Vitamin D was already a popular supplement. But after a widely publicized study linked low levels to COVID-19 morbidity and the lauded Dr. Fauci stressed the nutrient's importance, consumer interest has only intensified. Our secret shopper hit up a natural foods store in the Upper Midwest to ask key questions about taking vitamin D.

NFM: With all the talk about vitamin D, how do I know how much to take or if I just need to spend more time outside?

Retailer: Vitamin D is super important, especially for folks who live in the northern U.S., because our winters are long and we don't get enough sunlight even in the summertime. Most of us, unless we take supplements, are probably low on vitamin D.

NFM: OK, but what is "low?" I hear a wide range of recommended amounts. How much vitamin D do I really need to take to stay healthy?

Retailer: You're right about the wide range. The Institute of Medicine recommends 600 IU daily for most people, but lots of doctors and nutritionists say we actually need a lot more, like at least 2,000 IU, to get the full benefits. That's easy to do with vitamin D supplements.

How did this retailer do?

The retailer is right that vitamin D is very important—it does all kinds of things, such as boost the immune system. But most of what we know about vitamin D comes from what happens when blood levels are low: We see more cancers, dementia, cardiovascular issues and respiratory diseases. Published research has shown that of people hospitalized for pneumonia, a very high percentage have low vitamin D levels. And then last summer, research found that 87% of people who died of COVID-19 in the hospital had low vitamin D levels.

In these studies, "low" is defined as less than 30 ng/mL, which is not what I consider adequate. The Institute of Medicine's number is based solely on research related to bone health, just one of myriad reasons why vitamin D is important. If your levels are less than 50 ng/mL, you still have an increased risk for neurological issues; if they're less than 60 ng/mL, you still have an increased risk of autoimmune diseases. That's why I say blood levels should be 60 to 80 ng/mL, and most functional medicine doctors agree on that.

As for the sun as a source, the retailer is correct that in the northern latitudes, anywhere north of Atlanta, UVB radiation is not strong enough year-round to prompt the body to make vitamin D. But the reality is there is also vitamin D deficiency in Brazil and India, so vitamin D status can't be solely related to where we live.

Basically, we all tend to need more vitamin D, and supplementation is the easiest way to get it. Testing is required to know exactly how much one should take for optimized blood levels, but for most people, 1,000 IU daily should be the entry point. Some experts in Europe say to start at 2,000 IU.