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A 5-Minute Guide To Vitamin D: Here's What It Is & Why It's So Important

Alejandra Carrasco, M.D.
Integrative & Functional Medicine Physician
By Alejandra Carrasco, M.D.
Integrative & Functional Medicine Physician
Alejandra Carrasco, M.D., is an integrative and functional medicine physician, best-selling author of Bloom, and founder of Nourish Medicine, a root-cause resolution integrative and functional medicine practice in Austin, Texas. She received her medical degree from the University of Texas Health Science Center.

Unless you live in the tropics year-round (and get outside) or take high-potency supplements proactively (and daily), chances are you aren't getting enough vitamin D—like not even close. And though it's an issue much of the health world is now painfully aware of, vitamin D is scant in foods and one of the most common nutrients gaps1 worldwide.

As a result, almost all Americans are failing to consume just 400 IU of vitamin D from their daily diet, and clinical vitamin D insufficiency persists in almost half of the population (41 percent2, to be exact). Here's a look at what vitamin D is and the important roles it plays in the body, as well as how to test your levels and boost them if they're low (which is thankfully pretty easy to do!).*

What is vitamin D?

Vitamin D is both a vitamin and a preprohormone (and full-on hormone in its active form in the body) essential for healthy bodily function.* Often nicknamed "the sunshine vitamin," vitamin D3 is synthesized in our skin via UVB sunlight exposure.

We can also get some (though not enough to impact our vitamin D status in a meaningful way) vitamin D from specific foods.

Vitamin D supplements are also popular—and a necessity—for people who aren't able to get their fix from sunshine or food (which, it turns out, is pretty much all of us) or for those who are simply interested in achieving and maintaining a consistent state of vitamin D sufficiency throughout life.*

A recap

Vitamin D is both a vitamin and a hormone our bodies make and it's essential for healthy bodily function.*

Let's talk about vitamin D's benefits

While all nutrients play a crucial, symbiotic role in maintaining health, vitamin D is especially important as it is essential for nearly every single bodily system3 and function including, but not limited to, the musculoskeletal, immune, neurological, thyroid, gastrointestinal, renal, endocrine, and cardiovascular systems4.* Here's a look at vitamin D's many benefits:


It helps support thyroid health.*

Vitamin D plays a crucial role in thyroid health as it helps regulate the production of thyroid hormones5,* which have widespread, multifaceted functions in the body including metabolism, weight, mood, the sleep cycle, musculoskeletal health, reproductive health, and immunity. Thyroid hormones also play a role in keeping your hair, skin, and nails youthful and strong.

Pregnancy is another key period to be keenly aware of your thyroid health. To proactively support a healthy postpartum period of recovery, it's best to take care of your thyroid before, during, and after pregnancy―and, given its role in supporting thyroid health, getting enough vitamin D is a great place to start.*

Further studies have associated low levels of vitamin D with thyroid issues5.


It promotes a healthier pregnancy.*

Pregnancy, birth, and caring for a new baby require an increased amount of stress hormones and stamina (and often involve less-than-optimal sleep). This all puts an obvious strain on the body's physiological balance and nutrient reserves. Fetal vitamin D demands lead to increased maternal needs for this critical fat-soluble nutrient.

Repeated clinical research clearly demonstrates that vitamin D3 supplementation is critical for both maternal and infant vitamin D sufficiency6.* Further, healthy vitamin D status during this unique life stage in mom and baby is linked with better maternal and infant outcomes7, for example related to bone density, immune function, fetal growth, and maternal metabolic health, to name a few.*


It supports immunity.*

Optimal vitamin D levels have been shown to positively affect the innate and adaptive immune systems in a variety of ways, including supporting white blood cell function8, helping the immune system adapt to and ward off threats9, and more.*

Vitamin D deficiency has also been associated with a variety of autoimmune issues10.


It promotes gut health.*

Vitamin D has been shown to play a role in promoting beneficial gut bacteria, which, in turn, have a positive effect on gut function and metabolism.*

Further, a variety of studies have shown that vitamin D plays a key role in the health and integrity of the gut mucosal lining11, which acts as a barrier between your digestive system and bloodstream.*


It supports healthy bones.*

You probably already knew this one, but vitamin D is also critical for your bones because it helps the body absorb and maintains homeostatic balance of calcium and phosphorus, which are essential structural elements of our bones.* For example, in the small intestines, vitamin D works to shuttle the calcium we consume from food into the bloodstream.

The bottom line: Maintaining sufficient vitamin D levels is a must for regulating mineral balance and promoting bone density, strength, and health for life.*

How to find out if you're vitamin D deficient

The safest and smartest way to optimize your vitamin D consumption is via high-quality supplementation. To monitor your baseline vitamin D status and track the success of your supplement, you can monitor your blood levels. To do that, have your doctor order a 25-hydroxyvitamin D test, which is also known as the 25(OH)D test. This is the biomarker for vitamin D status in your whole body.

The optimal (not just "normal") amount you want to see, is 50 ng/ml plus. In some cases, even higher levels may be appropriate. The cutpoint to avoid (because it's the cutoff for vitamin D insufficiency12) is 30 ng/ml.

I recommend having your 25(OH)D levels tested twice a year—and perhaps even more often if your health care provider thinks it useful. The benefit of knowing your serum vitamin D status is to track progress and have peace of mind that your vitamin D ritual is achieving and maintaining sufficient levels throughout life.*

How to get more vitamin D

The amount of vitamin D you'll need to overcome deficiency or insufficiency and achieve true sufficiency depends upon your individual makeup, body composition, baseline lab test results, and other vitamin D inputs, so check with your health care provider for personalized recommendations on how to keep your levels in check.

As a quick rule-of-thumb, to achieve a 50 ng/ml or higher 25(OH)D lab test result, this will require 5,000 IU of vitamin D3 for most normal-weight adults. Individuals with more weight will need proportionately more vitamin D.12

One quick way to up your vitamin D is to get moderate sunlight exposure13 each day. That means never letting your skin burn, and I like to personally follow the Vitamin D Council's recommendations for skin type, location, etc., as follows:

  • Those with very light to light skin likely need 10 to 15 minutes of unprotected sun exposure.
  • Those with naturally tan skin can get unprotected sun for 15 to 20 minutes.
  • Those with darker skin can safely get one to two hours of unprotected sun exposure.

From there, be sure to protect yourself with nontoxic sunscreen. However, since many people can't safely get enough exposure to move the needle (whether because of an indoor job or where in the world they live), the sun isn't the only answer. And others seek to avoid cumulative sun exposure for other obvious skincare and health risk reasons.

A few foods also contain vitamin D, such as shiitake and button mushrooms (leave them in the sun to elevate their vitamin D levels), mackerel, sockeye salmon, cod liver oil, sardines, and eggs—but consuming them in high enough quantities to meet your nutritional requirements and move the needle on your 25(OH)D levels is a tall, tall order.

So what's the solution? Vitamin D supplements, a key tool for most. I suggest choosing a high-quality, high-potency supplement that contains vitamin D3 (which is the body's preferred form) and optimizes the fat-soluble absorption of this critical micronutrient.

mbg tip

Want specific recommendations? Here's a list of our favorite.

A recap:

  1. Most Americans2 have a dietary vitamin D gap and many are vitamin D insufficient or deficient.
  2. Vitamin D is both a vitamin and a preprohormone essential for nearly every single bodily function.*
  3. The serum 25-hydroxyvitamin D test, also known as the 25(OH)D test, is the best way to determine your vitamin D levels and status.
  4. Research and clinical consensus demonstrate the optimal 25(OH)D is 50 ng/ml, and that higher levels may be appropriate for some individuals.
  5. Vitamin D deficiency has been associated with a wide variety of health issues, affecting the immune system gut, thyroid, and more.
  6. Vitamin D is important across all life stages, and especially important for women during preconception, conception, pregnancy, and postpartum.
  7. Though you can get some vitamin D from food and sun exposure, supplements remain necessary for most people to achieve and maintain vitamin D sufficiency throughout life.*
  8. As a functional medicine doctor, I recommend getting your 25(OH)D levels checked at least twice annually to be intentionally aware of your vitamin D status and to ensure sufficiency.
If you are pregnant, breastfeeding, or taking medications, consult with your doctor before starting a supplement routine. It is always optimal to consult with a health care provider when considering what supplements are right for you.
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