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

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.
Portrait of a Young Woman Standing in The Sunshine

Image by David Prado / Stocksy

Unless you live in the tropics year-round or take supplements proactively, chances are you aren't getting enough vitamin D. And despite widespread educational campaigns to raise awareness, vitamin D deficiency persists in more than half of the population. Here's a look at what vitamin D is and the important role it plays in the body, as well as how to test your levels and bring them up if they're low (thankfully, it's pretty easy to do!).

What is vitamin D?

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Vitamin D is both a vitamin and a hormone essential for healthy bodily function. Often nicknamed "the sunshine vitamin," vitamin D mostly enters our bodies via the sun, which is absorbed by our skin and converted into a usable form by cholesterol. You can also get some vitamin D from specific foods, and vitamin D supplements are popular for people who aren't able to get their fix from sunshine or food (which, it turns out, is a lot of us).

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What does vitamin D do?

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 system and function including the musculoskeletal, gastrointestinal, renal, endocrine, and cardiovascular systems.* Here's a look at how it affects each one:

1. It helps support thyroid health.*

Vitamin D plays a crucial role in thyroid health as it regulates the production of thyroid hormones, which help protect you from thyroid diseases and maintain a healthy weight, mood, sleep cycle, and immunity.* Certain thyroid hormones also play a role in keeping your hair, skin, and nails youthful and strong.

Further studies have shown low levels of vitamin D are associated with chronic thyroid conditions such as Hashimoto's thyroiditis and Graves' disease.*

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2. It promotes a healthier pregnancy.*

Pregnancy, birth, and caring for a new baby require an increased amount of stress hormones, sleep deprivation, and stamina. This puts a huge strain on the body's hormonal balance and nutrient reserves, which often results in a compromised thyroid and adrenals. If I could give one piece of preventive advice for a healthy postpartum recovery, it would be to take care of your thyroid before, during, and after your pregnancy―and getting enough vitamin D is a great place to start.

3. It supports immunity.*

Optimal vitamin D levels have been shown to positively affect the innate and adaptive immune system in a variety of ways including boosting genetic expression of white blood cells, helping the immune system adapt and ward off infection, and managing inflammation.* There is reason to believe that supplementing with vitamin D could help reduce the risk of COVID-19, but more research needs to be done.

Vitamin D deficiency has also been associated with a variety of autoimmune diseases including multiple sclerosis, lupus, diabetes, and rheumatoid arthritis.

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4. It reduces adverse health conditions caused by inflammation.*

While some inflammation can be a good thing―like the protective swelling that occurs after a physical trauma―chronic inflammation is problematic, as it causes body breakdown. (Inflammation is now recognized as a causal factor in nearly every chronic condition, from heart disease to depression.)

Vitamin D helps manage inflammation by modulating the release and proliferation of cytokines―chemical messengers that initiate inflammation.*

5. It enhances gut health.*

Vitamin D has been shown to play a crucial role in promoting beneficial gut bacteria, which, in turn, has a positive effect on metabolism.* Further, a variety of studies have shown vitamin D plays a key role in the health of the gut mucosal lining, acting as an anti-inflammatory, anti-infective, and immune booster to protect against viruses and other pathogens.

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6. It promotes healthy bones.

You probably already knew this one, but what makes vitamin D good for your bones? The answer lies in the way it helps us absorb calcium, the structural element of our bones.* In the small intestines, vitamin D works to shuttle the calcium we consume from food into the bloodstream. Maintaining adequate calcium and vitamin D levels throughout life, as part of a well-balanced diet, may reduce the risk of osteoporosis.*

To boost vitamin D's calcium absorption, try also adding vitamin K2 to your diet or supplement routine. Vitamin K2 acts as an essential "GPS" for the calcium liberated by vitamin D, ensuring it gets to the right places in your body―like your bones―while staying out of the wrong places―like your arteries.

7. It plays a role in healthy genetics and gene expression.*

Vitamin D has been shown to be responsible for up to 3% of what's known as "gene transcription," the process in which your inherited genes are expressed or activated.* In other words, vitamin D plays one of the most essential roles in nourishing your body, protecting your genes, preventing acute and chronic disease, and maintaining your overall health.*

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Labs to request to test for vitamin D deficiency.

The safest and smartest way to optimize your vitamin D consumption is to monitor your blood levels. To do so, have your doctor specifically order a 25-hydroxy vitamin D test, also known as the 25-OH D test. The optimal (not just "normal") amount you want to see is 45 to 65 ng/mL, and in some cases, even higher levels are appropriate.

How to get more vitamin D.

The amount of vitamin D required to replenish deficiencies depends upon your lab tests, so you'll want to check with your provider for individual recommendations on how to keep your levels in check.

A quick way to up your vitamin D intake is to get more unfiltered, unprotected sunlight each day (see below for more on this). But if you live in a place where the sun isn't shining year-round or work a job that requires you to spend a lot of time inside, this can be a challenge. In that case, you may need to supplement. I suggest choosing a vitamin D3 supplement, and working closely with your doctor to monitor your levels.

Some foods also contain vitamin D, such as shiitake and button mushrooms (leave mushrooms in the sun to elevate their vitamin D levels), mackerel, sockeye salmon, cod liver oil, sardines, and eggs, but chances are you aren't consuming them in high enough quantities to meet all of your nutritional requirements.

Vitamin D and sun exposure.

There are many healing benefits of the sun, but is there a right way to get vitamin D, especially in the summer? In my experience as a physician and based on the current research, moderate sun exposure is best. Therefore, I recommend getting some unprotected sun exposure every day based on the Vitamin D Council's recommendations for your skin type, location, etc., as follows, and to never let your skin burn. Then be sure and protect yourself with a nontoxic sunscreen:

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

To recap:

  1. Most of us who live in North America are at high risk of vitamin D deficiency.
  2. The 25-hydroxy vitamin D test, also known as the 25-OH D test, is the best way to determine your vitamin D levels.
  3. Vitamin D is both a vitamin and a hormone essential for nearly every single bodily function.
  4. Optimal/functional ranges lie between 45 and 60 ng/mL, and higher levels may be appropriate for some individuals.
  5. Vitamin D deficiency has been associated with a wide variety of conditions including autoimmune diseases, chronic inflammatory conditions (like heart disease, metabolic syndrome, and MS), gut health, thyroid conditions, fertility (in men and women), pregnancy complications, and developmental issues in children.
  6. Vitamin D is especially important for new mothers during preconception, conception, pregnancy, and postpartum.
  7. Your best source of vitamin D is the sun. But be careful about overexposure.
  8. Your next best source is a D3/K2 supplement, as recommended by your doctor or health care practitioner.
  9. You can also get vitamin D from foods such as mushrooms, mackerel, sockeye salmon, cod liver oil, grass-fed beef, sardines, and eggs.
  10. As a functional medicine doctor, I recommend getting your levels checked at least twice annually, possibly more if you're on a high-dose supplementation protocol.

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