If you're a woman and you cycle or swim on a regular basis, you're doing your heart and cardiovascular system a lot of good.
Unfortunately, this non weight-bearing exercise is not the best type of exercise to maintain and perhaps enhance bone density and strength -- yes even cycling. You need to get more impact into your exercise program.
Although this also applies to men as well, as discussed in Velo News recently, women are even more at risk of declining bone density.
Impact exercises like running and jogging and appropriate weight training exercises are highly recommended. Here's a roundup.
I am really glad to see you covering this topic. I read about another easy exercise not too long ago that uses back weights to strengthen bone. Described here… Stronger back muscles can lower future fracture risk