Well, that depends on what part of the body you're talking about. Due to the heavy resistance on the legs, cyclists undoubtedly have significantly increased bone density there.
The studies I've read show the most significant loss of bone density (compared to other athletes) occuring in the spine. None has showed an increase in leg density and one showed that the bone density in the hips and femurs actually decreased during the cycling season when compared to pre-season values.
This is not really surprising if you work out the actual forces involved in the pedal stroke:
The peak power output for a world class pro cyclist is about:
- 24 W/kg sustained for 5s (this would be reached during a brief, hard burst)
- 11 W/kg sustained for 1min (this might be a 1km track sprint)
- 6.4 W/kg sustained for an hour (this is typical time-trial pace).
- 4.7 W/kg over 30min on the final climb of a mountain stage of the Tour de France.
This means that a 65kg pro cyclist riding at 80rpm (a fairly low cadence) produces an average force of:
- 54 kg/leg over 5s (7 reps)
- 24 kg/leg over 1min (80 reps)
- 14 kg/leg over 1hr (4,800 reps)
- 10 kg/leg climbing a mountain. (2,400 reps)