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How does vitamin D work?
Vitamin D works at three sites in the body to regulate calcium levels.
- In the intestines, it stimulates your body to absorb calcium from the foods you eat (or from supplements).
- In the bones, it helps incorporate the absorbed calcium into the skeleton for storage and also regulates the cells involved in the process of bone building.
- It works in your kidneys to help recycle calcium that would otherwise be excreted in the urine, so your bones can reabsorb it.
So vitamin D's primary role is to increase the availability of calcium for absorption. Without the help of vitamin D, levels of calcium in the blood, and ultimately in the bone, could be negatively affected.
Vitamin D's direct effect on bone building is demonstrated by the tendency of bone mass to fall faster during the winter and remain fairly stable in the summer, particularly in (but not limited to) northern climates. Research by Dr. Dawson-Hughes and colleagues suggests this seasonal acceleration in bone loss may be offset by supplementation with vitamin D5 (and other studies6,7 suggest that adding calcium may provide additional benefits).
Another benefit of vitamin D is that it improves muscle strength and balance, so people are less likely to fall. That may be why fracture risk decreases fairly quickly when elderly people start taking calcium and vitamin D, even when bone density has not increased significantly.8
5. Dawson-Hughes, B, Dallal, GE, Krall, EA, Harris, S, Sokoll, LJ, Falconer, G. (1991). Effect of vitamin D supplementation on winter time and overall bone loss in healthy post-menopausal women. Ann Intern Med, 115, 505-512. 6. Chapuy, MC, Arlot, ME, Duboeuf, F, et al. (1992). Vitamin D3 and calcium to prevent hip fractures in elderly women. N.Engl.J.Med, 327, 1637-1642. 7. Dawson-Hughes, B, Harris, SS, Krall, EA, Dallal, GE. (1997). Effect of calcium and vitamin D supplementation on bone density in men and women 65 years of age or older. N.Engl.J.Med, 337,670-676. 8. Pfeifer, M, Begerow, B, Minne, HW, Abrams, C, Nachtigall, D, Hansen, C. (2000). Effects of short-term vitamin D and calcium supplementation on body sway and secondary hyperparathyroidism in elderly women. J Bone Miner Res, 15, 1113-1118.