Calcium intake alone is not enough to maintain strong, healthy bones and to prevent osteoporosis.
Unlike calcium, vitamin D is not readily available from your diet.
The recommended intake of vitamin D is 400 to 800 IU daily.
The main source of vitamin D is through exposure to summer sunlight.
As people age, their skin becomes less able to make vitamin D.
Winter sunlight in most of the United States is too weak to produce vitamin D.
Even in the summer, sunblock and some clothing can prevent enough sunlight from getting through.
Very few foods are natural sources of vitamin D.
Some foods, such as milk and some brands of orange juice and breakfast cereals, are fortified with vitamin D.
Too Little Vitamin D Leads to Low Calcium Absorption and Low Phosphate
Calcium and phosphate are minerals that make bones strong.
Even if you are eating a diet rich in calcium or taking a calcium supplement, your body cannot absorb calcium properly unless you have enough vitamin D.
Too little vitamin D may lead to bone loss and osteoporosis.
Severe vitamin D deficiency may cause muscle weakness, which can lead to falls and a greater risk of fracture.
SOURCE:
http://www.fosamax.com/alendronate_s...d.jsp?WT.svl=2