Vitamin D is an essential nutrient for health, and its importance in bone health, immunity, and mood is well-known. However, it’s generally not the first thing that comes to mind in the preconception period – even though the current research tells us that it is especially important for fertility, pregnancy, and early childhood! Vitamin D is also commonly deficient, yet routine testing for adequate Vitamin D levels during preconception and pregnancy is not currently being done.
What is Vitamin D?
The name “Vitamin D” is a bit of a misnomer. Vitamin D is actually a group of fat-soluble compounds which act as hormones in the body. Vitamin D receptors are found in many bodily tissues, and the scope of its effects in the body are vast. Functions of Vitamin D include:
Regulation of metabolism
Calcium and magnesium regulation, and bone health
Gene transcription
Cardiovascular health
Insulin sensitivity
Mood (especially seasonal affective disorder)
Prevention of breast and colon cancer
Hormonal regulation
Immune system regulation (see my previous post about Vitamin D + immunity and autoimmunity)
Deficiency of Vitamin D
Vitamin D deficiency is very common, especially in northern countries. Statistics Canada reports that 40% of Canadians have insufficient Vitamin D levels (< 50 nmol/L) in the winter time. There are a few reasons why deficiency is so prevalent. Firstly, because our primary source of Vitamin D comes from a biochemical process that requires skin exposure to sunlight, lack of adequate sunshine (and UV light) is a big factor. This is affected by time spent outdoors, but also latitude, skin color, age, sunscreen and covering up. Another reason is that food sources of Vitamin D are limited, and often lacking in western diets. As a fat-soluble vitamin, absorption of Vitamin D can also be hindered by digestive concerns such as Irritable Bowel Disease and Celiac Disease.
Preconception & Vitamin D
Vitamin D status is beginning to be understood as an important piece of the fertility puzzle. Its role in ovarian physiology includes FSH sensitivity, as well as progesterone production and release (learn more about these hormones in my previous post), suggesting it may play a role in follicle development (which is essential for fertility). Vitamin D status was also shown to be correlated with serum Anti-Mullerian Hormone (AMH) levels, a marker of ovarian reserve.
In women with Polycystic Ovary Syndrome (PCOS), a known cause of fertility struggles, Vitamin D is commonly deficient. In these cases, Vitamin D supplementation has been shown to lower abnormally high AMH levels and improve insulin resistance.
Vitamin D and IVF
Vitamin D status has been shown to correlate with IVF outcomes, with studies showing lower levels associated with poorer response to IVF. Additionally, a meta-analysis found live birth rates following IVF were also higher in women whose Vitamin D levels were replete.
Pregnancy Outcomes
Low maternal Vitamin D status has been associated with several adverse pregnancy outcomes, including preeclampsia (a dangerous elevation in blood pressure occurring during pregnancy), gestational diabetes, and pre-term birth, as well as low birthweight. These outcomes have been confirmed by multiple studies, and a 2016 Cochrane review suggested that supplementing Vitamin D in pregnancy may play a role in prevention of preeclampsia, preterm birth, and low birthweight.
Infant Health
Low maternal Vitamin D status during pregnancy can have health effects after birth as well, and supplementation in pregnancy can protect against Vitamin D deficiency in infants. Studies have shown that in utero exposure to Vitamin D is inversely associated with the risk of asthma and recurrent wheeze in infants (meaning that lower Vitamin D levels increase the risk). An analysis study of two randomized controlled trials found Vitamin D supplementation to be protective of asthma and recurrent wheeze in infants, while another study showed this protective effect in children at 3 years old. Low gestational Vitamin D status has also been linked to poor bone health and rickets, as well as Type 1 Diabetes.
Testing for Vitamin D
The simplest way to assess for Vitamin D deficiency is through a blood test. There are several markers of Vitamin D status, but the most accurate test is called “25-hydroxyvitamin D” or “25(OH)D.” While there is some debate as to what an ideal Vitamin D level is, Statistics Canada considers levels between 30-50 nmol/L insufficient, while levels below 30 nmol/L are considered deficient. Several studies suggest that sufficient levels start at 75 nmol/L, while optimal levels fall between 90-120 nmol/L.
Since Vitamin D is so important in the preconception period, it is best to get your Vitamin D levels checked before trying to conceive. Unfortunately, routine testing of Vitamin D is not currently recommended by the American Society of Obstetricians and Gynecologists (ACOG) and is not frequently done. If your healthcare provider has not checked your levels, then you may need to specifically request this test.
Optimizing Vitamin D levels
Sunlight:
There are several ways to get Vitamin D, but as mentioned above our primary source of Vitamin D is made when our skin is exposed to sunlight. It can be difficult to know how much sunlight exposure is needed and how to balance that with safe sun exposure. While it’s not a perfect science, there are a couple of tools that can be helpful:
QSun App and dminder App: both of these apps let you track and estimate how much Vitamin D you are making, while providing information on UV index to promote safe sun exposure. They both consider location (and latitude), skin type, sunscreen, time of year, etc.
Food:
Food sources are limited for Vitamin D, but adding in foods with even small amounts of Vitamin D can help to optimize your levels. Top food sources are fatty fish such as tuna, mackerel, and salmon, cod liver oil, and eggs. Some food sources such as dairy and are also commonly fortified with Vitamin D.
Oral Supplementation:
Studies have shown that in the preconception period, dietary sources of Vitamin D are not enough and supplementation is often necessary. Vitamin D3 is the most biologically active form of Vitamin D (and the form of choice for supplementation), and as a fat-soluble vitamin should be taken in a fat (oil) base. How much to supplement will vary depending on your current levels. In Canada, the recommended daily allowance (RDA) for Vitamin D is 800 IU per day for adults, and 600 IU per day in pregnancy and breastfeeding. A typical prenatal or multivitamin will give this amount, however several current studies suggest this may not be enough. One double-blind, randomized clinical trial found that supplementing 4000IU per day in pregnancy to be most effective in maintaining 25(OH)D levels of 80 nmol/L at the time of delivery (these levels were met in 82% of mothers supplementing with 4000IU per day, versus 71% supplementing 2000IU per day, and only 50% supplementing 400IU per day). A second study confirmed the safety of supplementing 4000 IU per day and suggested it was protective of preterm labour and birth.
Intramuscular injection:
Vitamin D injection provides an alternative to oral supplementation with several advantages. One is that rather than taking a daily supplement, injections are only needed 1-3 times per year (administered in clinic). The other is that blood levels of Vitamin D have been shown to be more consistently maintained after intramuscular Vitamin D over oral supplementation. Injections are recommended only once Vitamin D levels have been assessed for deficiency.
Importantly, because it is a fat-soluble vitamin, toxicity can theoretically occur. While this is rare, it speaks the importance of having your levels checked before beginning supplementation. If you are pregnant or plan on becoming pregnant, consider talking to your health care provider about having your Vitamin D levels checked and to discuss if supplementation or injections might be beneficial for you.
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