Health Why Vitamin D is Central to a Strong Immune System
Mar 23, 2022 3 min. read

You know that vitamin D is important. You’ve likely been hearing for decades this essential nutrient supports the growth of healthy bones and teeth. But how does it work?

Vitamin D stimulates the intestinal absorption of calcium and phosphorus. At certain levels, these two minerals promote normal bone growth. They also form the bony structure that makes up tooth enamel.

But vitamin D's importance goes way beyond structural. More recently, new research has connected optimal vitamin D levels with an overall improvement in immune function and reduction in inflammation.

What vitamin D is and how it works

Before we talk more about how much our immune systems love vitamin D, let’s understand what it is.

Vitamin D is a group of fat-soluble steroids that occur in humans in two forms: ergocalciferol, known as D2, and cholecalciferol, known as D3. In the body, vitamin D is synthesized mainly through the skin via a chemical reaction that depends on sun exposure. If there’s no sunlight (sorry, northern-hemisphere friends), vitamin D can also be obtained through food and supplementation.

But vitamin D in our bodies is considered biologically inactive, regardless of the form, until it goes through a chemical reaction that involves the liver and, subsequently, the kidneys to produce a biologically active form of vitamin D that can be used by the body.

And... voilà! Strong bones! Strong teeth!

The sunshine vitamin and the common cold

According to researchers, vitamin D may have a place as a go-to cold and flu remedy too. Protection against viruses has traditionally focused on supplementation of vitamin C and zinc. More recently, botanicals such as elderberry, turmeric, and ginger have gained popularity. Vitamin D may be next.

The evidence that vitamin D may support immune system regulation began in 1983, when researchers demonstrated that macrophages—a type of white blood cell that kills and removes harmful and dead cells—were involved in vitamin D production. Another research study validated this idea by demonstrating that vitamin D3 may help modulate the cells that regulate the immune system to produce an anti-inflammatory response.

A step further: gut health and inflammation 

Studies also show that vitamin D can impact the gut microbiota, increase the integrity of the gut lining, and further strengthen the connection between gut health and a robust immune system.

A 2020 randomized double-blind trial, which aimed to investigate the effects of various doses of oral vitamin D3 on gut microbiota, tested varying supplementation levels—600, 4,000, and 10,000 IU/day—of oral vitamin D3 for eight weeks. The findings demonstrated a dose-dependent increase in two specific gut bacteria, both of which help maintain a complex and beneficial gut microbiome. The study also showed that these bacteria maintained the expression of regulatory T cells, which help distinguish between invading cells from your own cells so your immune system doesn’t have to go to war against itself. End result: a reduction in system-wide inflammation.

An essential vitamin with a common deficiency

Though the research around immune health is vast and ever evolving, vitamin D levels are clearly critical to keeping your immune system in top fighting form. But get this: In 2015, vitamin D was declared one of the five nutrients of concern in the USDA’s Dietary Guidelines for Americans 2015–2020, along with calcium, potassium, and iron. According to the most recent set of data from the National Health and Nutrition Examination Survey, the prevalence of vitamin D deficiency among US adults was over 40 percent, with increased risk factors noted for females, African Americans, and those in the obese category with a BMI above 30.

Despite these high numbers, there are many ways to make sure you're hitting your
recommended dietary allowance (RDA) of 15-20 mcg per day. The sun, natural and fortified foods, and supplementation are all viable sources!


25 mcg vitamin D3 per serving

Charoenngam, N., Shirvani, A., Kalajian, T. A., Song, A., & Holick, M. F. (2020). The Effect of Various Doses of Oral Vitamin D3 Supplementation on Gut Microbiota in Healthy Adults: A Randomized, Double-blinded, Dose-response Study. Anticancer Research, 40(1), 551–556. Retrieved from

Forrest, K. Y. Z., & Stuhldreher, W. L. (2011). Prevalence and correlates of vitamin D deficiency in US adults. Nutrition Research, 31(1), 48–54. Retrieved from

Sheikh, V., Kasapoglu, P., Zamani, A., Basiri, Z., Tahamoli-Roudsari, A., & Alahgholi-Hajibehzad, M. (2018). Vitamin D3 inhibits the proliferation of T helper cells, downregulate CD4+ T cell cytokines and upregulate inhibitory markers. Human Immunology, 79(6), 439–445. Retrieved from

US Department of Health and Human Services & US Department of Agriculture. (2015). 2015–2020 Dietary Guidelines for Americans, 8th ed.
Retrieved from

Wexler, H. M. (2007). Bacteroides: The Good, the Bad, and the Nitty-Gritty. Clinical Microbiology Reviews, 20(4), 593–621. Retrieved from

author Medically reviewed by Sarah Muhammad, MS, RD
Medically reviewed by Sarah Muhammad, MS, RD

Sarah Muhammad is a Registered Dietitian and nutrition coach at Wellory. Follow Sarah on Instagram at @nutritionwithintentionrd, where she shares her expertise about diabetes management, gut health, and functional nutrition.