The advice about supplements can get as finicky as the mineral itself. Calcium isn't easily absorbed in large amounts, so it's best to break up your daily intake into two to three doses and to take it with meals, says Heaney. For the calcium to reach your bones, you need to make sure you're getting 200 international units (IU) of vitamin D a day if you are 50 or younger; 400 IU, ages 51 to 69; 600 IU, 70 and older. Your body probably produces enough of the vitamin if you simply step out into the sunlight every day. But you may need to take a D supplement if you're supervigilant about sunscreen or if you're African-American, since melanin acts as a shield. You may have heard that supplements with calcium citrate are absorbed more easily than the calcium carbonate found in antacids, but Heaney says the evidence for both is equally strong. And if you can't be bothered with rules, just down your daily quotient all at once. It's better than not taking any at all.