As we age, our bone density drops, and we need more calcium. But, you also need to take magnesium, D3, and K-2 with the calcium for it to be absorbed properly. Additionally, your body can't absorb more than about 500mg of calcium at a time, so taking a 1000mg calcium tablet doesn't do you any more good than taking the 500. Dietary calcium, particularly dairy, is the best source of calcium. Barring that, I've found that some of the algae based calcium supplements are really good, but must be taken at meals. I use algaecal, but only on days that I'm unsure that I'll meet my dietary requirements alone. Plus, when training I take extra calcium. The muscles use calcium also, so running a lot requires a lot of calcium, due both to excreting a lot through sweat, but also just muscle contractions.