I too am concerned with staving off osteoperosis. I've watched my mother shrink several inches in height and suffer multiple bone breaks starting in her 50's.
My research leads me to disagree with the conventional "consume more calcium" approach. I read a book on osteoperosis by a specialist which made this recommendation although the author acknowledged no data supported it. Data does support a diet connection and weight-bearing exercise. While the problem is lack of calcium in the bones, it is not clear that ingesting more calcium is the solution. In fact, countries with low dairy & calcium consumption (Asia) have less osteoperosis problems. This may be due to more exercise or other factors, but another explanation seems to be the overall diet. Specifically, it seems that excess protein (dairy or meat) and refined food (white flour & sugar) cause minerals to be leached out of our body that are needed for digestion, including calcium. This was explained most clearly in "The China Study" and the less scholarly "Sugar Blues." We can try to compensate for the mineral imbalance with vitamin supplements, but it seems more reliable and cheaper to rely on whole food and limiting protein to modest portions.
I also heard that rowing does not qualify as "weight-bearing exercise," neither does walking, and that jumping and running are best for the legs. However, since strength training does count, I think rowing does provide weight-bearing benefits, just for different bones. I suspect rowing helps strengthen wrist bones, for example, which are vulnerable in falls. Also, it is weight bearing for the legs, shins and back in ways that jumping and running are not. Perhaps this can be enhanced by including a few minutes of full effort strokes similar to a weight lifting set. Since jumping and running aren't my favorite sports and put a lot of stress on the knees, I mostly limit myself to gentler sports with including brisk walking. For hard work, I can always dig rocks out of the garden.
