During the early days of the pandemic, vitamin D was everywhere in the news. People noticed that patients with low vitamin D seemed to do worse, and sales of vitamin D supplements jumped. Several years and many studies later, we have a much clearer picture. The short version: vitamin D matters for your immune system, being low is worth fixing, but taking extra when you already have enough does not seem to give you extra protection.
What vitamin D does in the immune system
Most people think of vitamin D as a bone vitamin, and it is. It helps your body absorb calcium. But immune cells also have receptors for vitamin D, and they use it to help control how they respond to germs. In lab studies, vitamin D helps cells make natural antimicrobial compounds and helps keep inflammation in check.
That is the biology. The more useful question is what happens when real people take vitamin D supplements.
What the studies in people found
Colds and other respiratory infections
A large analysis that pooled data from many trials found that vitamin D supplements slightly reduced the risk of respiratory infections such as colds. The benefit was clearest in people who started out with low vitamin D levels and who took a modest dose every day or every week. Large single doses taken occasionally did not seem to help.
COVID-19
Here the results were disappointing for people hoping for a simple fix. Several well-run trials gave vitamin D to people before or after infection and found no meaningful reduction in infection rates or severe illness in the general population. Low vitamin D may be a marker of poorer overall health rather than a direct cause of worse outcomes.
The pattern
Put together, the research suggests vitamin D works like many nutrients: if you are low, getting back to a normal level helps. If you are already in the normal range, more does not add extra benefit.
Are you likely to be low?
You are more likely to have low vitamin D if you:
- Live far from the equator, especially in winter.
- Spend most of the day indoors.
- Have darker skin, which makes less vitamin D from the same amount of sun.
- Are over 65.
- Are overweight, because vitamin D is stored in fat tissue.
- Have a condition that affects fat absorption, such as celiac disease or Crohn's disease.
- Cover most of your skin outdoors or always wear sunscreen, which is good for your skin but reduces vitamin D production.
A simple blood test called 25-hydroxyvitamin D tells you your level. It is worth asking your doctor about if you fall into one of these groups.
How much do you need?
The recommended daily amount for most adults is 600 IU (15 micrograms), rising to 800 IU (20 micrograms) after age 70. The safe upper limit for adults is 4,000 IU a day unless your doctor recommends more to correct a deficiency. We cover dosing in detail in how much vitamin D should you take?.
Food, sun or supplements?
Few foods contain much vitamin D. Fatty fish such as salmon and trout, egg yolks, and fortified milk, cereals and orange juice are the main sources. Sun exposure lets your skin make vitamin D, but how much depends on the season, time of day, where you live and your skin tone, and too much sun raises skin cancer risk.
That is why many people, particularly in winter, find a supplement the most reliable option. Vitamin D3 is the form most people choose, and it tends to raise blood levels a little more effectively than D2. See vitamin D2 vs D3 for the comparison.
Other habits that support your immune system
No single vitamin replaces the basics:
- Sleep seven to nine hours a night.
- Stay up to date on recommended vaccines.
- Stay active.
- Eat plenty of fruits, vegetables, beans and whole grains.
- Wash your hands, especially during cold and flu season.
The bottom line
Vitamin D supports normal immune function, and correcting a deficiency is worthwhile. Taking a moderate daily dose makes sense if you are low or at risk of being low. Megadoses do not provide extra protection and can cause problems. For a full overview, see our vitamin D guide and the NIH's vitamin D fact sheet.