bones. Recreational riders have little to worry about. Title of the clip makes one wonder.

From the article (which was interesting):

Even more encouraging, most recreational cyclists probably don’t need to worry too much about their bones. “The studies to date have looked primarily at racers,” Smathers says. “That’s a very specialized demographic.