Vitamin D and immune function

2020 has been The Year of the Three “I’s”

  1. Immune function
  2. Infection
  3. Inflammation

We know nutrition and diet are key players in immune function and inflammation. Unfortunately, we can’t predict how poor nutritional status affects immune function because we don’t have standard ways to measure nutritional status. This lack of information leaves the door open for some medical professionals to downplay the importance of nutrition. They’ll say things like “there’s no evidence that [this nutrient] helps immunity.” Well there’s no evidence it doesn’t. There’s no evidence, period, because no one has done a study.

Despite lack of evidence, some people tout one or another single nutrient as the cure for viral infections like Covid 19. Vitamin D is one of those nutrients. Vitamin D activates special receptors on immune cell membranes, which regulates immune function. Deficiency can interfere with immune response. The 3rd US National Health and Nutrition Examination Study (NHANES III) found that low blood levels of vitamin D were linked to self-reported incidence of upper respiratory infections. Vitamin D has been linked to lower susceptibility to flu, and studies suggest that deficiency is associated with worse inflammation. Indirect vitamin D therapy, by way of sun exposure, was historically a key part of tuberculosis treatment.

While vitamin D clearly plays a key role in immune function, it’s important to understand that it is not a magical cure, and not the Be-All and End-All of immune function. It’s one part of the puzzle. Taking big whopping doses of vitamin D as a short-term cure or preventative for a virus is not likely to be helpful.

Big whopping doses could also back fire. Vitamin D is fat soluble, so it’s stored in tissues. If you’re taking too much, all that stored vitamin D can play havoc with your liver and other organs. It’s a good idea to get a blood test before buying supplements, so you know if you need to supplement at all.

Special Considerations for Older Adults

  • While intense summer sunlight can activate vitamin D production in your skin, this process doesn’t work very well in older skin. You may spend lots of time outdoors, but that may not be doing anything for your vitamin D status. And if you wear sunscreen, all bets are off; sunscreen blocks the vitamin-D-producing rays. Finally, those intense rays of sunshine are not available in northern locations during the dark days of winter flu season, when you might need the immune-enhancing effect of vitamin D the most.
  • Because vitamin D is fat soluble, absorption depends on fat in your food. If you typically take vitamin D on an empty stomach or with a low fat meal, you won’t be absorbing as much (or any).
  • Many drugs taken by older people can interfere with vitamin D. The muscle pain and inflammation caused by statin drugs can be worse for people with low vitamin D. People on statins seem to need more vitamin D than non-statin-users in order to raise blood levels to adequate levels. Other heart medications can be impacted by vitamin D; your pharmacist should be able to advise you about drug interactions.
  • Obesity has a very negative impact on vitamin D blood levels. All those fat cells act like black holes, sucking fat soluble vitamin D into their high fat interiors. And once inside the fat cell, the vitamin D does not come out. So obese people have a harder time getting their blood levels up.

Smart Supplementing

  1. Get a vitamin D blood test so you know where you’re starting from. From what I’ve read over the past 15+ years, levels between 30 mg/ml and 50 mg/ml are adequate. Higher levels don’t provide any known benefit for normally healthy people.
  2. Take supplements with a meal that includes fat
  3. The better supplements are gel caps, not tablets, of vitamin D3. Vegans may prefer vitamin D2 (ergocalciferol), although it is less well absorbed.
  4. It can take 4-6 weeks for your blood level to respond fully to supplements.
  5. Doses are not one-size-fits-all. Your doctor might recommend some standard amount, but your system might not absorb it as well as another person. Again, medication use, your diet and obesity (or high amount of body fat) can impact absorption.


Vitamin D is one nutrient, out of several, that play critical roles in immune function. Loading up on D will not make up for deficits in the other nutrients, and will not magically cure any disease.