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Hormonal contraception use may lead to overestimation of vitamin D concentration
Healthy women using hormonal contraception were more likely to have higher levels of total 25-hydroxyvitamin D vs. women not using hormonal contraception, although levels of free 25-(OH)D were similar between the groups, according to findings from a post hoc analysis of a randomized controlled trial.
The use of hormonal contraceptives has been associated with higher levels of vitamin D binding protein and higher concentrations of total 25-(OH)D in most, but not all studies, Stefan Pilz, MD, PhD, of the division of endocrinology and diabetology at the Medical University of Graz, Austria, and colleagues wrote in the study background. However, the association between hormonal contraception use and levels of free 25-(OH)D levels is largely unknown.
“Considering that roughly 50% of women of childbearing age in Europe and the U.S. use [hormonal contraception], it is of great importance to evaluate the impact of [hormonal contraception] on parameters of vitamin D status,” Pilz and colleagues wrote. “If only total — but not free 25-(OH)D — is significantly increased by [hormonal contraception] use, this could suggest that vitamin D status based on total 25-(OH)D concentrations might be overestimated in women taking [hormonal contraception].”
Researchers analyzed data from 201 healthy women aged 18 to 45 years with BMI between 17 kg/m² and 30 kg/m² who did not take vitamin D supplements in the 2 months before baseline. Researchers randomly assigned women to a daily multi-micronutrient supplement containing 200 IU (n = 100) or 800 IU (n = 101) vitamin D3 for 8 weeks (176 women completed the trial and provided consent for further research). Women were stratified based on use of hormonal contraception. Researchers measured total serum 25-(OH)D; measurements of free 25-(OH)D were performed in a subgroup of women. Researchers used analysis of covariance to assess differences in total, free and percentage of free 25-(OH)D between users (n = 101) and nonusers (n = 100) of hormonal contraception. Primary outcome measure was the difference in free 25-(OH)D between users and nonusers of hormonal contraception after vitamin D3 supplementation. Secondary outcomes were differences in the total and percentage of free 25-(OH)D between groups according to hormonal contraception use.
Researchers found that women using hormonal contraception were younger and had higher total 25-(OH)D levels but a lower percentage of free 25-(OH)D vs. nonusers of hormonal contraception; however, there was no significant difference in free 25-(OH)D according to hormone use. The difference in total 25-(OH)D and percentage of free 25-(OH)D between users and nonusers of hormonal contraception persisted after adjustment for age and BMI (P < .001 for both).
At 8 weeks, researchers found that differences in the total and percentage of free 25-(OH)D remained significant between users and nonusers of hormonal contraception (P < .005 for all), with no differences in free 25-(OH)D concentrations. Results persisted in analyses excluding women who used a hormone-release vaginal ring or gestagen injections as birth control methods, according to researchers.
“The clinical significance of these [hormonal contraception]-induced changes in vitamin D metabolites have to be further elucidated, but it could be hypothesized that vitamin D status based on total 25-(OH)D concentrations might be overestimated in [hormonal contraception] users,” the researchers wrote. “This notion points toward a possible impact of our findings when considering that in the Western world, approximately every second a woman of reproductive age is taking [hormonal contraception].” – by Regina Schaffer
Disclosures: The authors report no relevant financial disclosures.