Vitamin D Benefits: Why is Vitamin D Important for Immunity?


Around one billion people worldwide are deficient in vitamin D, an essential nutrient with numerous important functions in the body. In this article, we’ll explore the benefits of vitamin D, why it’s important for immunity and other bodily systems, and how to make sure you’re getting enough.

For most people, the best way to get enough vitamin D is taking a supplement because it is hard to eat enough through food.

Vitamin D Benefits

Despite its name, vitamin D is actually a hormone, since the body can synthesize it on its own in reaction to sunlight. This hormone plays a crucial role in supporting the immune system by activating immune cells and increasing their function to protect us from infections.

Vitamin D also helps to reduce inflammation and inhibit cancer cell growth, as well as increase intestinal absorption of calcium, magnesium and phosphate. This means vitamin D is essential for bone density, as well as brain health, heart health, and overall longevity.

What Happens if You Don’t Get Enough Vitamin D?

Research suggests that low levels of vitamin D may be linked to diabetes and heart disease. Studies show that people with high blood pressure who are deficient in vitamin D have nearly twice the rate of heart attack as those with adequate levels, although the exact reason behind this is unclear.

Vitamin D is also important for brain health. Recent studies have confirmed a link between vitamin D deficiency and cognitive impairment and dementia. Some studies even suggest vitamin D may help protect us from developing multiple sclerosis, although more research is needed.

Researchers are learning more about how vitamin D deficiency plays a role in upper respiratory infections, but it’s clear that vitamin D deficiency can increase the likelihood that you get sick. In a recent study, people with severe complications from COVID had low vitamin D levels. Normal vitamin D levels should be around 50 nmol/L (20 ng/mL), and subjects in this study had levels below 30 nmol/L (12 ng/mL). When test subjects supplemented with vitamin D, their symptoms quickly improved.

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How to Get the Benefits of Vitamin D

There are multiple ways to get your daily dose of vitamin D: through spending time outdoors, diet, and supplementation. Your skin makes vitamin D when it’s directly exposed to sunlight. While this may be the easiest way to get vitamin D in some climates or seasons, it’s not always possible. And if you spend the bulk of your time indoors, your body likely doesn’t produce much vitamin D on its own.

“Part of the difficulty of maintaining vitamin D levels is because there are not a large variety of foods that contain much Vitamin D,” says Kristin Gustashaw, MS, RD, LDN, CSG, clinical dietitian at Rush University Medical Center. With that said, it’s still a good idea to add foods with vitamin D to your diet.

Best Food Sources of Vitamin D:

  • Egg yolks
  • Dairy products like milk, cheese, and yogurt
  • Mushrooms
  • Fatty fish, especially wild-caught salmon and mackerel
  • Canned tuna in water
  • Sardines
  • Beef or calf liver
  • D-fortified products, such as almond, soy, and oat milks, certain cereals, bread, and orange juice

Gustashaw recommends that you get out in the sun for at least 15-30 minutes a day–when the sun is available. “Consider the amount you get from the sun as extra protection,” she says Gustashaw. “Be sure to get a constant source from your diet and supplementation all year round.”

Supplements can also be a great source of vitamin D. According to Gustashaw, “Adults should get a minimum of 600 IU of the vitamin each day and 800 IU if over age 70. Children should get 600 IU each day. And infants 0-12 months 400 IU/day.”

Vitamin D vs D3

You may have heard of multiple kinds of vitamin D. Vitamin D is fat-soluble and helps to regulate our calcium and phosphorus levels, while vitamin D3 is the natural form produced by our bodies in response to sunlight.

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As for vitamin D supplements, there are two kinds: D3 and D2. Each has a different molecular structure, and D2 comes from plants, whereas D3 comes from animals. Some research shows that vitamin D3 supplements are easier for your body to absorb. If you’re 65 or under, your doctor may recommend 600 IU (international units) of vitamin D3 daily. If you’re older than 65, you may need 800 IU daily.

For a severe deficiency, your doctor may suggest a higher dosage. But too much vitamin D can lead to health problems, including nausea, vomiting or kidney stones, so don’t overdo it.

Why is Vitamin D Important for Immunity?

Researchers are still uncovering the link between vitamin D deficiency and respiratory infections like COVID-19, but it’s clear that vitamin D is extremely important for immunity. That’s because vitamin D inhibits the damaging inflammatory response of some white blood cells, while boosting immune cells’ production of microbe-fighting proteins. It promotes immune responses by enhancing the production of antimicrobial agents, and it controls the influence of the specific immune responses.

So if you want to stay healthy this season–and all year long–be sure to get enough of this critical nutrient.

How to Determine if You’re Deficient in Vitamin D

“We know that a large percentage of the population has suboptimal levels of vitamin D,” explains Gustashaw. “This can possibly lead to symptoms including fatigue, tiredness, hair loss, delayed wound healing, decreased immune health, muscle pain and more, with no other known causes.”

Gustashaw says you can determine your levels through a blood test. A low level is 25-hydroxyvitamin D less than 30 ng/mL; a severe deficiency is 25-hydroxyvitamin D less than 5 ng/mL.

If you determine that you do have a lower than desired level, it’s always best to talk to your healthcare provider or dietitian about the best way to boost your intake. And be mindful of any medications you may be taking that may affect vitamin absorption, including steroids (e.g., prednisone); the cholesterol-lowering drug cholestyramine; and the seizure medications phenobarbital (Luminal) and phenytoin (Dilantin).

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Some people are more likely to have a vitamin D deficiency than others. Americans with darker-colored skin are less efficient at producing their own vitamin D. Older adults may experience deficiency, because their skin may not make vitamin D as well as it did years earlier and they may not get outdoors as often.

What if I Take Too Much Vitamin D?

The main consequence of vitamin D toxicity is a buildup of calcium in the blood, called hypercalcemia. Early symptoms of hypercalcemia include nausea, vomiting, diarrhea, constipation, and weakness. Excessive thirst, an altered level of consciousness, high blood pressure, calcification in the kidney tubes, kidney failure or hearing loss may also develop.

Hypercalcemia caused by regularly taking high amounts of vitamin D supplements may take a few months to resolve. This is because vitamin D accumulates in body fat and is released into the blood slowly.

If you are supplementing with vitamin D, it’s important to also ensure sufficient intake of vitamin K and magnesium. These may reduce the risk of adverse effects from a higher vitamin D intake. Vitamin D and magnesium are also important to take together because the bioavailability of vitamin D largely relies on magnesium being present.

An occasional high dose of vitamin D is sometimes used to treat a deficiency, but always consult with your doctor or dietitian before taking a large dose. As with many other things in nutrition, more does not always equal better.



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Carrie Solomon

Carrie Solomon is a freelance health writer, copywriter, and passionate wellness enthusiast. She’s on a mission to help wellness-focused companies educate, engage, and inspire their audiences to make the world a healthier, happier place. Learn more about her at or on LinkedIn.

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