The two go hand in hand: if you don't get enough D, it won't matter how much calcium you get, because your bones can't absorb it properly. But if you don't get enough calcium, there's nothing for the vitamin D to help your bones absorb.
The National Academy of Sciences has developed recommendations for how much calcium and vitamin D you need at every age:
Young children 1-3 years old should get 700 milligrams (mg) of calcium a day.
Children 4-8 years old should get 1,000 mg per day.
Teenagers should get 1,300 mg of calcium a day.
Adults up to age 70 should get 1,000 mg per day. Women 51 and over should get 1,200 mg/day.
Women and men 71 and over should get 1,200 mg per day.
To "unlock" that calcium, the National Academy of Sciences recommends 600 international units (IU) of vitamin D per day from age 1 through age 70 and 800 IU after age 70. But some experts are now saying we need even more vitamin D.
Some osteoporosis experts say that we should be getting 800 to 1,200 IU of vitamin D per day. This is particularly important, because the primary source of natural vitamin D -- exposure to sunlight -- carries the potential risk of skin cancer. As more of us slather on sunscreen and stay in the shade, we need to make sure we get enough vitamin D from other sources.
To find out how much vitamin D you personally need, consider a blood test for vitamin D (25-hydroxy vitamin D) from your doctor. It measures how much vitamin D is in your body.
Experts think that vitamin D may do more to protect you from osteoporosis than only helping you absorb calcium.
"Particularly in older individuals, vitamin D deficiency makes you more likely to fall down," says Ethel Siris, director of the Toni Stabile Osteoporosis Center at Columbia University Medical Center. "If you get enough vitamin D, you not only improve your calcium, but you're less likely to fall and get a fracture."