Many of Us Are Deficient in Vitamin D — Here’s What That Means for Our Immune Systems

Roughly 35 percent of American adults are considered deficient in vitamin D, according to a report in ​StatPearls​.

Vitamin D has been linked to having effects — both good and bad — on the immune system.

And during a time when everyone wants to fortify their immune systems, it's important to understand the role vitamin D plays.

A quick overview: Vitamin D is actually a hormone, which the body produces after sun exposure, according to the Hormone Health Network.

Sunlight, by the way, is the best source of vitamin D (it's actually hard to get enough from food sources). And vitamin D is fat-soluble, meaning you need fat in your diet to help your body absorb it.

Vitamin D is known for its important role in promoting and maintaining bone health because it helps the body absorb calcium, says Caroline Jouhourian, MD, a gastroenterologist at Lowell General Hospital in Lowell, Massachusetts.

There are two types of vitamin D: D3 and D2. Vitamin D3 is what your body makes when exposed to sunlight, Dr. Jouhourian says.

It may also be be more effective at improving and sustaining vitamin D levels, Melaina Bjorklund, RD, a clinical dietitian at Penrose-St. Francis Health Services previously told LIVESTRONG.com.

D2 is present in some plants such as mushrooms and yeast.

While we know that vitamin D plays a role in immune health, it's extremely complicated, and a lot remains to be seen, Dr. Jouhourian says.

We have two types of immune systems: the innate, which we're born with, and the adaptive (also known as acquired), which evolves over time as it's exposed to various pathogens. It's the acquired immune system that will fight bacterial and viral infections, Dr. Jouhourian explains.

If you're deficient in vitamin D, it's suspected that your acquired immune system might suffer. In fact, not getting enough vitamin D is linked to a higher risk for chronic autoimmune diseases, including multiple sclerosis, type 1 diabetes and Crohn's disease, Dr. Jouhourian says.

For example, low levels of serum vitamin D (circulating in the bloodstream) affects the risk of developing MS and the disease activity in people with MS, according to a June 2018 review in ​Neurology Therapy​.

People living at higher latitudes — farther away from the equator where sunlight is prevalent year-round — have lower levels of vitamin D and an increased risk for infectious diseases and many chronic illnesses, including autoimmune diseases, some cancers, heart disease, schizophrenia and type 2 diabetes, according to a January 2013 paper in ​Dermato-Endocrinology.​

On the flip side, too much vitamin D might negatively affect the innate immune system, but there hasn't been enough research that looks further into that, Dr. Jouhourian says.

The acquired immune system is responsible for fighting illnesses brought on by bacteria and viruses, including COVID-19.

That means those who are deficient in vitamin D might not fight infections as well as folks with healthy levels, Dr. Jouhourian says, noting that more research is needed. In fact, vitamin D deficiency is associated with a higher risk of getting COVID-19, per a September 2020 study in ​JAMA Network Open​. And Black people with vitamin D levels of 30 to 39.9 ng/ml (nanogram per milliliter) had a 2.64 times higher risk of testing positive for COVID-19 than people with levels of 40 ng/ml or greater in a March 2021 study in ​JAMA Network Open​.

However, vitamin D supplementation might have a positive effect when it comes to fighting respiratory infections, according to a February 2017 meta-analysis and systematic review in the ​British Medical Journal​.

The researchers reviewed 25 studies and found that vitamin D supplementation reduced the risk of acute respiratory tract infection for all participants. Those who were very deficient saw even more of a benefit to supplementation.

"There is no supplement that can protect someone from contracting COVID-19."

But this doesn't mean supplementing with vitamin D will fortify your immune system, which is very important to note (especially during a global pandemic when people will try almost anything to protect themselves).

"At this point, there is not enough data to conclude that [a supplement] would protect you, but there is also not enough data to say it won't help," Dr. Jouhourian says.

She goes on to explain that even if a supplement might help fight infection, it won't protect you from ​acquiring​ an illness like COVID-19. It's also important to remember that vitamin D has been heavily studied in its role in preventing and fighting osteoporosis, not immune health.

"There are several relationships between vitamin D and various health outcomes that have been explored, like the relationship between vitamin D status and cancer, but none have been proven other than the relationship between vitamin D and bone health," says Frances Largeman-Roth, RDN, author of ​Smoothies & Juices: Prevention Healing Kitchen​.

General recommendations from the National Institutes of Health say adults should get 600 IU of vitamin D every day, while those over age 70 should get 800 IU.

But, Dr. Jouhourian says, knowing how much vitamin D someone needs for a healthy immune system is unclear. And because vitamin D toxicity can occur at levels of more than 4,000 IU per day, more is ​not​ better.

The only way to know for sure what your vitamin D levels are is through blood work, Dr. Jouhourian says. She recommends people looking to supplement with vitamin D work with a dietitian or health care professional.

Hands down, the best way to get vitamin D is from the sun.

When exposed to sunlight, your body produces its own vitamin D. Experts recommend about 15 to 20 minutes of sun exposure every day, between 10 a.m. and 3 p.m., when the sun is strongest.

Dr. Jouhourian says it's possible to get the sunlight you need even while wearing sunscreen. Largeman-Roth advises her clients to get about 15 minutes of sun without coverage.

"I am all about skin cancer and wrinkle prevention, but you need to let your skin get exposed a bit in order for it to make vitamin D," she says.

People with darker skin may need a little extra time in the sun in order for the UVB rays to penetrate the skin and activate the body's vitamin D-making process, according to the paper published in ​Dermato-Endocrinology​.

It's possible to get some vitamin D from certain foods, including fatty fish (salmon, tuna and cod, for example), cod liver oil, egg yolk, mushrooms (if grown under UV light) and fortified products, including milk, yogurt and orange juice, Largeman-Roth says.

Supplements can also be an effective way to boost your vitamin D levels, if you need a little extra help, Largeman-Roth says.

When choosing a supplement, look for those certified by a third party, such as United States Pharmacopeia (USP). But when it comes to boosting vitamin D levels for your immune system, Largeman-Roth reminds us that supplements won't protect against COVID-19.

"There is no supplement that can protect someone from contracting COVID-19," she says.

"The best we can do is to maintain a healthy diet, get regular exercise — preferably outside — wear masks when we need to, practice social distancing and wash our hands regularly."