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Let the Sunshine In: New Studies Illuminate Vitamin D

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Two recent studies in The Journal of Clinical Endocrinology & Metabolism shed light on various factors that affect vitamin D levels in the body. The links between vitamin D and certain forms of contraception as well as how it can be impacted by endocrine-disrupting chemicals have been given a closer look by endocrine researchers.

Because of its wide-reaching implications for the functioning of many body tissues found in cardiovascular, digestive, immune, and reproductive systems, maintaining adequate vitamin D levels is critical for overall human health, especially so for women. Yet many U.S. women are vitamin D deficient, especially among the African American population. Moreover, getting adequate vitamin D can be difficult for some, as it does not occur in many foods.

Two recent studies on vitamin D examine exogenous factors that can influence its levels: one on what can have a negative impact, the other, a positive.

EDCs Strike Again

In “Relationships Between Urinary Phthalate Metabolite and Bisphenol A Concentrations and Vitamin D Levels in U.S. Adults: National Health and Nutrition Examination Survey (NHANES), 2005–2010,” published in The Journal of Clinical Endocrinology & Metabolism, a team including John D. Meeker, ScD, CIH, and Lauren E. Johns, a PhD candidate at the University of Michigan School of Public Health in Ann Arbor, Mich., took a look at what effect endocrine-disrupting chemicals (EDCs) might have on vitamin D in 4,667 NHANES participants for whom urinary phthalate metabolites, urinary BPA, urinary creatinine, and serum 25(OH)D date were available. “Two previous studies have suggested the potential for environmental exposure to other EDCs (e.g., pesticides) to alter circulating vitamin D levels in humans,” Johns says. “Phthalates and bisphenol A (BPA) have been shown to disrupt thyroid and reproductive hormones in humans. Because the active vitamin D metabolite in our bodies is similar in structure and function to these other hormones, it is plausible that phthalates and BPA could also impact vitamin D levels in our body through some of the same mechanisms.”

They found that phthalate metabolite levels inversely correlate with total adult circulating vitamin D levels, resulting in a 1.09% decrease. Furthermore, the degree of reduction of vitamin D levels due to environmental exposure to phthalates was sex specific, with stronger associations found in women. BPA level also inversely correlated with vitamin D level, resulting in a 3.71% decrease in women only.

The mechanisms underlying how EDCs act on vitamin D levels in the endocrine system remain largely unknown, but these researchers and others suggest that one possibility is by directly acting on the metabolic enzymes involved in converting inactive vitamin D prohormones to their circulating metabolites. “It is also possible that phthalates and BPA may indirectly influence circulating vitamin D concentrations through the disruption of calcium homeostasis. BPA in particular has been shown to alter the expression of proteins involved in calcium signaling processes as well as serum calcium levels,” Johns says.

“Although the role of vitamin D in improving a couple’s ability to get pregnant and have a healthy pregnancy are still being examined, we do know that pregnancy is a time of increased demand for vitamin D. Women who are stopping their birth control to start planning a pregnancy should work with their healthcare provider to ensure that their vitamin D levels remain adequate while trying to conceive and during pregnancy. Pre-conception planning and counselling should account for a possible decline in serum vitamin D levels after estrogen-containing contraception is stopped.” – Quaker Harmon, MD, PhD, fellow, National Institutes of Health’s National Institute of Environmental Health Sciences, Research Triangle Park, N.C.

Protecting Vitamin D Status

This was the first study to investigate environmental exposure to phthalates (found, for example, in perfumes, hairspray, deodorants, and lotions) and total 25(OH)D, although vitamin D has been investigated in relation to BPA. The findings remain consistent. “While additional studies are required to determine the clinical impact of our findings, the widespread use of these chemicals coupled with the essential role that vitamin D plays in numerous physiological processes in our bodies could mean that the public health implications of these findings may be quite large. It is likely that the downstream clinical outcomes potentially associated with subclinical alterations in circulating vitamin D concentrations are sex-dependent and depend on physiological state (i.e., pregnancy status, adolescence, etc.),” Meeker says.

Once again, the best advice clinicians can give at this point is EDC avoidance when possible. Meeker explains, “Although the widespread use of these chemicals (and their alternatives) precludes complete avoidance of exposure, it may be possible to reduce exposure to some degree by limiting use of plastic-containing products, reducing consumption of heavily processed/packaged foods, and finding more natural alternatives to personal care products.”

Not Such a Bitter Pill

In “Use of Estrogen-Containing Contraception Is Associated with Increased Concentrations of 25-Hydroxy Vitamin D,” also published in The Journal of Clinical Endocrinology & Metabolism, a team including Quaker Harmon, MD, PhD a fellow at the National Institutes of Health’s National Institute of Environmental Health Sciences in Research Triangle Park, N.C., took a closer look at the association between increased levels of circulating vitamin D and estrogen-containing contraceptives found in previous studies, possibly due to alterations in the metabolic pathway of vitamin D. “Previous studies were not able to control for many important variables including time outside,” Harmon says. “We were interested in following up the previous findings in a larger population of young women.”

Measuring serum vitamin D levels from 1,662 African American women ages 23–34 in the Detroit area who had been recruited for the Study of Environment, Lifestyle & Fibroids, the researchers found robust support for the association between estrogen-containing contraception and vitamin D levels. Even after controlling for 43 variables, including time outside and supplement use, an independent 20% increase was identified.

“Phthalates and bisphenol A (BPA) have been shown to disrupt thyroid and reproductive hormones in humans. Because the active vitamin D metabolite in our bodies is similar in structure and function to these other hormones, it is plausible that phthalates and BPA could also impact vitamin D levels in our body through some of the same mechanisms.” – Lauren E. Johns, a PhD candidate at the University of Michigan School of Public Health in Ann Arbor, Mich.

That’s the Good News

The other side of this coin is that “Women who are planning pregnancy and stop their estrogen-containing contraception may experience a decrease in vitamin D levels,” Harmon says. “Although the role of vitamin D in improving a couple’s ability to get pregnant and have a healthy pregnancy are still being examined, we do know that pregnancy is a time of increased demand for vitamin D. Women who are stopping their birth control to start planning a pregnancy should work with their healthcare provider to ensure that their vitamin D levels remain adequate while trying to conceive and during pregnancy,” she says. “Pre-conception planning and counselling should account for a possible decline in serum vitamin D levels after estrogen-containing contraception is stopped.”

To determine the mechanism by which estrogen-containing contraceptives increase serum vitamin D levels among other things, Harmon and team “are continuing to follow the vitamin D levels in this group of women to track changes over time. We are also working with women from another study to examine changes in vitamin D across the menstrual cycle.”

In the meantime, for any woman who is concerned about her vitamin D levels, they suggest consulting a healthcare provider to determine current levels and possible interventions to increase levels if needed, such as by increased exposure to sunlight and/or by supplementation.

  • Horvath is a freelance writer based in Baltimore, Md. She wrote last month’s cover story on the underlying causes and possible solutions regarding obesity in children.

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