How Vitamin D Helps Fight the Cold, Flu, and Other Viruses

Thanks to Aimee McNew at at PaleoHacks for this great article!

Move over, vitamin C—there’s a new flu buster in town 

It turns out that vitamin D, also known as the sunshine vitamin, is every bit as important as its antioxidant counterparts whenever the immune system is under duress, like during a cold or flu.  

If you’re one of those people who keeps getting sick and you don’t know why, read on. Here’s how this underestimated sunshine vitamin can help boost immunity and fight viral infections.  


The Basics of Vitamin D 

First, it’s important to understand that vitamin D isn’t a “vitamin” at all, but rather a prohormone that is synthesized in response to interaction between cholesterol, the skin, and sun exposure.  

Vitamin D is widely studied for all kinds of health benefits, like bone health, calcium absorption, heart health, and immunity. [1-10] While experts differ on how much vitamin D is needed to provide protection and avoid symptoms of insufficiency or deficiency, nearly everyone agrees that you can’t get enough vitamin D from food alone. [8] This might explain why even if you’re sipping on that gut-healing bone broth, it might not be enough to keep you fully protected from the flu.  


Why You Need Vitamin D to Boost Immunity 

When you take vitamin D, you lower your chances of getting sick from viral infections, like colds and the flu, by at least 10 percent. But people who are deficient in vitamin D actually see a greater protective benefit when they supplement, reducing their risk by 50 percent. [11]  

Most Americans get far less than the recommended daily allowance for vitamin D of 600 IU, setting the stage for rampant deficiencies. Certain people, like those with Crohn’s disease, celiac disease, osteoporosis, or women who are pregnant or breastfeeding, have an even higher risk of being deficient because of issues with absorption in the intestines and because of greater demands. 

Cold and flu viruses are contagious respiratory infections that affect the nose, throat, and lungs with symptoms like congestion, coughing, wheezing, sore throat, body aches, and fever. The flu, in particular, can have serious consequences in people who are at risk: the elderly, infants, young children, people with suppressed immunity, and those with other chronic disorders. On any given year, approximately 10 percent of the population gets the flu, but during epidemic flu seasons, it can be as high as 20 percent. [12, 13] There are several overlapping factors that are associated to both the severity of the flu and being at risk of vitamin D deficiency, including: 

  • Age (under age 2 or over age 65) 
  • Pregnancy 
  • Chronic disease (diabetes, heart disease, autoimmunity, etc.)
  • Suppressed immunity (HIV, AIDS) 


Vitamin D Reduces Risk of the Flu 

While flu vaccines are common, they only address a few strains based on the best guesses by experts of which strains will predominate. Sometimes experts guess wrong, rendering the flu vaccine less effective during those seasons. What doesn’t change, however, is the immune system’s reliance on vitamin D levels, making it a more secure way to boost immunity and avoid the flu. Research even shows that optimal vitamin D levels boost the effectiveness of the flu vaccine. [14] 

One study even drew a direct link between vitamin D levels and the risk of getting the flu: Those with the lowest serum concentrations had the highest chances of getting viral infections[15]  

Receptors for vitamin D are located on cell surfaces in the immune system, allowing vitamin D to bind to them. When this happens, vitamin D reduces inflammatory cytokines that perpetuate illness. It also boosts the proteins that fight bacteria and viruses; which are essentially the natural versions of antibiotics that protect the body from infection. 

Vitamin D boosts the innate branch of the immune system – the body’s first line of defense against infection – which mounts an attack against foreign invaders. The response time for this branch of the immune system is strongly associated with our ability to ward off illness or shorten the duration of sickness. [16] When vitamin D levels are low, immune cells respond slower and the innate immune response is not as rapid as it should be. 

Even if you do get sick, increase your vitamin D levels immediately for faster recovery. When your levels are too low, your body will struggle longer to bounce back. 

Research shows that for every 4 ng/mL increase in your vitamin D concentrations in the blood, you lower your odds of getting the flu by seven percent. It’s not coincidental that flu season peaks as our natural ability to produce vitamin D falls due to reduced sun exposure in the colder months. [17]  


How to Supplement with Vitamin D 

If you cannot get sun exposure regularly—as many Americans do not from September through March each year—then supplementation is essential to maintain steady levels. Vitamin D3 is considered to be the natural form and is most effective at raising serum concentrations[18]. Dosage recommendations range from 400 IU to upwards of 10,000 IU, depending on health conditions and other factors, like absorption. [19] 

The Vitamin D Council suggests that adults take between 5,000 and 10,000 IU per day, depending on body weight. Children should take 1,000 IU daily per 25 pounds of body weight (up to 125 pounds). The best bet when it comes to supplementing with vitamin D is to have your doctor test your levels and advise you on the proper dosage for your health factors. 

Vitamin D supplements absorb best when paired with fish oil or fatty foods to optimize absorption. [20]  

Since vitamin D is a fat-soluble nutrient – meaning your body has limited ability to excrete it – levels can become toxic in the body with over-supplementation, so get your blood levels checked before supplementing. A healthy blood range is above 30 ng/mL, with insufficiency falling between 20 and 29 ng/mL, and deficiency anything below 20 ng/mL. [21]  


Bottom Line 

Vitamin D is a critical nutrient for immune support. When the body is low in this nutrient, immune system cells will have slower response times to viral invaders, sickness duration will be longer, and overall immunity will be suppressed. 

Vitamin D is one of the few nutrients that doesn’t have an abundance of food sources. Wwere designed to produce most of the vitamin D we need in the skin from sun exposure. But between food and supplements, as well as sun exposure in the warmer months, we can optimize our vitamin D levels for overall health. 



1.Klibanski, A., et al., Osteoporosis prevention, diagnosis, and therapy. Jama-Journal of the American Medical Association, 2001. 285(6): p. 785-795. 

2.Holick, M.F., Vitamin D: importance in the prevention of cancers, type 1 diabetes, heart disease, and osteoporosis. American Journal of Clinical Nutrition, 2004. 79(3): p. 362-371. 

3.Dawson-Hughes, B., et al., Estimates of optimal vitamin D status. Osteoporosis International, 2005. 16(7): p. 713-716. 

4.Bischoff-Ferrari, H.A., et al., Estimation of optimal serum concentrations of 25-hydroxyvitamin D for multiple health outcomes. American Journal of Clinical Nutrition, 2006. 84(1): p. 18-28. 

5.Holick, M.F., High prevalence of vitamin D inadequacy and implications for health. Mayo Clinic Proceedings, 2006. 81(3): p. 353-373. 

6.Jackson, R.D., et al., Calcium plus vitamin D supplementation and the risk of fractures. New England Journal of Medicine, 2006. 354(7): p. 669-683. 

7.Holick, M.F., Vitamin D deficiency. New England Journal of Medicine, 2007. 357(3): p. 266-281. 

8.Holick, M.F. and T.C. Chen, Vitamin D deficiency: a worldwide problem with health consequences. American Journal of Clinical Nutrition, 2008. 87(4): p. 1080S-1086S. 

9.Ross, A.C., et al., The 2011 Report on Dietary Reference Intakes for Calcium and Vitamin D from the Institute of Medicine: What Clinicians Need to Know. Journal of Clinical Endocrinology & Metabolism, 2011. 96(1): p. 53-58. 

10.Gillespie, L.D., et al., Interventions for preventing falls in older people living in the community. Cochrane Database of Systematic Reviews, 2012(9): p. 408. 

11.Martineau, A.R., et al., Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data. BMJ, 2017. 356: p. i6583. 

12.Molinari, N.A., et al., The annual impact of seasonal influenza in the US: measuring disease burden and costs. Vaccine, 2007. 25(27): p. 5086-96. 

13.Shrestha, S.S., et al., Estimating the burden of 2009 pandemic influenza A (H1N1) in the United States (April 2009-April 2010). Clin Infect Dis, 2011. 52 Suppl 1: p. S75-82. 

14.Gruber-Bzura, B.M., Vitamin D and Influenza-Prevention or Therapy? International journal of molecular sciences, 2018. 19(8): p. 2419. 

15.Laaksi, I., et al., An association of serum vitamin D concentrations < 40 nmol/L with acute respiratory tract infection in young Finnish men. Am J Clin Nutr, 2007. 86(3): p. 714-7. 

16.Cannell, J.J., et al., On the epidemiology of influenza. Virol J, 2008. 5: p. 29. 

17.Berry, D.J., et al., Vitamin D status has a linear association with seasonal infections and lung function in British adults. Br J Nutr, 2011. 106(9): p. 1433-40. 

18.Nair, R. and A. Maseeh, Vitamin D: The “sunshine” vitamin. Journal of pharmacology & pharmacotherapeutics, 2012. 3(2): p. 118-126. 

19.Heaney, R.P., Vitamin D in health and disease. Clin J Am Soc Nephrol, 2008. 3(5): p. 1535-41. 

20.Korkor, A.B. and C. Bretzmann, Effect of fish oil on vitamin D absorption. Am J Kidney Dis, 2009. 53(2): p. 356. 

21.Holick, M.F., et al., Evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab, 2011. 96(7): p. 1911-30. 


About Aimee McNew

Aimee McNewAimee McNew with PaleoHacks is a Certified Nutritionist who specializes in women's health, thyroid problems, infertility, and digestive wellness. She ate her way back to health using a Paleo diet, lost 80 pounds, and had a healthy baby after numerous miscarriages. She focuses on simple nutrition practices that promote long-lasting results.

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“5” Comments

  1. 30 ng/ML seems way too low for optimum health. Based on the papers I’ve read, it seems like striving for 50-70 ng/ML would be best. I know that I tend to get sick more in the winter if my serum D level falls below 50 ng/ML. I have to supplement with 5000 IU daily to maintain a serum D level of 50 ng/ML during the winter months.

    • Hi Rob, thanks for the feedback! We tend to agree that closer to 50 ng/ML is better, but the current recommendations still say that above 30 ng/ML is the healthy range. It’ll be interesting to see if that changes over the next few years

  2. There was a paper out a few years ago that found that serum D in african hunter gatherers was of the order of 115 nmol/l (46 ng/ml). However, having posted this I must also mention that too much vitamin D can elevate calcium to high in the blood which can result in health problems – the serum levels of vitamin D required to do that are much, much higher (ie above 200nmol/l). A serum level of 115 nmol/l equates to something like 5,000 IU/day depending on body weight and so on. The 5,000 being from all sources ie ingested in food &/or supplements and synthesised after UV-B in sunlight sun strikes the skin. UV-B striking the skin results in a range of molecules ie various structural isomers of Vitamin D resulting. In other words, there may be added benefits from getting some of your daily dose of Vitamin D via sunlight.

    Martine F et al (2013) Vitamin D status indicators in indigenous populations in East Africa. Eur J Nutr 52(3): 1115–25

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