How vitamin D supports the immune system

[7 MIN READ]

In this article:

• Vitamin D is necessary for good health and plays a role in strengthening your bones and building new ones. Research also has shown a relationship between vitamin D and immune function.

• Some clinical trials have shown an association between low levels of vitamin D and infections, inflammation and autoimmune diseases. But the effects of vitamin D on preventing these conditions isn’t clear, and more research is needed.

• To maintain healthy vitamin D levels, spend time in the sunlight and eat foods that contain this nutrient. You can also talk to your doctor about taking a vitamin D supplement.

The role of vitamin D in supporting the immune system

Having a strong immune system, especially during cold and flu season, can help reduce your risk of getting sick. Here we consider the role of vitamin D in supporting the immune system.

What is vitamin D?

Vitamin D helps your body absorb calcium and phosphorus to build and strengthen your bones. Your muscles and nervous system also need vitamin D to function properly. And scientific evidence suggests that vitamin D may also play an important role in your immune system. 

Vitamin D has two forms:

• Vitamin D2 (ergocalciferol) is in plants and fungi like mushrooms
• Vitamin D3 (cholecalciferol) is in animal products like tuna and liver 

Our bodies also create vitamin D as a response to the sun’s ultraviolet B rays on our skin. 

“Vitamin D is a little different from other vitamins that we get primarily from our diet,” says Sunil Adwani, M.D., an internal medicine physician at Providence Manhattan Beach Primary Care. “While we get it from certain foods, vitamin D also is produced in our skin from sun exposure.”

Vitamin D and immune function

Our immune system has two types that work together: 

• Innate immune system: Immune cells called white blood cells (WBC) destroy bacteria, viruses and anything foreign in our bodies. Vitamin D acts on these WBC (or macrophages) so they produce proteins and enzymes to kill these germs.
• Adaptive immune system: If the innate immune system doesn’t destroy specific germs, the adaptive immune system kicks in with T cells, B cells and dendritic cells. It has a memory and recognizes germs that have been in your body before. Vitamin D helps regulate this immune response and decrease inflammation. 

You need sufficient levels of vitamin D for both immune systems to function at their best.

The relationship between vitamin D and the immune system

Some studies have shown an association between low levels of vitamin D and infections, inflammation and autoimmune diseases.

• Infections: Some research shows that people with low levels of vitamin D more frequently have colds, flu and viral illnesses.
• Inflammation: Studies show that vitamin D in the adaptive immune system limits cells from producing inflammation and making antibodies.
• Autoimmune diseases: Low levels of vitamin D are associated with conditions like Type 1 diabetes, rheumatoid arthritis, multiple sclerosis and lupus. With autoimmunity, the immune system is overactive and the body attacks itself.  

It hasn’t been proven that supplementing vitamin D can prevent infections, inflammation and autoimmune diseases. More research is needed.

“It’s not clear that giving vitamin D to people with a deficiency can prevent issues like infections, autoimmune conditions, heart disease or cancer,” Dr. Adwani points out. “Doctors disagree about this, and we don’t know at this point if we should check vitamin D levels in everyone.”

The impact of vitamin D deficiency

According to the National Institutes of Health, about one in four people has low vitamin D levels. The daily recommendation is 15 micrograms for people who are 19-70 years old, and 20 micrograms for those over age 70.

Causes of vitamin D deficiency

Vitamin D deficiency is commonly due to a lack of exposure to sunlight, particularly in the winter in some areas. “Many of us live largely indoors,” Dr. Adwani says. “When we do go outside, we may be covered up or wearing sunscreen. So we’re limiting our exposure to the sun.”

You may also have a vitamin D deficiency if you don’t get enough through the foods you eat. Few foods contain vitamin D, so a restricted diet can limit your intake.

People at risk for vitamin D deficiency

Groups of people that have an increased risk of low vitamin D levels include:

• Breastfed infants: Breast milk does not supply enough vitamin D.
• Dark skinned individuals: Darker skin makes less vitamin D from sunlight exposure.
• Gastric bypass patients: This procedure can limit your body’s ability to absorb vitamin D.
• Obese individuals: Vitamin D accumulates in excess fat but isn’t easily available to the body.
• Older adults: As you age, you don’t make as much vitamin D from sunlight.
• People with inflammatory bowel disease: Crohn’s disease and ulcerative colitis make it harder for your body to absorb vitamin D.
• Vegans: People who don’t eat animal products or can’t tolerate milk, eggs and fish may have reduced vitamin D intake.

Symptoms and risks of vitamin D deficiency

According to Dr. Adwani, many people with vitamin D deficiency don’t have any symptoms. They may not know they have a deficiency without a blood test. In severe cases, however, symptoms of vitamin D deficiency can include:

• Bone pain
• Fatigue
• Frequent illness
• Muscle aches, cramps or weakness

And over time, the risks of prolonged vitamin D deficiency include:

• Osteoporosis: bones that become brittle and less dense and can fracture easily
• Osteomalacia: weak and softened bones, along with muscle weakness in adults
• Rickets: soft bones and skeletal deformities in infants and children.

How to maintain healthy vitamin D levels

One of the simplest ways to increase your vitamin D levels is to go outdoors in the sunlight. Sunscreen is important to help prevent skin cancer, but it limits vitamin D production. If possible, try to balance its use with spending 10-30 minutes in the sun daily to help your skin make enough vitamin D.   

Diet

Eating foods that naturally include vitamin D or are fortified with it raises your level. These include:

• Cereal
• Cheese
• Cod liver oil
• Eggs
• Fatty fish like salmon, tuna, mackerel and trout
• Milk
• Mushrooms
• Orange juice

Dietary supplements

Dietary supplements come in various dosages and may be combined with other nutrients like calcium or magnesium. Multivitamins also typically contain vitamin D. Talk with your doctor about what’s best for you.

“I recommend vitamin D supplements for the elderly, pregnant women, people with osteoporosis and infants who are exclusively breastfed to support healthy levels,” Dr. Adwani says.

But too much vitamin D can be harmful. While rare, excessive vitamin D can cause symptoms including:

• Confusion
• Dehydration
• Kidney stones
• Loss of appetite
• Muscle weakness
• Nausea
• Vomiting

“Today, people are more proactive and informed about their health,” Dr. Adwani says. “So they should talk with their doctor about vitamin D and immunity. Supplementing vitamin D doesn’t appear to have much downside, and in fact, may have some upside.”

Contributing caregiver

Sunil Adwani, M.D., is an internal medicine physician at Providence Manhattan Beach Primary Care in Manhattan Beach, California. 

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Related resources

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Boost your immunity — this winter and beyond

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This information is not intended as a substitute for professional medical care. Always follow your health care professional’s instructions.