Vitamin D for Fertility (and the Importance of Nailing Your Dose)

I’m like you: curious and constantly doing my own research.

But sometimes, the research we happen upon in our quest to be well-informed can be misleading, even downright scary. This is bound to happen for many reasons, especially in the world of supplements and nutrition. When something as important as pregnancy is involved, we are certainly not looking to take any chances. Brands will spin information for marketing purposes, selecting specific studies to present as set-in-stone research. In contrast, other studies or trials that may be less convenient to a brand’s product and message get pushed into the background, out of sight. This kind of subjective presentation of facts is how misinformation begins to spread.

I am proud to say that FullWell Prenatal was built on transparency.

That’s why I thoroughly address your questions and try to dive deeply into every ingredient in FullWell Prenatal here on our blog. And lately, I’ve been getting a flood of panicked questions about everything related to vitamin D. 

So are you getting enough vitamin d? Probably not. But don’t panic. 

As I look around at what’s out there on vitamin D, I see a lot of misinformation and scare tactics. Additional stress is the last thing you (and your partner!) need while trying to conceive or working toward your healthiest pregnancy, so let me clear a few things up. 

First, the facts.

What vitamin D is and how it works in the body

Vitamin D is a fat-soluble vitamin that is unique because our bodies make it. It is one of the few micronutrients that isn’t readily abundant through diet and our environment. We were made to be self-sufficient when it comes to vitamin D! It is an essential nutrient to support immune function and efficient cell division and is necessary for keeping bones healthy by increasing the intestinal absorption of calcium. 

The most natural form is produced by our own bodies, in our very skin, from a form of cholesterol, 7-dehydrocholesterol. Energy from sunlight (specifically UVB rays) converts 7-dehydrocholesterol into vitamin D3. In the liver, vitamin D picks up additional oxygen and hydrogen to become 25-hydroxyvitamin D, or 25(OH)D. This is the chemical that many healthcare practitioners use to measure vitamin D in the body (more on how to ask for vitamin D labs later, stay with me). But this form of vitamin D isn’t fully bioavailable until it has hit the kidneys. There, it picks up one last set of oxygen and hydrogen molecules to become 1,25 dihydroxy vitamin D. This is the “active” or most bioavailable form of the vitamin. Its proper name is 1,25(OH)2D or calcitriol, but simply referring to it as vitamin D is good enough for what we are outlining here, so don’t let yourself get overwhelmed by the chemical abbreviations!

Vitamin D is your body’s most potent bone protector. When levels dip low enough, it can lead to low bone calcium stores, which increases bone fragility and the risk of fractures. 

Your body relies on Vitamin D in many other ways as well. Muscles need it to move, nerves need it to carry messages between your brain and body, and the immune system needs vitamin D to fight off invading bacteria and viruses.

So structurally, for all of us, it is critical to get enough vitamin D. The problem is that many Americans don't.

It doesn’t matter who you are or what stage of life you are in: vitamin D deficiency is incredibly common. More often than not, women’s vitamin D levels are much lower than they should be. When it comes to conception, this can play an enormous role in the struggle to get pregnant, and even after conception, it is common for deficiencies to occur and start to throw mom and baby off-kilter.

Who is at risk for Vitamin D deficiency? What risk factors play a role? 

• Skin color | People with darker skin, are at increased risk for deficiency due to higher levels of the pigment melanin (!)
• Weight and body composition | Those who are significantly overweight or obese are more likely to suffer from vitamin D deficiency due to volumetric dilution. People who have undergone gastric bypass surgery can also become vitamin D deficient.
• Diet | Certain animal products are naturally high in vitamin D, so those who do not consume them - mainly fish or dairy - are not getting any help through food
• Avoidance of the sun or wearing sunblock | Sunblock is necessary to protect against the development of skin cancer over prolonged exposure to UV rays. Still, you actually need some exposure to UVB rays and natural sunlight to synthesize vitamin D. Skin exposed to sunshine indoors through a window will also not produce vitamin D.
• Geographical latitude | Small daylight windows + colder weather = a lifestyle with less time in the sun. I test a lot of New Englanders in my private practice at Boston Functional Nutrition, and I've yet to see any healthy vitamin D levels on the first try!
• Conditions that limit fat absorption | People with gastrointestinal disorders such as Crohn’s disease, celiac disease, or liver disease can become deficient quickly because vitamin D is fat-soluble: its absorption depends on the gut's ability to absorb dietary fat.

(!) A critical note: Women of color have a significantly higher rate of preterm birth. While many factors play a part in this, ensuring adequate vitamin D levels dramatically reduces the risk.⠀

Most of us live with at least one of the above risk factors, but many of us don't know whether or not we are getting enough vitamin D because the symptoms can often be mild or slow to present. Many adapt to living with their symptoms and don’t even identify them as problems.

Symptoms of Vitamin D Deficiency

It might be time to check your labs if you are experiencing the following:

• COMPROMISED IMMUNE FUNCTION | You're getting sick more often
• CHRONIC FATIGUE | You feel a little (or a lot) tired all or most of the time
• BONE LOSS & SKELETAL PAIN | You're prone to small, gnawing injuries like stress fractures or back pain

If you're trying to conceive, know that vitamin D affects both you and your partner’s fertility.

Vitamin D plays a big part in ovulation and men’s fertility.

For women, vitamin D plays a vital role in ovulation.

As you now understand, vitamin D’s main claim to fame is its role in bone health (e.g., maintaining calcium and phosphorus homeostasis). There is some evidence that - in conjunction with certain sex steroid hormones that we know to be the regulators of reproduction - vitamin D can have positive effects on the reproductive process for women. The female reproductive system (your uterus, endometrium, ovaries, and placenta) holds vitamin D receptors, indicating that it plays a pivotal role in female fertility. This is thought to be true for women of all ages, but studies show that vitamin D can significantly benefit those who are older or have previously had trouble conceiving. 

Late reproductive-age fertility

Some studies have shown that when 25(OH)D was increased for women of late reproductive age (35+), it increased their odds of conceiving and carrying out a healthy pregnancy. If you or your partner are trying to conceive and are over 35, proactively discussing your specific vitamin D needs with your doctor will ensure you’re doing everything you can to set yourselves up for success.

Vitamin D and IVF

Considering the importance of vitamin D in pregnancy and implantation, it’s no wonder that many women who have thought about or are currently undergoing IVF are concerned with getting enough. After all, the amount of vitamin D in the follicular fluid can significantly affect the success of assisted reproductive technology procedures. Adequate levels have been shown to determine the number and quality of oocytes (or immature egg cells). It has also been documented that high levels of vitamin D in follicular fluid and serum is related to a higher chance of conceiving following IVF. 

For men, vitamin D has been positively associated with semen quality.

Similar to women, in recent years, the presence of vitamin D receptors and vitamin D metabolizing enzymes in men have been found in the gonads, reproductive tract, and spermatozoa. This indicates a reproductive role exists, linking to positive predictive markers of semen quality and higher expression in spermatozoa from normal than infertile men.

Make sure you and your partner check out my latest post on men's fertility to ensure you’re both doing everything you need for a healthy conception.

Once you’ve conceived, vitamin D levels affect your pregnancy.

Vitamin D is a critical tool your body will use to help your baby's bones, kidneys, heart, and nervous system develop. It can also have a massive impact on your and baby's health throughout pregnancy, delivery, and postpartum.

When a woman experiences vitamin D deficiency during pregnancy, it increases her risk of

• preeclampsia
• having a baby with a low birth weight
• gestational diabetes
• high blood pressure
• recurrent pregnancy loss
• preterm delivery
• poor bone development in baby that may persist into childhood
• postpartum depression

How to ask for the proper labs for vitamin D

You don’t want to simply request that your practitioner run labs for "vitamin D," whether you’re trying to conceive or already pregnant. Many providers mistakenly order vitamin D-1, 25, which is not the correct way to assess vitamin D status accurately. Remember, this is the form of vitamin D that hasn’t made it to your kidneys and is not readily bioavailable, so your body can’t use it all just yet. That number might even look relatively high, especially if you’ve already started proactively supplementing with vitamin D. Still, vitamin D-1, 25 does not reflect your actual vitamin D status, so be sure the correct test is ordered. I’ve had clients come to me from other providers who have been advised to stop taking vitamin D out of fear of toxicity. To me, this is alarming. Your practitioner ordering the wrong test can scare you into halting vitamin D supplementation when you desperately need it, so you must be clear that you want to be checked for vitamin D, 25-Hydroxy (vitamin D 25-OH). 

So is a vitamin D supplement enough?

This will surprise you, but many prenatal vitamins don't contain enough vitamin D for pregnant women. Vitamin D dosage can be controversial since vitamin D can be toxic if your levels are too high. But if you’ve read any of my other articles on specific vitamins (A, E, B, for example), you remember one key takeaway: it is tough to overdo many of these vitamins while pregnant or breastfeeding.

Very few foods naturally have vitamin D. Fortified foods provide most of the vitamin D in American diets.

• Fatty fish such as salmon, tuna, trout, and mackerel are among the best sources
• Beef liver, cheese, and egg yolks provide small amounts
• Mushrooms provide some vitamin D. In some mushrooms that are newly available in stores, the vitamin D content is being boosted by exposing these mushrooms to ultraviolet light
• Almost all of the U.S. milk supply is fortified with 120 IU of vitamin D per cup, but foods made from milk, like cheese and ice cream, are usually not fortified
• Vitamin D is added to many breakfast cereals and some brands of orange juice, yogurt, and soy beverages, so check the labels

As an expert tasked with striking the perfect balance of nutrients to keep pregnant women at peak health (no pressure), here is how I think about it. 

Most prenatal vitamins contain somewhere around 400 IUs of vitamin D; however, a double-blind, randomized clinical trial found that Vitamin D supplementation of 4,000 IU/day for pregnant women was safe and most effective in achieving sufficient levels of vitamin D in all women and their neonates, regardless of skin color. It was the lowest amount needed for 82% of women to have adequate vitamin D levels at birth.

Once pregnant, the body's demand for Vitamin D is so high that 4,000 IU daily is the absolute minimum amount of Vitamin D you want to take for your and your baby's health. When you weigh the risks associated with Vitamin D deficiency in pregnancy that I listed above, you can see why I chose to pack FullWell Prenatal Vitamins with more vitamin D than any other prenatal on the market. I believe in this data and the importance of Vitamin D for your and your baby’s health.