I once heard a doctor explain the reasons why so many of us are low in vitamin D, and all the ways that being deficient compromises our health, then conclude with the totally illogical, “But I don’t recommend everyone get tested.”

Huh? Why not? There is no downside to testing for this ultra-important vitamin (other than about $65 and a prick in the arm), but considerable upside to ensuring your levels stay in optimal range.

Besides balancing calcium and keeping bones strong, vitamin D is vital to a robust immune system — it boosts our ability to fight bacteria and viruses and lowers our risk of respiratory infection. So, does that mean it helps fight Covid-19? 


Early in the pandemic, researchers weren’t sure; there was too little known about vitamin D’s effect on the ACE2 receptor — the door that SARS-CoV-2 uses to gain access to our cells.

But now that more than 100 studies have been published on the link between vitamin D and Covid, a clearer picture has begun to emerge: People deficient in vitamin D appear to have two to five times the risk of getting infected with Covid, having a severe case, and dying. Pediatric studies show that kids deficient in D are five times more likely to have a severe case of Covid.

If you think that because you spend lots of time outdoors (like my husband and me) or are a ridiculously strong athlete (husband), your level of D must be perfectly fine, thank you very much.

We were both wrong — especially me.

My first test showed I was sorely deficient at 12 ng/ml. (See chart for levels.) It took me two years of supplementing with D3, the common oral form, starting with 1000 IU/day, then bumping to 3000, then 5000 IU/day, before I was safely in optimal range. Most people don’t need this much, but periodic testing shows I do.

TIME FOR A BOOST: Just because you spend lots of time outdoors or have a well-rounded diet doesn’t mean that your body has sufficient levels of vitamin D, as 40% of Americans are deficient.

I share with you this cautionary tale for a couple of reasons: 1) It’s impossible to know your vitamin D level without getting tested, and 2) If you’re low, it may take time and experimentation before you’re at adequate levels. And don’t wait until you have Covid to start popping supplements; the liver needs five or more days to convert vitamin D3 — whether from supplements, food, or the sun — to the circulating form used by the body. If you want the vitamin to be one of the many things that might help you fight the virus (on top of — obviously — lifestyle, a healthy diet, and getting vaccinated) then the vitamin should already be in your system at optimal levels. 

So why are so many of us low in vitamin D? For starters, we’re staying out of the sun to protect ourselves from skin cancer. Second, it’s hard to get enough D from food alone. Third, as we age, we make less D from the sun. Lastly, a great many people live north of the 35-degree latitude line running from L.A. to Atlanta and can’t make vitamin D from the sun in the winter. Collectively, these factors explain why 40% of Americans are deficient in D. 

To be clear, vitamin D doesn’t prevent Covid, nor does having adequate D guarantee you won’t become infected or even die from the virus. And other hormones and vitamins may be involved in ways we don’t yet understand. Yet the bulk of the published research indicates that D vitamin appears beneficial. 

In a study from Spain, Covid patients were given large doses of vitamin D in a form called 25(OH)D, starting on day one of hospitalization. Fifty percent of the control group required ICU admission, but only 2% of the vitamin D group did. 

A study of almost 200,000 Americans found that those with the highest vitamin D levels in the year prior to the pandemic had a 6% risk of Covid infection, while those with the lowest levels had more than double that risk. When researchers controlled for age, sex, race, and latitude, a person’s vitamin D level explained 96% of the risk of getting infected. In this study, the greatest protection was seen at 50 to 60 ng/ml, which is higher than the average person’s level. But large population studies show that having a level of 30 ng/ml, which is the low end of “sufficient,” keeps most people out of the danger zone of serious illness or dying.

To learn your vitamin D status, ask your doctor for a test or use a walk-in center like Cash Clinical in Reno. If your level is below sufficient, ask your doctor about supplementing.  

What do the Numbers Mean?*

-20ng/ml Deficient

21-29 ng/ml Insufficient

30-100 ng/ml Sufficient  (40-60 ng/ml is considered optimal)

100+ ng/ml Toxic

*Vitamin D tests measure your blood level of 25(OH)D in nanograms/ml


  • Linda Lindsay

    Linda Lindsay has been writing health articles for Moonshine Ink since 2003. She has a degree in natural resources from Colorado State University, and has worked for the Yosemite Institute, Outward Bound, the Park Service, and Forest Service. She came to Tahoe in 1984 to check it out for a winter and never left. She lives in the Prosser area with her husband, daughter, two dogs, and a cat.

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