You need adequate vitamin D to absorb calcium and maintain good bone health. Without it, your risk goes up for bone health conditions like osteoporosis and osteomalacia. Your body also needs vitamin D for healthy nerves, muscles and immune system function. If your doctor suspects you are low on vitamin D, she is likely to order a blood test to measure the levels of this vitamin in your body. Inadequate blood levels of vitamin D are common worldwide, possibly affecting about half of the population on earth, according to Harvard Medical School.
Blood Testing
The blood test to measure vitamin D measures total 25-hydroxyvitamin D, which is the amount of vitamin D-2 and D-3 in your blood. You need your doctor to help you interpret results. There are no optimal blood levels established for vitamin D, so there is no consensus on the level that indicates deficiency, according to the U.S. Office of Dietary Supplements, or ODS. Also, optimal concentrations for blood levels of vitamin D are likely to vary depending on your age. Other factors, such as your blood calcium or magnesium levels also are factors when screening for possible health risks because these vitamins work together in your body.
Levels
Blood levels of vitamin D are reported in both and nanograms per milliliter -- ng/mL -- or nanomoles per liter -- nmol/L. According to ODS, blood levels that are less than 30 nmol/L or 12 ng/mL signal vitamin D deficiency leading to poor bone health. Blood levels lower than 50 nmol/L or 20 ng/mL are inadequate for overall health and good bone health, according to ODS.
Health Risks
If you are low in vitamin D, you are at risk for osteomalacia, which causes weak bones. Symptoms of low vitamin D levels, like muscle weakness and bone pain, often stay undetected in early initial stages of the deficiency, according to ODS. Vitamin D deficiency also may raise your risk for cardiovascular disease, some cancers, immune diseases like multiple sclerosis, and infectious diseases like seasonal flu and tuberculosis. This needs more research to determine what role, if any, vitamin D plays in the development of such conditions, notes Harvard Medical School. Children who have low blood levels of vitamin D may suffer rickets in which bone tissue doesn't properly mineralize. This causes skeletal deformities and soft bones.
Trends and Recommendations
During the late 1990s and early 2000s, blood concentrations of vitamin D have declined among men in the United States. However, they have not declined among women. The decline links with less milk intake, more use of sunscreen and increases in body weight, according to ODS. Low blood levels of vitamin D usually occur when vitamin D intake is less than recommended over time, you have limited sunlight exposure, your digestive tract does not adequately absorb vitamin D or when your kidneys cannot convert vitamin D to its active form. Deficiency also links to lactose intolerance, milk allergies, veganism and ovo-vegetarianism, notes ODS. The recommended dietary allowance, or RDA, for vitamin D is 600 IU, or 15 mcg, for adults younger than 70. Over age 70, your RDA is 800 IU, or 20 mcg. These RDAs are controversial, however, according to Harvard Medical School. You may need more than these RDAs for disease prevention and bone health, according to the experts at Harvard.
Considerations
Your blood levels of vitamin D increase when you consume additional vitamin D, but this relationship is not linear. The reason for this remains unclear at the time of publication, according to ODS. However, increasing your blood levels of vitamin D so they become higher than 50 nmol/L requires more vitamin D than raising your levels of vitamin D to 50 nmol/L from a level that is lower than 50 nmol/L, note the experts at ODS.
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