Vitamin D is sometimes referred to as a hormone because the activated forms are hormones. You utilize this essential vitamin in nearly all parts of your body. Vitamin D is crucial for absorbing other nutrients like calcium, iron, magnesium, phosphate, and zinc. Vitamin D also regulates immune function, cell growth, neuromuscular function and affects thousands of genes. Your baby relies on you to supply adequate vitamin D to support their growth and development during pregnancy.
Despite the importance of vitamin D, data shows that most pregnant mothers do not get enough vitamin D. Deficiency is associated with elevated risks for preeclampsia, adverse health outcomes for your child later in life, and even a cesarean section. Most prenatal vitamins contain far less vitamin D than the evidence suggests during pregnancy. Find out what the current evidence says about vitamin D during pregnancy and how to ensure you have enough vitamin D to support you and your baby.
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What is Vitamin D?
Vitamin D is a fat-soluble vitamin that is dissolved in fat and stored in fatty tissue. This allows your body to store it and use it as needed. One of the most critical things vitamin D does is help with the absorption of other essential nutrients like calcium, iron, magnesium, phosphate, and zinc. Vitamin D also regulates immune function, cell growth, neuromuscular function and affects thousands of genes. You have receptors for vitamin D in numerous tissues, including the placenta, prostate, breast, colon, lung, bone, parathyroid, pancreas, immune system, and vascular wall.
Types of Vitamin D
There are two main types of vitamin D. D3 (cholecalciferol) is generated in our skin and the skin of animals. D2 (ergocalciferol) is from plants. We get biologically inert vitamin D from sun exposure, food, and supplements. To be activated, it must undergo chemical conversion processes in the body. The first occurs in the liver and converts vitamin D to calcidiol. The second occurs primarily in the kidney and forms calcitriol.
Vitamin D is sometimes referred to as a hormone because the activated forms are hormones.
Vitamin D Deficiency
Our modern lifestyles have significantly impacted our natural access to vitamin D. Our ancestors spent more time outside; those who lived in colder climates far from the equator had diets rich in fish. Today, we spend a lot of time indoors, live in places with limited sunlight part or most of the year, use sunscreen to block UVB rays, and eat diets that are not rich in vitamin D. As a result, vitamin D deficiency is more common than ever. An estimated 1 billion people worldwide have a vitamin D deficiency across all ethnicities and age groups.
The most substantial risks of vitamin D deficiency are rickets in children and osteoporosis in adults. Rickets causes issues with calcification, softening, or distortion of bones. Osteoporosis is a condition in which bones become brittle and weak, often resulting in fractures. Many developed countries began fortifying foods with vitamin D in the 1930s to decrease the prevalence of rickets. Since then, scientists have linked vitamin D deficiency to everything from cancer to COVID-19. We should see more research in the future clarifying the effects of vitamin D deficiency. Due to the processes in the body that utilize vitamin D, it would make sense that a deficiency could contribute to many health outcomes.
There are some groups with an increased risk of vitamin D deficiency. This includes older adults because your skin does not convert vitamin D as efficiently as you age. People with limited sun exposure, due to geographic location, lifestyle are at a higher risk. People who wear clothing that covers most of their bodies and even people who are diligent about wearing sunscreen limit their vitamin D from sunlight. Sunscreen with an SPF of 15 can decrease vitamin D production by 99%. People with more melanin in their skin produce less vitamin D, so people with darker complexions are at a higher risk of deficiency. Also, individuals who are obese have lower vitamin D levels because fat cells can contain more vitamin D, making it less bioavailable in other cells.
There are also conditions involving fat malabsorption that can put people at risk for deficiency. As well as genetic polymorphisms that can affect the pathways in which we convert vitamin D to active forms.
The only way to test for vitamin D deficiency is with a blood test that measures the serum levels of 25-hydroxy vitamin D (25[OH]D). This indicator increases in proportion to vitamin D synthesis and intake. Most doctors do not routinely screen patients for vitamin D. Unfortunately, most expecting mothers will have a vitamin D deficiency. A study that examined vitamin D deficiency during pregnancy found ninety-seven percent of African-Americans, 81% of Hispanics, and 67% of Caucasians were deficient.
Vitamin D in Pregnancy
During pregnancy, your baby entirely relies on your vitamin D levels for their vitamin D. Calcitriol is the bioactive form of vitamin D. Your calcitriol levels are elevated during pregnancy to make sure that you can supply enough calcium to your developing baby. An estimated 25-30 grams of calcium transfer to your baby’s skeleton by the time they are born, the majority of this transfer is in the last trimester. Vitamin D is more critical to your baby as their bones are developing.
You can imagine that your baby needs vitamin D for more than bone development. Vitamin D deficiency in pregnancy is a risk factor for preeclampsia. Deficiency is associated with increased odds of primary cesarean section. A study examined how maternal vitamin D deficiency could affect a baby later in life. Researchers found an association with impaired lung development in six-year-olds, neurocognitive difficulties at age 10, increased risk of eating disorders in adolescence, and lower peak bone mass at 20 years. There is an abundance of evidence to support ensuring you are getting adequate vitamin D during pregnancy.
How Much Vitamin D Do You Need?
To understand how much vitamin D you need, you should understand how these recommendations work. There are two metrics to understand. The first is your measured 25(OH)D levels, and the second is the amount of vitamin D you get daily.
The Food and Nutrition Board of the Institute of Medicine, National Academy of Sciences, set recommendations for your serum 25(OH)D concentrations. Of course, the only way to know this would be to get tested. They consider below 30 nmol/L (12 ng/mL) deficient. Between 30 to 50 nmol/L (12–20 ng/mL) are inadequate for bone and overall health. Over 50 nmol/L (20 ng/mL) is adequate for bone and overall health in healthy individuals. Levels above 125 nmol/L (50 ng/mL) are linked to potential adverse effects.
Ideally, you have your 25(OH)D levels checked to determine where you are on the spectrum. If your levels are not ideal, you can supplement with additional vitamin D to increase your 25(OH)D levels. Unfortunately, there is not a simple equation to find out how much vitamin D you need, over what period, to increase your levels a certain amount.
Recommended amounts of vitamins are in the Dietary Reference Intake. The Food and Nutrition Board of the Institute of Medicine, National Academy of Sciences, come up with these values. The values vary by age and sex and if you are pregnant or breastfeeding. There are two values set for vitamin D. The Recommended Dietary Allowance (RDA) is the average daily intake level sufficient to meet the nutrient requirements of nearly all (97%-98%) healthy people. The Tolerable Upper Intake Level (UL) is the maximum daily intake unlikely to cause adverse health effects. In the case of vitamin D, the recommendations are the same from age 1-70 regardless of whether you are female, pregnant, or breastfeeding. The RDA is 15 mcg (600 IU). The UL is 100mcg (4,000 IU). If you are deficient in vitamin D, your doctor or midwife would likely recommend supplementing with an amount in this range.
Other countries and some professional societies have different guidelines for vitamin D intake. The Endocrine Society calls for higher serum levels, more than 75 nmol/L (30 ng/mL), to maximize the effect of vitamin D on calcium, bone, and muscle metabolism. The Endocrine Society states, “We suggest that pregnant and lactating women require at least 600 IU/d of vitamin D and recognize that at least 1500–2000 IU/d of vitamin D may be needed to maintain a blood level of 25(OH)D above 30 ng/ml .” Their tolerable upper intake level is also 100 mcg (4,000 IU).
At this point, you should be confused as to how much vitamin D you need. The RDA is 600 IU, the Endocrine Society says at least 600 IU, and that at least 1500–2000 IU/d of vitamin D may be needed. Both organizations state to keep levels under 4,000 IU.
Vitamin D Toxicity
The majority of vitamins are water-soluble, which means if your body has too much, they will pass through your urine as waste. Four vitamins are fat-soluble and are vitamins A, D, E, and K. Excess fat-soluble vitamins are stored in fat and have a higher risk for toxicity because your body cannot get rid of them as quickly.
You cannot get too much vitamin D from sunlight. You can, however, get a sunburn and should be taking steps to protect your skin. It is possible to take too much supplemental vitamin D. The typical cause of vitamin D toxicity is accidental overdose or an error in processing fortified foods. In the 1990s, a dairy made an error in processing, and a batch of milk had about 58,141 IUs in only 8 ounces. Investigators discovered the mistake after eight patients ended up in the hospital.
Vitamin D toxicity leads to hypercalcemia, a buildup of calcium in the blood, and an imbalance in the regulation of bone metabolism. The good news is that vitamin D toxicity is usually treatable without severe complications. Acute vitamin D toxicity is generally associated with doses of vitamin D above 10,000 IU/day, resulting in serum 25(OH)D concentrations >150ng/mL. This is far above the RDA of 600 IU and the higher suggestions from the Endocrine Society.
The Ideal Amount of Vitamin D in Pregnancy
Let’s examine some evidence to help you understand how much vitamin D you should be getting during pregnancy. A double-blind randomized clinical trial included pregnant women in weeks 12-16. The expecting mothers were given 400, 2000, or 4000 IU of D3 per day until their birth. The researchers found vitamin D supplementation of 4,000 IU/day for pregnant women was safe and most effective in achieving sufficiency in all women and their neonates regardless of race. The current estimated average requirement was comparatively ineffective at achieving adequate circulating 25(OH)D, especially in African Americans. A separate randomized control trial confirmed that expecting mothers need 4,000 IU per day.
A Cochrane Review of vitamin D supplementation in pregnancy compared women who received placebo or no intervention to women who supplemented with vitamin D. This review found supplementation with vitamin D during pregnancy probably reduces the risk of preeclampsia, gestational diabetes, reduces the risk of severe postpartum hemorrhage, and having a baby with low birth weight.
The American College of Obstetricians and Gynecologists does not recommend routine vitamin D supplementation during pregnancy beyond that contained in a prenatal vitamin. They note that when vitamin D deficiency is identified during pregnancy, most experts agree that 1,000–2,000 international units per day of vitamin D are safe. Unfortunately, ACOG also does not support routine serum vitamin D levels testing.
Sources of Vitamin D
There are two ways your body gets vitamin D, either from the sun or from your diet.
Vitamin D From Sunlight
It is estimated that 90% of our vitamin D comes from the sun. Plus, remember that since vitamin D is fat-soluble, you can store it. This means your body can build vitamin D stores when you typically get more sun exposure in the summer.
The sun emits both UVA and UVB rays. UVA rays contribute to skin damage and premature aging. UVB rays are needed for your body to produce vitamin D. When UVB rays hit the surface of your skin, your body uses photosynthesis to start processing it. Next, you transport it to your liver for additional processing and process it in your kidneys. This processing allows you to convert sunlight into the active hormones your body needs.
The amount of vitamin D you produce from sunlight varies depending on where you live, the time of year, time of day, how much skin is exposed, your skin, etc. If you live near the equator, you receive more sunlight. If you live above 37 degrees latitude, you probably aren’t getting enough vitamin D from sunlight in the winter months. This is about if you cut California in half at San Francisco and went across the northern borders of Arizona, New Mexico, Texas, and Philadelphia on the east coast.
UVB rays do not penetrate through glass, but UVA rays do. If you are spending time next to a window in the sunlight, you can get sunburned, but it will not help you produce vitamin D. Sunscreen can also inhibit your vitamin D production from sunlight. SPF blocks UVB rays, and broad-spectrum sunscreen blocks UVA and UVB. Any sunscreen with SPF will prohibit you from making vitamin D.
Remember that your skin tends to be more sensitive when you are pregnant, including to sunlight. Plus, if you have melasma, darker patches of skin on your face, or stretch marks, the sun may worsen those conditions.
It was difficult to find guidelines for how much time you need to spend in the sun to achieve adequate levels of vitamin D. Plus, you can’t know your vitamin D levels without a test. There are a lot of calculators online that vary a lot, and I was unable to find reliable information as to whether they are accurate.
The highest levels of UV light occur from 10:00 am to 2:00 pm. Spending one hour outside at 9:00 am will expose you to the same amount of UV light in 15 minutes at 1:00 pm. In general, if you are in the sun long enough for your skin to get pink, that is too long. You do not need to be naked in the sun. Exposure to your arms, legs, face, or back is good. It may be unrealistic to spend time outside in the sun year-round in some places. This is when you need to look to diet or supplements to ensure you are getting adequate vitamin D.
Vitamin D From Diet
Getting enough vitamin D solely from your diet is challenging. One of the foods highest vitamin D is fish. 3 oz of wild-caught salmon has 570 IU but farmed salmon has about ¼ of the vitamin D of wild salmon. If you are looking for wild-caught, avoid Atlantic salmon, which is farm-raised, or look for salmon specifically labeled wild-caught. Fish oil is also high in vitamin D. Some fish oil supplements may include vitamin D. There are also many foods fortified with vitamin D. An 8 oz glass of milk or orange juice fortified with vitamin D has 120 IU.
Unless you eat fish every day or consume a lot of milk, it is unlikely you are getting enough vitamin D from diet alone. While many other foods contain some vitamin D, it is in much smaller quantities. One egg yolk contains 44 IU of vitamin D. To reach the RDA of 600 IU, you would have to eat over one dozen eggs every day.
If you eat a plant-based diet, getting enough vitamin D from food alone is very challenging. One of the highest plant foods containing vitamin D is mushrooms treated with UV light or grown outdoors. Most mushrooms at the grocery store, like the white button mushroom, are grown in dark rooms and contain minimal vitamin D. If you are looking for mushrooms high in vitamin D, you need to ensure they were exposed to sunlight or UV radiation from lamps. D levels begin decreasing after about a week in the fridge, and you reduce vitamin D content in the cooking process. For more information on mushrooms as a source of vitamin D, see this review.
Vitamin D in Your Prenatal Vitamin
Most prenatal vitamins contain 400 IU of vitamin D. Some high-quality vitamins (like the Zahler Prenatal +DHA) will include higher amounts reflecting the evidence that 400 IU is likely not enough.
Taking a Vitamin D Supplement
Before considering a supplement, you may want to request to get your levels tested. You can look at how much vitamin D is in your prenatal vitamin or any other supplements you are taking. Also take into account whether you eat a diet rich in vitamin D. As with any supplement, please run it by your doctor or midwife first.
Vitamin D supplements come in liquid, capsule, or pill form. It is a relatively inexpensive supplement. As with any vitamin or supplement, quality does matter. A report that analyzed 19 of the best-selling vitamin D supplements in the U.S. found all 19 products exceeded their claimed vitamin D3 content, averaging 22% over their stated label claims. 6 of the 19 products exceeded their label claims by greater than 40%.
Some D3 supplements also include K2. The purpose of vitamin K is to help calcium transported by the vitamin D to be absorbed by your bones rather than accumulate in your arteries. I have two brands of vitamin D in my home right now. One is from Athletic Greens; you get a free one-year supply of vitamin D with your first purchase. The other is from Thorne. Both of these combine D3 with K2, which is my personal preference. If you choose to take a vitamin D supplement, since it is fat-soluble, you should take it with a meal.
Vitamin D and Your Baby
Your baby is entirely reliant on you for their vitamin D. Most infants will deplete the vitamin D stores acquired from the mother by approximately eight weeks of age. For the first six months of your baby’s life, their diet is solely breastmilk or formula. Breastmilk is naturally low in vitamin D, and infant formula is fortified with vitamin D. Plus, doctors recommend keeping a baby out of direct sunlight for the first six months of their life.
In 2009 the American Academy of Pediatrics doubled their recommended guidelines for vitamin D in children to at least 400 IU per day. They state infants who are exclusively or partially breastfed should receive 400 IU of supplemental vitamin D daily, beginning in the first few days of life. In 2020 the AAP published a study that examined infant vitamin D levels, and only 27.1% of infants met the vitamin D intake guidelines.
If you are exclusively breastfeeding, supplementing with vitamin D will increase the vitamin D transferred to your baby through breastmilk. A small study on breastfeeding mothers found that a maternal intake of 4000 IU/d could substantially improve maternal and neonatal nutritional vitamin D status.
Vitamin D and Respiratory Infections, Including COVID-19
It is well documented that vitamin D is necessary for immune function. Since COVID-19, some scientists and medical professionals have been outspoken about the potential benefits of vitamin D supplementation. Governments and health organizations have not joined in recommending vitamin D. Instead, their efforts have been masking, social distancing, and vaccines. Part of the pushback to recommend vitamin D is over concern that benefits of vitamin D are overstated, insufficient evidence of efficacy, fear of toxicity with overdoses, or that people will think vitamin D will make them immune to COVID-19. Meanwhile, some research shows that it may be worth taking a look at vitamin D.
An in-depth review makes a strong case that COVID-19 risk and severity is increased with vitamin D deficiency. Interestingly, the risk groups for severe COVID-19 perfectly match the risk groups for vitamin D deficiency. The thinking is that vitamin D is known to modulate the immune system, helping prevent both an under-reaction that allows upper respiratory infections to be contracted and the over-reaction referred to in COVID-19 as the “cytokine storm.” This review is a preprint, and it has not been peer-reviewed. It is the most comprehensive resource for all research on vitamin D and COVID-19. This review included over 141 articles that overwhelmingly support the assertions that vitamin D sufficiency increases resistance to viral infections and helps prevent every symptom of severe COVID-19 that results in fatalities.
Weighing the Risks and Benefits of Supplementing with Vitamin D
Let’s tie all of this up and give you some takeaways to weigh the risks and benefits of supplementing with vitamin D. It is clear that getting sufficient vitamin D is essential for your health and the health of your baby. Deficiency is widespread, and most people, including most pregnant mothers, have insufficient vitamin D levels.
The best way to know what your vitamin D levels are is to get a test. The following best way is to evaluate how likely you are to get adequate exposure to sunlight and vitamin D from your diet or prenatal vitamin. Examining your sun exposure involves taking into account many factors. Think about how much time you spend outdoors, where you live in relation to the equator, the time of year, your sunscreen use, how much bare skin is exposed outdoors, and whether you have high levels of melanin in your skin. You can determine how much vitamin D is included in your prenatal vitamin by reading the label. It is likely to have 400 IU, and higher-quality vitamins will have more. Then you can look at your diet to determine whether you are eating foods high in vitamin D every day.
Once you have an idea of how much vitamin D you are getting, you can look at the recommendations to determine whether you may want to consider an additional supplement. While the recommendations are confusing, evidence supports levels above the RDA of 600 IU. Some of the research in pregnancy supports 4,000 IU per day.
Vitamin D is a relatively inexpensive supplement and takes little energy to incorporate into your life. The doses required to reach toxic levels are at least 10,000 IU per day over an extended period. Overall, there is a minimal downside to taking vitamin D, assuming you are not overdoing it. None of the research reviewed for this episode found adverse effects to supplementing with vitamin D under the toxicity threshold levels. The upside is that you are ensuring you and your baby have adequate vitamin D to support everything from healthy bones to immune function.
Talk to Your Doctor, Midwife, or Pediatrician
As always, talk to your doctor, midwife, or pediatrician for their recommendations if you are considering any supplement. If you have concerns about your vitamin D levels, advocate for yourself to be tested, especially if you are in a high-risk group. Depending on your care provider, your health insurance may not cover testing, which could be an out-of-pocket cost. If you have any questions about vitamin D, please discuss them with your care provider.
Thank you to the amazing companies that have supported this episode.
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