In blood tests, the accepted normal range for vitamin D levels runs from 20 to 50 nanograms per milliliter, says WebMD. A person with less than 12 nanograms per milliliter has a deficiency. Nevertheless, there is no medical consensus about what vitamin D level is necessary for optimal human health.
Since fat cells extract vitamin D from the blood, obesity is a common cause of vitamin D deficiency, according to WebMD. Medical illnesses such as Crohn's disease and celiac disease can prevent the digestive tract from absorbing sufficient amounts of vitamin D. To preserve bone health, the Institute of Medicine recommends that people over age 70 take substantially more vitamin D than younger people.