Scientists Say This Supplement May Help Protect You From the Flu

Given that 18% of Americans have inadequate vitamin D levels, this is worth paying attention to.

Meet the experts: Amesh A. Adalja, M.D., an infectious disease expert and senior scholar at the Johns Hopkins Center for Health Security; Thomas Russo, M.D., a professor and chief of infectious diseases at the University at Buffalo in New York; Jessica Cording, R.D., C.D.N., author of The Little Book of Game-Changers; William Schaffner, M.D., an infectious disease specialist and professor at the Vanderbilt University School of Medicine

This isn’t the first study to link low levels with a higher risk of respiratory infections, raising questions about whether you should be focused on your own levels of the nutrient. Here’s what doctors and a dietitian want you to keep in mind.

Why may vitamin D lower the risk of severe respiratory infection?

The researchers didn’t dive into that, but doctors aren’t shocked by these findings. “Vitamin D plays a role in optimal immune system function,” says Amesh A. Adalja, M.D., an infectious disease expert and senior scholar at the Johns Hopkins Center for Health Security.

Vitamin D enhances both your innate immunity—your body’s first line of defense against pathogens—and your adaptive immunity, which is the part of your immune system that “learns” to fight certain germs after you’ve been exposed to them, explains Thomas Russo, M.D., a professor and chief of infectious diseases at the University at Buffalo in New York. “Appropriate vitamin D concentrations are important for an optimal immune response,” he says, noting that even having a “moderate” deficiency in vitamin D may impact your immune system.

But it’s important to keep two things in mind with this particular study: One is that it’s observational, so researchers simply found an association between having low vitamin D and a greater risk of serious infection. The other is that they focused on those with a severe deficiency.

“The literature around vitamin D [supplementation] is yes and no in terms of whether it’s helpful,” says William Schaffner, M.D., an infectious disease specialist and professor at the Vanderbilt University School of Medicine. “It would appear the more severe your vitamin D deficiency is, the more likely your immune system is not functioning optimally.”

How much vitamin D do you need?

The recommended daily allowance (RDA) of vitamin D depends on your age and life stage. However, most adults should get 600 international units (IU) daily, according to the National Institutes of Health (NIH). (People who are 70 and older should have 800 IU of vitamin D.)

What are the signs I’m not getting enough vitamin D?

Vitamin D deficiency is more common in people who are lactose intolerant, follow a vegan diet, or those who have limited exposure to sunlight, per the NIH. Symptoms of low vitamin D can be vague or even hard to detect at all, but these are commonly linked to the status, according to Jessica Cording, R.D., C.D.N., author of The Little Book of Game-Changers:

• Fatigue
• Brain fog
• Low mood
• Getting sick a lot
• Bone fractures (usually only in severe cases)

How to get more vitamin D

Exposure to the sun is the most common way people get vitamin D, but it’s also possible to get the nutrient from foods. These are the biggest vitamin D sources, per the NIH:

• Cod liver oil
• Trout
• Salmon,
• Mushrooms
• Fortified milk
• Fortified cereal

But you can also get vitamin D from a supplement, Dr. Russo points out.

Should you take a vitamin D supplement for immune health?

It depends. It’s always important to check with a healthcare provider before taking any kind of supplement, Dr. Schaffner says. In the case of vitamin D, your doctor will likely recommend that you get a blood test to measure your vitamin D levels to see if you have a deficiency, Dr. Adalja says.

Why is this important? Vitamin D is a fat-soluble vitamin, which means that it can build up in your body if you take in too much. “Don’t start taking excessive amounts or you could get into trouble with high calcium levels in your blood,” Dr. Russo says.

But doctors also say that many people in temperate climates are likely to have lower-than-optimal vitamin D levels. If that describes you, “supplementation with a daily vitamin D pill is a generally good practice,” Dr. Adalja says. (Again, just check in with your healthcare provider first to be sure.)

In terms of a flu-prevention strategy, Dr. Russo says that things like getting vaccinated, avoiding people who are obviously sick, and wearing a mask in crowded indoor settings are still better. “A vitamin D supplement is one more piece that may help,” he says. “There’s little harm as long as you don’t overdo it.”

But Dr. Schaffner stops short of recommending that everyone take a vitamin D supplement for immune health. “If you have a special concern or interest in this, you can get a blood test,” he says. “Your doctor can tell you if you ought to take a supplement based on that.”