Although women with darker skin tones tend to have lower levels of the biomarker used to measure Vitamin D levels, called 25-hydroxyvitamin D or 25OHD, the study found that older African-American and Caucasian women responded in the same way when they received vitamin D supplements.
Unlike many vitamins that are absorbed primarily from foods, the body's main source of vitamin D is sunlight. Vitamin D deficiency, which is primarily caused by inadequate exposure to sunlight and very poor diet, can result in abnormalities in calcium, phosphorus and bone metabolism. Vitamin D deficiency can cause rickets in children or a bone-softening condition called osteomalacia and muscle weakness in adults.
In a double-blind study that gave varying vitamin D doses to African-American and Caucasian women of similar body size, levels of the 25OHD biomarker were very similar. The findings suggest that vitamin D absorption and metabolism is the same in both groups. Researchers concluded that African-American women tend to have lower levels of the biomarker 25OHD because they naturally produce less vitamin D in the skin after sunlight exposure.
"African-American women don't have to worry about taking larger doses of vitamin D to compensate," said J. Chris Gallagher, MD, of Creighton University School of Medicine in Omaha, Neb., and lead author of the study. "They should follow the current medical guidelines for vitamin D supplementation suggested recently by the Institute of Medicine."
The Endocrine Society has issued a separate set of clinical practice guidelines governing vitamin D dosage.
Previously there was no research in minority populations to determine whether the same guidelines applied to them. Researchers designed this study to develop dosing guidelines for older African-American women. The study, which looked at vitamin D doses in 110 African-American women between the ages of 57 and 90, was the first randomized controlled dose response study conducted in this population.
"We saw a real need to study optimal vitamin D doses in African-American women and help their health care professionals make informed medical decisions," Gallagher said.
Although exposure to sunlight increases vitamin D levels, concerns about melanoma and other types of skin cancer necessitate avoidance of excessive exposure to the sun.
Other researchers working on the study include: M. Peacock of Indiana University School of Medicine, V. Yalamanchili of Creighton University School of Medicine and L. Smith of the University of Nebraska Medical Center.
The National Institutes of Health's National Institute of Aging and Office of Dietary Supplements funded the research.
The article, "Effects of Vitamin D Supplementation in Older African American Women
," appears in the March 2013 issue of JCEM