Vitamin D Supplement Needs Similar in Black and White Women

Joe Barber Jr, PhD

February 12, 2013

The absorption and metabolism of oral vitamin D is similar between black and white women, according to the findings of a randomized, double-blind placebo-controlled trial. The results suggest that the lower serum 25-hydroxyvitamin D (25OHD) levels observed in women with darker skin color are a result of differences in vitamin D production in skin and not differences in metabolism.

J. Christopher Gallagher, MD, from Creighton University School of Medicine in Omaha, Nebraska, and colleagues present their findings in an article published online February 5 in the Journal of Clinical Endocrinology & Metabolism.

"In a recent publication of the Institute of Medicine, it was pointed out that there were insufficient data to estimate the recommended dietary allowance (RDA) for other ethnic groups," the authors note in describing their motivation for the study.

"One of the issues noted by the Institute of Medicine...was the lack of dose-response studies in ethnic populations."

In the current study, the authors measured serum 25OHD levels in 110 black women who were given oral vitamin D supplementation at 1 of 8 dose levels for 1 year and who met the following inclusion criteria: age between 57 to 90 years, postmenopausal status, and vitamin D insufficiency with serum 25OHD levels of 20 ng/mL or less. Exclusion criteria included significant comorbidities, terminal illness, chronic renal failure, and a history of cancer (except skin cancer), within the past 10 years.

The team found that an oral vitamin D dose of 800 IU/day was sufficient for maintaining a serum 25OHD level exceeding 20 ng/mL, as recommended by the Institute of Medicine. Moreover, when the team compared the data with data from a parallel study in white women, which recruited more quickly, they saw no significant interactions among time, race, and dose (all P > .10), indicating that the effect of vitamin D dose on serum 25OHD levels is not race-dependent.

The authors also measured serum pituitary hormone (PTH) levels because secondary hypoparathyroidism is associated with vitamin D insufficiency. Serum PTH levels were 6.4 ng/L higher at baseline than after 12 months of vitamin D supplementation. Multivariate analysis, adjusted for age, body mass index, total calcium intake, and other known covariates, demonstrated that a 1-ng/mL increase in serum 25OHD levels was associated with a −0.0036 decrease in log10 PTH (95% confidence interval, −0.0057 to −0.0015; P = .0008), similar to previously reported findings in white patients.

The limitations of the study included its small size and the inclusion of mostly older women.

"Vitamin D 800 IU daily increased serum 25OHD [to] 20 ng/mL (>50 nmol/L) in 97.5% of African American women, similar to results reported by us in Caucasian women, suggesting that the RDA for vitamin D is the same as that proposed by the [Institute of Medicine]," the authors write. "The implication therefore is that the absorption and metabolism of vitamin D are similar in African American and Caucasian women. Although serum levels of serum 25OHD are usually much lower in African Americans, it is most likely because of decreased formation of vitamin D in skin."

Bayer provided calcium supplements at no cost for the study. The authors have disclosed no relevant financial relationships.

J Clin Endocrinol Metabol. Published online February 5, 2013. Abstract