I don't know too much about the validity of the bone density concern, but the first time this story did the rounds of the mainstream newspapers it was amazing how little time it took a car-driving colleague who is "greener-than-thou" in every other area of her life to let me know that my commuting was liable to make my bones crumble to dust. Um yeah. Thanks. Riding a bike has also lowered my blood pressure and my weight, increased my fitness level and improved my quality of life, saved me a ton of gas money and lowered my environmental impact. And it has helped me to set a good example for my kids. No way this study is getting me off my bike.