I recall that the research I've seen on this in the past showed reduced bone density etc. only in serious endurance runners (e.g. multiple marathons, ultra-mara

I recall that the research I've seen on this in the past showed reduced bone density etc. only in serious endurance runners (e.g. multiple marathons, ultra-marathon) and indicated improvements in your "average" runner. Bone density is definitely an issue, especially for women.