Vitamin D & infertility: What your doctor wants you to know

Many people may be surprised to learn that Vitamin D is not actually a vitamin at all. It's actually in the family of steroid hormones.

Technically speaking, vitamins are nutrients that the body doesn't make on its own, but are required for normal body processes. Therefore, we need to take these in from our environment or our food instead. 

Vitamin D is not strictly a vitamin because our bodies can actually make it through exposure to sunlight.

People became aware of vitamin D a long time ago when it was realized that some people who had dietary deficiencies wound up with bone problems, like rickets. But back then, when people spent all their time outdoors, deficiency in Vitamin D wasn't much of an issue. 

But now that people spend more time indoors, and we're more conscious about wearing sunscreen and limiting sun exposure, we’re seeing lower vitamin D levels in some people.

Some food products like milk are supplemented with Vitamin D, and it's naturally occurring in fish like salmon, but otherwise, we don't get much Vitamin D from our diets.

Vitamin D plays an important role in the immune system and may have an impact on COVID-19. 

There've been several recent studies looking at serum levels of vitamin D in relation to susceptibility or severity of COVID-19. We've seen that patients in an ICU with COVID-19 tend to be deficient in vitamin D at a higher rate.

It appears that deficiency in vitamin D causes an increased susceptibility to respiratory infections in general — not just COVID-19 but also influenza.

Vitamin D also appears to play a role in fertility and reproductive health.

Our understanding of the role of vitamin D in reproduction and fertility is still evolving, and studies about it are not conclusive.

As a steroid hormone, Vitamin D has receptors all over the body, not just in the bone. Some of these receptors are in the female reproductive tract, including the ovaries, uterus, uterine lining and endometrium.

We know that women who are pregnant and have low vitamin D levels appear to have a higher risk of things like miscarriage, preterm delivery, low birth weight, gestational diabetes and preeclampsia. 

Preeclampsia causes high blood pressure and a constellation of other issues that can happen typically late in the pregnancy and cause complications.

There have also been studies showing that some specific groups of women with infertility tend to have low vitamin D levels — such as patients with polycystic ovarian syndrome (PCOS).

Vitamin D also appears to be involved in the synthesis of other sex steroid hormones, like estrogen. It also seems to have an effect on AMH, which is a marker that we use to measure ovarian reserve, but also has a real function in the ovary.

There have been some studies correlating a lower success rate with IVF and low levels of vitamin D, but the quality of these studies are low and there are studies that conflict with this. 

Larger scale studies are needed to show causation, not just correlation, and to demonstrate that improving Vitamin D levels actually leads to improved IVF outcomes. 

But to undertake such a study, a pharmaceutical company would need to be willing to pay hundreds of millions of dollars to get drugs approved — however, vitamin D is cheap. You can get a large bottle of it at your local supermarket for just a few dollars, and of course, sunlight is free.

We don't know for sure if low Vitamin D causes problems with fertility and pregnancy health. 

For example, it could just be a poor diet in general that leads to those outcomes. 

It could also be that sunlight does more than just improve Vitamin D; it could hypothetically improve something else that the medical community doesn’t fully understand yet. That would mean that simply taking more Vitamin D from the supermarket may not solve it.

As we become more aware of this, it's likely that fertility doctors will begin incorporating Vitamin D more routinely into diagnostic testing.

Checking Vitamin D levels in infertile patients is part of a holistic approach to assessing and optimizing a patient’s overall health — just as we already check thyroid hormones in female patients that don't have an obvious goiter. (And low Vitamin D is much more common than thyroid deficiency!)