Vitamin D is a nutrient that acts as a hormone in your body.

Vitamin D is a fat-soluble vitamin, so it can be stored in your body’s fat and used by your body when it’s needed.

Vitamin D is sometimes called the sunshine vitamin because it can be made in your skin with UV rays from the sun.

Your immune system defends your body from invading germs.

It’s your first line of defense against infections.

If your immune system is strong, you have a better chance of fighting off infections and not getting sick.

You may be wondering how vitamin D plays a part in your immune system.

Many studies indicate that vitamin D is beneficial for your immune system, especially for protection against infections.

Important parts of your immune system, such as T cells, B cells, and macrophages, need vitamin D to function properly.

Vitamin D helps activate T cells, which are sometimes called the killer cells because they find and destroy harmful germs (1).

So, a low vitamin D level can increase your chance of infection if these killer cells aren’t activated to find and kill germs.

Vitamin D is the subject of a lot of research right now.

Many studies are seeing a connection between sunny climates, higher vitamin D levels, and lower rates of respiratory infections and some diseases.

There is a strong link between vitamin D and respiratory infections, including the flu, the common cold, and possibly COVID-19.

A 2009 study found that vitamin D may help the immune system fight off respiratory infections, like the common cold or the flu (2).

20% of the people in the study with the lowest vitamin D levels had recent respiratory illnesses.

But only 17% of those with the highest vitamin D levels had recently been sick with a respiratory infection.

A recent research review showed that taking a vitamin D supplement could prevent respiratory illnesses (3).

Taking a vitamin D supplement decreased the risk of respiratory infections in people with a vitamin D deficiency, but even people with good vitamin D levels had a lowered risk.

Not surprisingly, the most benefit was for people who were very deficient in vitamin D before they began taking the supplement.

Another study showed that children given a vitamin D supplement during the winter were 42% less likely to get the flu than those who did not take a vitamin D supplement (4).

Low levels of vitamin D can increase your risk of respiratory infections (5).

On the other hand, high vitamin D levels in the blood may decrease your risk of respiratory infections (6).

Taking vitamin D supplements daily or weekly showed more benefit than receiving high doses less often (7).

While there is not enough research yet to say for sure, it’s possible that low vitamin D levels may increase the risk of getting COVID-19 or may cause COVID-19 to be worse (8).

One research study showed that vitamin D levels in the blood were a predictor of death from COVID-19 (9).

The lockdown during the pandemic may have reduced the level of vitamin D in your body because you have had fewer opportunities to be in the sun.

So, you may want to talk to your doctor about taking a vitamin D supplement to boost your immune system.

Respiratory infections, such as the flu or the common cold, are more frequent in the winter months, but it’s not completely understood why.

Could vitamin D be the link? One theory is that vitamin D deficiency is more common in the winter because of less sun exposure and more clothes covering the skin (10).

And we just talked about how vitamin D deficiency can cause a weaker immune system, which can lead to more sickness.

So, it’s important to start your vitamin D supplement in the early fall to make sure you have a good vitamin D level in the winter.

Vitamin D deficiency is common in people with autoimmune diseases, such as multiple sclerosis (MS), type 1 diabetes, rheumatoid arthritis, and lupus (11).

Low levels of vitamin D can make the autoimmune disease worse (12).

If you have an autoimmune disease or are in a high-risk group, it’s important to talk to your doctor about testing your vitamin D level.

You can get vitamin D from exposing your skin to the sun (without sunscreen), from eating foods high in vitamin D, or from a vitamin D supplement.

Vitamin D deficiency is common in the United States, especially in people with darker skin.

Research estimates that up to 42% of Americans have a vitamin D deficiency, but 82% of black Americans and 69% of Hispanic Americans are deficient (13), (14), (15).

The only way to know if you have a vitamin D deficiency is to ask your doctor to measure your vitamin D level with a blood test.

Your vitamin D blood level should be at least 30 ng/ml (75 nmol/l) (16).

Only a few foods are naturally rich in vitamin D:

The only good plant source of vitamin D is mushrooms, so it’s especially hard for vegans or vegetarians to get enough vitamin D from food.

You can also get vitamin D from vitamin D-fortified milk, orange juice, or breakfast cereal.

If you don’t eat lots of fatty fish or don’t spend a lot of time in the sun (without sunscreen), you should consider a vitamin D supplement.

Vitamin D3 is the best choice for a vitamin D supplement because vitamin D3 raises vitamin D levels in the blood higher and keeps them there longer than vitamin D2 (17).

Keeping your vitamin D at a good level may boost your immune system overall and decrease your risk of respiratory infections, such as the flu and the common cold (18).