"In recent years there have been a number of studies showing that osteoporosis and osteopenia may well be associated with riding a bike. A quick check of PubMed found the following recent studies, most of which seem to support this possibility. There are some very interesting conclusions here for road cyclists, mountain bikers, triathletes and runners. Following the citation for each study is the quoted conclusion from the authors followed by my comments..."All forms of exercise are not equal. Your body expects certain inputs, both in diet and activity, and if those correct inputs are not provided, the system will go awry. Our bodies evolved to run, and running promotes bone density. Cycling provides most of the input of running, minus the impact. It appears that's an important loss.
"...Essentially, running, even if mixed with cycling as with triathletes, seems to maintain or even improve BMD."
I don't bike on the road, only off-road. Good news for me:
"Three groups were studied: 16 competitive mountain bikers, 14 competitive road cyclists and 15 recreationally active men (control group). [Bone Mineral Density] was significantly higher at all sites in the mountain cyclists compared with the road cyclists and controls. Mountain biking may not have as deleterious an effect on BMD as road cycling. The study also suggests that it may be more beneficial than being “recreationally active.” This may be due to the need to dismount and run with, and perhaps even carry, the bike over challenging sections on the course."
It may also be attributed to the fact that mountain bikers fall a lot. I broke three bones in my first 12 months of mountain biking: two ribs and a nose.
(And then three more bones (toes) in my first year of barefoot running. This is more bones than I've broken in my entire life. It's definitely occurred to me that I may have been undergoing increasing loss of bone density, as I've had lots of falls from motorcycle riding and roller-blading, and never any breaks. The dietary changes I've made should resolve that.)
From a Paleo perspective, carrying things through the woods is something that would have been a daily occurrence. Mountain bikers also take a lot of shocks just riding along, despite the suspensions a lot of the bikes have.
I'll also note that our Paleo ancestors had significantly denser bones than desk-bound weekend warriors like myself.
P.S. I'll note there's an interesting parallel between barefoot-style running and mountain biking in this study. While barefoot-style runners were probably not included in any of the studies (a safe bet), both barefoot-style running and mountain biking involve low-impact exercise. In barefoot-style running, the low impact is guaranteed by using your body's suspension system properly (no heel-striking), and in mountain biking it's provided by the bikes' suspension and tires. So hard shocks like those occuring when heel-striking should not be required for increased BMD.