Vitamin D is a hormone that is essential for proper bone health. A precursor to Vitamin D called 7-Dehyrocholesterol is found in the skin. Ultraviolet B radiation (from sunlight) converts this precursor to Vitamin D. The effectiveness of this radiation may be affected by the melanin content in the skin (less absorption in Black people), use of sunscreen (less absorption), cloud cover, and the time spent indoors. Two additional reactions, the first occurring in the liver and the second occurring in the kidney results in the formation of “active” Vitamin D. This active form increases body calcium by increasing absorption from the intestines and increasing calcium release from bone, the largest store of calcium in the body. Calcium’s role in bone formation is analogous to the role of steel in a skyscraper, important in maintaining the rigidity and structural integrity of the bone.
Causes of Vitamin D deficiency include
- Decreased skin production (higher in Black people, the elderly, use of sunscreen)
- Inadequate dietary intake
- Decreased absorption (impaired fat absorption, ulcerative colitis)
- Increased loss of Vitamin D (associated with the use of certain medications)
- Severe liver disease
- Kidney disease
The prevalence of Vitamin D deficiency has been estimated to be approximately 50% (based on a 30 ng/mL cutoff). Severe deficiency can cause rickets in children. The lack of adequate calcium for bone development results in growth retardation, increased risk of fractures, bowed legs, abnormal skull development and bone pain. In adults osteomalacia may result, with similar consequences except for the growth retardation seen in rickets. Less severe deficiency can result in osteoporosis as a result of decreased calcium absorption and inadequate bone mineralization (with calcium). This increases the risk of fractures, especially among post menopausal women. There are some studies that indicate that inadequate Vitamin D levels may increase the risk of certain cancers (breast cancer, colorectal cancer and prostate cancer have been evaluated), and reduce the risk of heart disease. However these findings remain controversial and will likely need more studies before definitive recommendations can be given. Other studies have evaluated whether Vitamin D may reduce the risk of diabetes, hypertension and multiple sclerosis.
As mentioned, fish oil, fortified cereal and dairy products and egg yolk are good dietary sources for Vitamin D. Each of these may conflict with other dietary recommendations. For example, individuals may be allergic to fish, may be on a strict diet that does not allow for the extra carbohydrate intake associated with cereals or may be unable to tolerate dairy products. In such cases supplementation is recommended (always check your doctor first). The recommended dietary allowance (RDA) for Vitamin D is 600 international units daily for adults 70 and under and 800 units daily for adults over 70. For those with Vitamin D deficiency, the requirements will be higher, and will depend on the severity of the deficiency. There has been some controversy regarding the appropriate daily intake of Vitamin D. Some sources recommend much higher levels of intake than the published RDA values. If there is any doubt, it may be useful to have your Vitamin D level checked to determine whether your intake is adequate.