@ElizaBennett - That's so interesting! I breastfed my baby for about six months and I just assumed that it didn't do my bones any good. I'm planning another baby and hoping to make it past a year this time (I had work issues).
To pick up another issue from the article, isn't it only weight-bearing exercise that helps with preventing bone density loss? Swimming, for instance, is great exercise, but I don't think it helps your bones.
(When I was a kid, I was really into the space program. I remember learning that astronauts who are in space for a long time can lose bone density from not being exposed to gravity; that would actually be one challenge with a long mission, like going to Mars. And when you're swimming, of course you're weightless.)