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Hormonal contraception associated with increased vitamin D levels in black women
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Black women using estrogen-containing contraceptives were more likely to have higher vitamin D levels than those not taking such agents, regardless of whether they also took vitamin D supplements, according to recent findings.
“We found that women who are using estrogen-containing contraceptives tended to have higher vitamin D levels than other women,” Quaker E. Harmon, MD, PhD, of the NIH’s National Institue of Environmental Health Sciences in Research Triangle Park, North Carolina, told Endocrine Today. “This finding was not due to other factors such as spending more time outside or behaviors related to contraceptive choice. These findings suggest that the estrogen in the contraception may be responsible for the increased vitamin D levels.”
Quaker E. Harmon
Harmon and colleagues analyzed data from 1,662 black women aged 23 to 34 years from the Detroit, Michigan, area participating in a prospective study on risk factors for fibroid incidence and growth (Study of Environment, Lifestyle and Fibroids). Participants provided baseline blood samples (between 2010 and 2012) to measure serum 25-(OH)D and answered questionnaires on demographics, dietary and supplement intake, contraceptive use, reproductive and medical history and behaviors related to vitamin D status, including time spent outdoors and vacation time spent in a sunny location during winter. Contraceptive use was classified as use of an estrogen-containing product, use of a progestin-only product or none (including nonhormonal IUDs).
Researchers used linear regression modeling of cross-sectional data to estimate percent change in season-adjusted serum 25-(OH)D with estrogen use, after adjustment for other factors.
Mean serum 25-(OH)D concentrations for the cohort were low at 15.7 ng/mL. Women using an estrogen-containing contraceptive had elevated median 25-(OH)D levels at baseline; however, they were also more likely to report taking a vitamin D supplement (49%). When researchers stratified 25-(OH)D levels by supplement use and estrogen-containing contraceptive use, there was an overall increase in 25-(OH)D level among users of estrogen-containing contraceptives regardless of supplement use. Researchers found that current use of an estrogen-containing contraceptive was associated with a 20% increase in 25-(OH)D after adjustment (95% CI, 14-27), which was a greater increase than a found with low-dose vitamin D supplement (14% difference; 95% CI, 8-21).
Researchers noted that the interaction between estrogen and vitamin D supplement use was not statistically significant, indicating that the estrogen association remained constant across levels of supplement use.
“Our findings indicate women may run the risk of developing vitamin D deficiency just when they want to become pregnant,” Harmon said in a press release. “For women who are planning to stop using birth control, it is worth taking steps to ensure that vitamin D levels are adequate while trying to conceive and during pregnancy.” – by Regina Schaffer
Disclosure: The Intramural Research Program of the NIH’s National Institute of Environmental Health Sciences supported this research. The researchers report no relevant financial disclosures.
Perspective
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David B. Karpf, MD
I think it is somewhat unlikely that oral birth control pills actually raise vitamin D levels in pre-menopausal African-American women. Oral estrogen is known to induce a number of hepatic proteins via first pass portal clearance to the liver, including a number of hormone-binding proteins. This is why, for example, total thyroxine (T4) levels are typically raised in women who are pregnant or treated with estrogen, which is mediated by an increase in thyroxine-binding globulin. However, the free T4 and free triiodothyronine (T3) levels are not changed, so the patient remains euthyroid with a normal thyroid-stimulating hormone despite her elevated total T4.
The fact that the optimal vitamin D level appears to be lower in African-Americans (25-35 ng/mL) compared with whites (40-60 ng/mL) is believed to be due to a somewhat lower level of vitamin D-binding protein in African-Americans, resulting in a higher "free vitamin D" for any given level of total vitamin D in these individuals (Holick MF. New Engl J Med. 2013;doi:10.1056/NEJMe1312291).
If the apparent increase in vitamin D in this study was simply due to hepatic induction by oral birth control pills on hepatic production of vitamin D-binding protein, then one would expect levels to go back up when these subjects become pregnant (at least during the second and third trimesters, when estriol levels are sky high). Also, if the investigators measured free vitamin D levels, I wonder if they differed between the women on or off oral birth control pills.
Disclosure: Karpf reports no relevant financial disclosures.
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