Vitamin D is unique among vitamins in that it is possible to do entirely without vitamin D in your diet as long as you get enough exposure to sunlight. The problem is that many people do not, and also do not get enough vitamin D from their diet to make up the difference. If you are older than 70, overweight or obese, Hispanic or African-American, you are at higher than average risk for being vitamin D deficient.
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Vitamin D is needed to promote absorption of calcium from food, and thus for bone health. In adults, low blood levels of vitamin D are linked to increased risk of osteoporosis and broken bones, especially in the spine and hip. Higher blood levels of vitamin D also appear to reduce the risk of injury from falls, a major concern for the elderly, and may slow the development of dementia, and some types of cancer.
If you are having a blood sample collected for any reason, ask your doctor if the tests should also include vitamin D, as it is not a standard test. Joseph Zerwekh, Ph.D., reviewed vitamin D assay methods in the April 2008 issue of "The American Journal of Clinical Nutrition." The list of approved tests includes radioimmunoassay, enzyme-linked immunosorbent assay, chemiluminescent assay and liquid chromatography-mass spectrometry methods.
Your blood test result will consist of a number that represents the nanomoles per liter, which is abbreviated as nmol/L, or else as nanograms per milliliter, which is abbreviated as ng/mL. In addition to your result there will be a normal range, and a note if you are over or under the normal range. Deficiency is defined as less than 27.5 nmol/L, low as 27.5 to 50, adequate as 50 to 125, high as 125 to 400, and of concern as more than 400 nmol/L. Keep in mind that although laboratory reports may identify over 50 nmol/L as adequate, a review by Dr. Bischoff-Ferrari in the July 2006 issue of "The American Journal of Clinical Nutrition" supports values of greater than 75 nmol/L as necessary for ideal health.
If your result is below the normal range, discuss a plan to remedy this with your doctor. If it is marginally low, it may require nothing more than taking a daily supplement of 1000 to 2000 IU/day. If it is very low, a prescription for 50,000 IU taken orally once a week for 8 to16 weeks, followed by a second blood test, may be more appropriate.
You can put yourself at risk by taking too much vitamin D for too long. The Dietary Reference Intakes set a safe upper intake at 2000 IU/day. Too much above this for too long may raise your blood calcium to abnormally high levels, with side effects ranging from nausea and loss of appetite to calcified arteries and kidney stones.