Vitamin D, our “sunshine” vitamin is so much more than a nutrient. Vitamin D is a steroid hormone with receptors located in almost every tissue in the body. Although many of us associate vitamin D with bone health or preventing seasonal affective disorder (SAD), this special hormone plays a significant role in our immune system, insulin and blood sugar management, sex hormone systems and much more. Because of its influence within the testes, ovaries and uterus, proper vitamin D status is critical for fertility. 

Now that we’re diving into summer, many people think that their vitamin D needs are met, but are they really? There are multiple factors that can affect your ability to make vitamin D, even in the summer. 

Where do we get Vitamin D?

The most common way humans obtain vitamin D is from sun exposure. Ultraviolet-B (UVB) rays from sunlight react with a cholesterol in our skin, called 7-dehydrocholesterol, to create vitamin D3. D3 is then converted to a form of vitamin D called 25(OH)D in the liver. It is this form that we test for when checking vitamin D blood levels as it’s the major form found in the blood, and has a long half-life of about 2-3 weeks. 

From there, 25(OH)D is converted to the active form of vitamin D referred to as 1,25(OH)2D in the kidneys and other local tissues. It’s this 1,25(OH) form of vitamin D that functions as a classic steroid hormone in the body. 

There are two types of vitamin D supplements: D2 and D3. The D2 form can be made synthetically, or naturally from yeast and fungi. D3 supplements are typically created using lanolin from sheep’s wool, but more recently it has also been found to be produced by lichen microorganisms. Additionally, D3 is available in small amounts in certain foods such as egg yolks and oily fish. 

In general, D3 supplements seem to be superior to D2. Studies have shown that supplementing with the D3 form of vitamin D leads to greater increases in blood 25(OH)D levels compared to D2 supplementation. 

Vitamin D status in Canada

In general, most populations don’t meet the requirements for vitamin D. Newer and more advanced methods have even suggested that our current recommended requirement is likely underestimated.

How much vitamin D are we supposed to have? Currently, levels below 50nmol/L are considered deficient while 50-75nmol/L is considered insufficient. Many researchers and practitioners consider optimal levels to be between 100-150nmol/L. 

In Canada, 32% of the population have vitamin D concentrations <50nmol/L (considered deficient). Interestingly, Canadians in the age range of 20-39 years are more likely to be deficient than all other age groups.

What about in the summer? Well the numbers do shift in the summer compared to winter, but the magnitude of change may not be what you’d expect. In the winter months, 40% of Canadians are vitamin D deficient, compared to 25% in the summer. That 1 in 4 number is still pretty substantial. Cloud cover, smog, using sunscreens, staying indoors, all diminish or prevent exposure to UV radiation and the ability to make vitamin D from sunlight. 

In addition, it’s been proposed that another contributing factor to low vitamin D status is obesity. Since vitamin D is a fat-soluble vitamin, it can be stored in fat reserves and have decreased bioavailability in the body. 

Vitamin D and Fertility

In northern countries there is actually a seasonal change in pregnancy rates. Rates seem to peak in the summer and autumn when vitamin D concentrations are the highest. This finding was confirmed in a systematic review of over 2700 women undergoing advanced reproductive technology (ART). Compared to women with deficient or insufficient vitamin D levels, those with levels greater than 75nmol/L had more live births, more positive pregnancy tests, and more clinical pregnancies. 

Vitamin D plays a role in follicular development within the ovaries and is associated with AMH levels. It’s been suggested that vitamin D also improves the receptivity of the endometrial lining. One study showed that women with polycystic ovarian syndrome (PCOS) who supplemented with vitamin D during an IUI cycle had an increase in endometrial thickness. These women also had improvements to their serum cholesterol levels. 

PCOS is one of the leading causes of infertility and one of its major imbalances is insulin resistance. Vitamin D can actually improve the function of insulin by upregulating the expression of insulin receptors and enhancing the responsiveness of insulin in transporting glucose into the body’s cells. 

In a retrospective cohort study, women with PCOS who were dealing with infertility were more likely to achieve ovulation if their vitamin D levels were >75nmol/L compared to those with levels <50nmol/L. 

Other positive outcomes of vitamin D in fertility have been documented as well. In men with oligoasthenoteratozoospermia (OAT), a condition of low sperm count, poor motility and abnormal morphology, supplementing with vitamin D led to a significant increase in both sperm motility and pregnancy rate compared to men who didn’t supplement. 

Overall, having sufficient vitamin D levels is crucial for multiple aspects of our health, but especially for fertility and pregnancy. The preconception and pregnancy dose of vitamin D prescribed to you ensures an increase in serum levels of 25(OH)D, regardless of sun exposure. There are far too many factors that can prevent us from obtaining an adequate amount of sunlight to convert into vitamin D. Usage of sunscreens, while they help prevent skin cancer and skin wrinkling, also block out the UVB rays needed for vitamin D production. With heat waves, smog advisories and increasing humidity, many people choose to spend more time indoors as well. 

The take-home is that most of us are likely walking around with sub-optimal levels of vitamin D, if not deficient. Considering the importance of vitamin D in fertility and pregnancy, it’s best to stick to your prescribed dose of vitamin D and not rely on the summer sun. 

To learn more about how you can support your fertility, or to book a virtual  appointment with a Conceive Health ND, contact us today. 


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  4. Shieh A, et al Effects of high-dose vitamin D2 versus D3 on total and free 25-hydroxyvitamin D and markers of calcium balance. J Clin Endocrinol Metab. 2016; 101(8): 3070-8
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