Some recent studies indicated that too much cycling might cause bone loss (see NY Times synopsis below). Since we spend more time on the bike, as triathletes, than any other sport, I found this very alarming. Could this be true? In one study, competitive cyclists had less bone density than controls (men who were active but not competitive). Some even had osteopenia, the early stages of osteoporosis. These men were in their late 20s, not 70s! The 2nd study found that a different group of competitive cyclists lost a significant amount of bone throughout the race season (although, luckily, they regained some of it in the off season). A third study compared bone density between weight lifters, runners, and cyclists. Guess which group had the lowest? Yup. The cyclists. However, in another study, triathletes gained bone density throughout the race season. Huh? (But, yea!)
Why would cycling cause bone loss? Some have postulated it's due to the non-impact nature of the sport. Perhaps. It is known that weight lifting and running increase bone density. But that doesn't explain the net loss of bone. I think these studies have to be taken with a pound of salt because they were all done on competitive cyclists, a group of athletes known for not making the healthiest choices for their bodies (doping, anyone?). The amount of stress they put on their bodies in a season is extraordinary. In addition, extreme weight loss is known to cause bone loss (leading to osteopenia and osteoporosis) in people with eating disorders, especially anorexia. Competitive cyclists are notorious for starving themselves into an unhealthy "race weight", believing it will lead to faster performances (which is wrong--it depletes the immune system and causes fatigue, actually slowing you down). I would hypothesize that the competitive cyclists used in these studies suffered from bone loss due to their low weights and not the cycling itself. It would be interesting to repeat these studies in non-competitive cyclists.
That said, the study that looked at triathletes reported an increase in bone density over the season. Could it be that triathletes participate in more impact sports (e.g. running)? They have higher percentages of lean muscle mass? Or were they at a healthier weight?
Some food for thought....literally