When I am out running and pounding the pavement, I tell myself that among other things I am helping my bones.
While it is true that weight bearing exercise helps maintain bone density, sweating can translate into loss of calcium, the bone building mineral. A study at Purdue University found that women who exercise for an hour daily lose about 92 mg of calcium a day through sweat. Since many women are already falling short on this important bone fortifying mineral, losing almost 10 percent of your recommended daily intake (RDI), is just not good.
The answer, of course, is not to stop exercising but to ensure you are getting enough calcium daily. You should be getting 1000 mg a day, and while the mineral is much better absorbed from food sources, a calcium supplement will help. Check out What Does Your RDI Look Like: Calcium if you need ideas on how to eat for your bones.