Dr. Adit A. Ginde of the University of Colorado Denver School of Medicine in Aurora, said that evidence suggests that levels of 30 nanograms per milliliter to 40 nanograms per milliliter may be needed for optimum health.
Ginde and colleagues compared levels of serum 25-hydroxyvitamin D -- a measure of the amount of vitamin D in the blood -- from the Third National Health and Nutrition Examination Survey, collected between 1988 and 1994, to those collected during NHANES 2001-2004. Complete data were available for 18,883 participants in the first survey and 13,369 participants in the second survey.
"Overall, the mean serum 25-hydroxyvitamin D level in the U.S. population was 30 nanograms per milliliter during the 1988-1994 collection and decreased to 24 nanograms per milliliter during the 2001-2004 collection," the study authors said in a statement.
The study, published in the Archives of Internal Medicine, found the prevalence of levels lower than 10 nanograms per milliliter increased from 2 percent to 6 percent between the two study periods.
Racial and ethnic differences persisted throughout the surveys; among non-Hispanic blacks, the prevalence of 25-hydroxyvitamin D levels of less than 10 nanograms per milliliter increased from 9 percent to 29 percent and levels of more than 30 nanograms per milliliter or higher decreased from 12 percent to 3 percent.