Stopping birth control pills may lower vitamin D levels

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Women risk having their vitamin D levels fall when they stop using birth control pills or other contraceptives containing oestrogen.

Vitamin D is a hormone involved in the immune system and managing calcium levels in the blood. Proper calcium levels are necessary for bone health. The body produces vitamin D in a chemical reaction when the skin is exposed to sunlight. A smaller portion of the body’s vitamin D supply – about 10% – comes from food, including fatty fish and milk fortified with the vitamin. Chemical changes to vitamin D are needed to produce the active form.

During pregnancy, women produce increased amounts of the active form of vitamin D to support formation of the foetal skeleton. As a result, pregnant women face an increased risk of developing vitamin D deficiency, according to the Endocrine Society’s Clinical Practice Guideline on vitamin D deficiency.

“Our study found that women who were using contraception containing oestrogen tended to have higher vitamin D levels than other women,” said the study’s first author, Dr Quaker E Harmon, of the National Institutes of Health’s (NIH’s) National Institute of Environmental Health Sciences, North Carolina. “We could not find any behavioural differences such as increased time spent outdoors to explain the increase. Our findings suggest that contraceptives containing oestrogen tend to boost vitamin D levels, and those levels are likely to fall when women cease using contraception.”

For the cross-sectional data analysis, researchers analysed data from the Study of Environment, Lifestyle & Fibroids (SELF), a study of reproductive health in nearly 1,700 African-American women between the ages of 23 and 34. The women all lived in Detroit or the surrounding area. As part of the study, the participants answered questions about contraceptive use, as well as the amount of time they spent outdoors and any vitamin D supplements they took.

The 1,662 women provided blood samples, which were analysed to measure levels of 25-hydroxy vitamin D, the primary circulating form of vitamin D. After adjusting for seasonal exposure to sunlight, the researchers found the use of contraceptive pills, patch or ring containing oestrogen was associated with a 20% higher 25-hydroxy vitamin D level. While current birth control users tended to have higher levels of vitamin D in the blood, past contraceptive users had average levels of Vitamin D.

“Our findings indicate women may run the risk of developing vitamin D deficiency just when they want to become pregnant,” Harmon said. “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.”

The research was supported by the Intramural Research Programme of the NIH’s National Institute of Environmental Health Sciences.

Context: Small studies suggest exogenous estrogen may improve vitamin D status, but the etiology is unclear because women who use hormones may make lifestyle choices that differentially affect vitamin D status.
Objective: Our objective was to investigate the association between use of hormonal contraception and 25-hydroxy-vitamin D (25(OH)D).
Design: We 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.
Setting: At the enrollment clinic visit (2010–2012) into a cohort study of uterine fibroids, each subject provided a blood sample, had anthropomorphic variables and skin reflectance measured, and answered questionnaires on demographics, dietary and supplement intake, contraceptive use, reproductive and medical history, and behaviors.
Participants: A total of 1662 African American women, community volunteers, 23–34 years old, living in the Detroit, Michigan, area were included.
Interventions: None.
Main Outcomes and Measures: Serum 25(OH)D was measured.
Results: Serum 25(OH)D concentrations were low (70% Conclusions: The increase in 25(OH)D with use of estrogen-containing contraceptives raise mechanistic questions regarding the biological pathways involved, and highlights the need for studies that examine possible endogenous estrogen effects on vitamin D.

Quaker E Harmon, David M Umbach, Donna D Baird

The Endocrine Society material
Journal of Clinical Endocrinology & Metabolism

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