Vitamin D is a fat-soluble vitamin. It is found in food, but also can be made in your body after exposure to ultraviolet rays from the sun. Vitamin D exists in several forms, each with a different activity. Some forms are relatively inactive in the body and have limited ability to function as a vitamin. The liver and kidney help convert vitamin D to its active hormone form.
The major biologic function of vitamin D is to maintain normal blood levels of calcium and phosphorus. Vitamin D aids in the absorption of calcium, helping to form and maintain strong bones. It promotes bone mineralization in concert with a number of other vitamins, minerals, and hormones.
Vitamin D and Osteoporosis
It is estimated that over 25 million adults in the United States have, or are at risk of, developing osteoporosis. Osteoporosis is a disease characterized by fragile bones and results in increased risk of bone fractures. Having normal storage levels of vitamin D in your body helps keep your bones strong and may help prevent osteoporosis in elderly, non-ambulatory individuals, in post-menopausal women, and in individuals on chronic steroid therapy.
Researchers know that normal bone is constantly being remodeled (broken down and rebuilt). During menopause, the balance between these two systems is upset, resulting in more bone being broken down (resorbed), than rebuilt. Estrogen replacement, which limits symptoms of menopause, can help slow down the development of osteoporosis by stimulating the activity of cells that rebuild bone.
Vitamin D deficiency, which occurs more often in post-menopausal women and older Americans, has been associated with greater incidence of hip fractures. A greater vitamin D intake from diet and supplements has been associated with less bone loss in older women. Since bone loss increases the risk of fractures, vitamin D supplementation may help prevent fractures resulting from osteoporosis.
In a group of women with osteoporosis hospitalized for hip fractures, 50 percent were found to have signs of vitamin D deficiency. Treatment of vitamin D deficiency can result in decreased incidence of hip fractures, and daily supplementation with 20 mcg (800 IU) of vitamin D may reduce the risk of osteoporotic fractures in elderly populations with low blood levels of vitamin D. Your physician will discuss your need for vitamin D supplementation as part of an overall plan to prevent and/or treat osteoporosis when indicated.
Vitamin D and Cancer
Laboratory, animal, and epidemiologic evidence suggest that vitamin D may be protective against some cancers. Some dietary surveys have associated increased intake of dairy foods with decreased incidence of colon cancer. Another dietary survey associated a higher calcium and vitamin D intake with a lower incidence of colon cancer. Well-designed clinical trials need to be conducted to determine whether vitamin D deficiency increases cancer risk, or if an increased intake of vitamin D is protective against some cancers. Until such trials are conducted, it is premature to advise anyone to take vitamin D supplements to prevent cancer.
Vitamin D and Steroids
Corticosteroid medications are often prescribed to reduce inflammation from a variety of medical problems. These medicines may be essential for a person’s medical treatment, but they have potential side effects, including decreased calcium absorption. There is some evidence that steroids may also impair vitamin D metabolism, further contributing to the loss of bone and development of osteoporosis associated with steroid medications. For these reasons, individuals on chronic steroid therapy should consult with their physician or registered dietitian about the need to increase vitamin D intake through diet and/or dietary supplements.
Vitamin D and Alzheimer’s Disease
Adults with Alzheimer’s disease have increased risk of hip fractures. This may be because many Alzheimer’s patients are homebound, and frequently sunlight deprived. Alzheimer’s disease is more prevalent in older populations, so the fact that the ability of skin to convert vitamin D to its active form decreases as we age also may contribute to increased risk of hip fractures in this group. One study of women with Alzheimer’s disease found that decreased bone mineral density was associated with a low intake of vitamin D and inadequate sunlight exposure. Physicians evaluate the need for vitamin D supplementation as part of an overall treatment plan for adults with Alzheimer’s disease.