Immune Boosters – Vitamin D

Vitamin D is another nutrient that the body needs for proper immune system function. Just like vitamin C, vitamin D plays a role in both the body’s innate and adaptive immune responses, but it’s also needed for bone health (which I won’t talk about in this post).


A few things to know about vitamin D in general before I dive into its effects on immunity. Vitamin D is a fat-soluble vitamin, and it is unique in that your body can make it. This is nice because natural food sources of Vitamin D are not that easy to find. Ultraviolet light triggers the production of vitamin D from a molecule derived from cholesterol, which then has to be converted by your liver to 25 hydroxy vitamin D, the storage form of this vitamin, and then activated by your kidneys to the active form. Similarly, Vitamin D from food has to be activated by these two organs also.

The recommended dietary allowance (United States) (8)

Daily need

IU = international unit

0-12 months 400 IU
1-69 years 600 IU
>70 years 800 IU


Eggs contain a small amount of vitamin D

Like I said, good sources of vitamin D are scarce. These include salmon, cod liver oil and trout. Tuna fish, sardines, eggs and mushrooms (if exposed to UV light) also contain vitamin D, but in smaller amounts. Many products are being fortified with vitamin D, such as milk and some cereals.

Three to four cups of dairy, or 1 tablespoon of cod liver oil will provide 100% of the daily value. When it comes to eggs, you would have to consume ten large egg yolks to get enough of vitamin D for one day. Breast milk has no vitamin D, so if you have an infant who is solely being breastfed, do provide a supplement (and talk to your pediatrician). 

An easy way to fill up your vitamin D tank is to spend 5-30 minutes in the sun at least twice a week. Cloud coverage, being in the shade, darker skin tone and using sun screen decrease the amount of UVB radiation available to the skin by as much as 100% (sun screen), which makes it difficult to determine precise estimates and generalizations for how much time in the sun you need. Unfortunately, windows block UVB radiation, so sun bathing indoors does not help you to produce vitamin D. (8) Also, the time of the year matters a lot – even in San Francisco, your skin won’t produce vitamin D during the winter months no matter how much sun you get. Needless to say, being outdoors in the sun is getting even harder now with the “Shelter in Place” orders.

Some cereals are being fortified with Vitamin D, for example cream of wheat

Factors increasing deficiency risk (2,8):

  • Corticosteroid medications (like prednisone)
  • Obesity
  • Advanced age (reduces skin’s ability to produce vitamin D)
  • Fat malabsorption (such as inflammatory bowel disease, liver disease)
  • Gastric bypass surgery

Vitamin D toxicity is rare and is only caused by excessive supplementation for an extended period of time. In other words, eating vitamin D rich foods and sun can’t cause an overdose. However, keep in mind that sunburn is not desirable!

Vitamin D as an immune booster

When it comes to immunity, Vitamin D has anti-inflammatory effects: it inhibits inflammatory signaling molecules (cytokines) and boosts anti-inflammatory ones (1). 25 hydroxy vitamin D (the storage form) supports the production of antimicrobial peptides in response to both viruses and bacteria. Vitamin D metabolites (produced when vitamin D is broken down) have also been reported to support other antimicrobial mechanisms, such as stimulating autophagy (the removal of damaged cells and cell components) (3). A study found that cathelicidins and defensins (antimicrobial peptides) can lower viral reproduction and mitigate inflammation that injures the lungs and causes pneumonia (5). A very recent study done in rats reported also that vitamin D reduces lung injury. The results suggest that vitamin D has potential for the resolution of acute respiratory distress syndrome (ARDS), which is a very serious situation requiring mechanical ventilation. (6) 

Blood levels

The laboratory value that is used to monitor vitamin D level in the blood is 25 hydroxy vitamin D (8).

nmol/L ng/mL *Health status
<30 <12 Associated with vitamin D deficiency, leading to rickets in infants and children and osteomalacia in adults
30 to 50 12 to 20 Generally considered inadequate for bone and overall health in healthy individuals
≥50 ≥20 Generally considered adequate for bone and overall health in healthy individuals
>125 >50 Emerging evidence links potential adverse effects to such high levels, particularly >150 nmol/L (>60 ng/mL)
100-150   Interestingly, a recent article recommends that people at risk for influenza and COVID-19 should take 10,000 IU of cholecalciferol daily, for some weeks, followed by 5000 IU daily, to reach these levels. However, the researchers are calling for studies to evaluate these recommendations (5).

Of note, there has been much debate among experts over what sufficient levels are (9). Perhaps a sufficient level is different for different people, and in different circumstances. (8) Above are the official numbers at this time.

A systematic review and meta-analysis noted that for those whose vitamin D level was less than 25 nmol/L (10 ng/mL, considered deficient), taking a vitamin D supplement had a protective effect against respiratory infections. The same article reported that large one time doses didn’t seem effective, but taking a supplement daily or weekly was helpful in improving blood levels. One study also saw a reduction in the incidence in influenza in those who took vitamin D supplements. (3)

On the other hand, in children in rural India, vitamin D supplementation (1000 IU daily) didn’t show any beneficial effects – however, the children’s average 25 hydroxy vitamin D level to begin with was good, 59.7 nmol/L (~24 ng/mL) (4), considered sufficient. Additionally, a recent article summarizing studies looking at the effect of vitamin D level in the body on viral respiratory infections found conflicting results. The author concluded that higher blood levels didn’t necessarily associate with lower incidence of infections. (7)

To supplement or not?

Day at the beach – vitamin D production in action!

I do think that just like with vitamin C, it’s important to make sure you are getting enough vitamin D to support appropriate blood levels. In my experience, most of my patients and people I’ve talked to (including dietitians!) have lowish blood levels, so doing a better job with getting enough sunshine, eating more of vitamin D rich foods, or taking a supplement would be in order.

If it’s not possible for you to be outdoors in the sun, or you don’t eat fish, or drink fortified cow’s, or plant milks, supplementation can help. How much should you take? It seems to me that about 1000 IU of vitamin D3 (cholecalciferol) per day might be appropriate as a maintenance dose, and more if you have a deficiency. If you are concerned about your levels, it wouldn’t hurt to check with your doctor.


Thank you for reading!





  1. Vitamin D and the Immune System 2011.
  2. Hughes, Norton. Vitamin D and Respiratory Health 2009.
  3. Martineau et al. Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data. 2017.
  4. Mandlik et al. Occurrence of infections in schoolchildren subsequent to supplementation with vitamin D-calcium or zinc: a randomized, double-blind, placebo-controlled trial. 2020.
  5. Grant et al. Evidence that Vitamin D Supplementation Could Reduce Risk of Influenza and COVID-19 Infections and Deaths 2020.
  6. Zheng et al. Vitamin D attenuates lung injury via stimulating epithelial repair, reducing epithelial cell apoptosis and inhibits TGF-β induced epithelial to mesenchymal transition.
  7. Lee. Controversial Effects of Vitamin D and Related Genes on Viral Infections, Pathogenesis, and Treatment Outcomes. 2020.
  8. Vitamin D. NIH.
  9. Tello, MD. Vitamin D: What’s the “right” level? 2016.

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