An epidemic of vitamin D deficiency has been emerging over the last decade among all racial groups in the United States. In fact, the prevalence of vitamin D insufficiency nearly doubled from 1994 to 2004. Among women of reproductive age, more than 40 percent are insufficient in vitamin D.
Vitamin D’s most important role in the human body is to keep bones healthy and strong by helping the body absorb calcium. However, recent research has pointed to additional ways that vitamin D deficiency factors into our overall health, including its role in chronic diseases like diabetes, obesity, autoimmune disease, cardiovascular disease and cancer, as well as an association with a higher risk of both overall and cancer mortality. Instead of only affecting cells that live in the bone, we now understand that Vitamin D is able to affect many different types of cells in different organs in the body. The way it does this is by turning genes within that cell, “on and off.” In other words, vitamin D affects the way a cell carries out its function, and it can control the growth or the death of that cell.
Many studies have recently focused on the relationship of vitamin D levels to cancer. Low Vitamin D levels have been associated with a 30 to 50 percent increased risk of colon, prostate and breast cancer. Indeed, vitamin D’s newly discovered roles in the human body involving immunity and the ability to turn on or off certain genes, provides a logical explanation as to how vitamin D could contribute to cancer risk and mortality. However, while these studies do show a link between vitamin D levels and cancer risk, they do not prove that low levels of vitamin D cause these cancers—and there have not yet been any studies that show a clear benefit in taking vitamin D.
In the world of reproduction, the importance of vitamin D was initially shown in experiments with mice. Mice who are either deficient in vitamin D, or who lack the vitamin D receptor, can demonstrate underdevelopment of the uterus and inability to form normal mature eggs, resulting in infertility. If pregnancy is achieved, the fetuses of these mice show impaired growth. Reproduction is normalized in mice with vitamin D supplementation, but not with calcium alone, suggesting that vitamin D’s role in female reproduction is not related to helping the body absorb calcium.
In humans, the vitamin D receptor is present in many female organs, including the ovary, uterus, and placenta. The active form of vitamin D (calcitriol) has many roles in female reproduction. Bound to its receptor, calcitriol is able to control the genes involved in making estrogen.v The uterine lining produces calcitriol in response to the embryo as it enters the uterine cavity, shortly before implantation. Calcitriol controls several genes involved in embryo implantation. Once a woman becomes pregnant, the uterus and placenta continue to make calcitriol, which helps organize immune cells in the uterus, so that infections can be fought without harming the pregnancy. Poor vitamin D status has been associated with certain pregnancy complications such as gestational hypertension and diabetes.
Women about to undergo in vitro fertilization (IVF) can provide valuable insight into the role of vitamin D, since it is possible to examine each aspect of reproduction, from egg development to implantation of the embryo. A recent study found that women with higher vitamin D levels were significantly more likely to achieve pregnancy from IVF compared to women with lower levels of vitamin D. This study was repeated in another IVF center, which confirmed a four-fold difference in pregnancy rates between vitamin D replete and deficient women. In another study looking at the recipients of donor eggs, vitamin D levels in the recipients were associated with clinical pregnancy, emphasizing that the critical role of vitamin D in pregnancy may be within the uterus. Though more research is needed, it appears that vitamin D levels are associated with IVF success, and that its most important role in reproduction may be at the uterine lining. No studies have yet evaluted whether giving vitamin D improves IVF outcomes. But the data that does exist at this point suggests that a role for vitamin D supplementation may exist as a means of improving one’s natural fertility both among the fertile and the infertile.
The research that implicates vitamin D deficiency as playing a role in not only fertility but in overall health is compelling, and it is backed by sound physiologic hypothesis. Before broad treatment changes are instituted, however, further research needs to be completed to assess the health benefits of supplementing vitamin D to attain replete levels. Nevertheless, this is an exciting new field of research that could lead to simple and inexpensive approaches to improve health. Regardless of potential fertility benefits, patients should be counseled regarding appropriate vitamin D supplementation for overall health benefits, including bone health, pregnancy health, and chronic disease risk reduction.