Improving your health is often related to your immune system. In cases with poor immune response, individuals may get sick more often. On the flip side, when the immune system is too aggressive or attacking the wrong targets, you can get experience inflammation, allergies, and autoimmune disease.

While inflammation is a healthy part of a normal immune response, it can become excessive and contribute to chronic disease. Some research even indicates that excessive inflammation is part of the aging process itself.

To maintain a proper balance of immune function the research suggests that several components can come into play. Examples include omega-3 fats for helping balance inflammatory responses, vitamin C as an antioxidant, and vitamin D.

‌‌‌‌What is Vitamin D?

Since its discovery, vitamin D has been recognized as an important factor in calcium metabolism and bone health. However, it wasn’t until the late 1990s that researchers started to uncover more of its effect on the immune system.

Vitamin D, often dubbed the sunshine vitamin, is actually misnamed. In the body, vitamin D is a prohormone. While we can get sparing amounts of vitamin D from food, most vitamin D is produced from sunlight and cholesterol in the skin. This creates the prohormone compound cholecalciferol, the same compound used in vitamin D capsules. Cholecalciferol is then converted into a hormone, referred to as active vitamin D, in the liver and kidneys.

‌‌‌‌Vitamin D and Immune Function

Ongoing research into vitamin D is finding effects beyond those initially related to bone health. Vitamin D is critical for white blood cell production of antimicrobial compounds called cathelicidins. These compounds, found inside white blood cells, directly kill bacteria, viruses, and fungi, helping clear infections.

On the flip side, vitamin D appears to have direct anti-inflammatory activity, potentially helping calm overactive immune responses. In the body, vitamin D acts to reduce “toll-like receptors” (TLRs). These receptors are a key part of the inflammatory process. By reducing TLRs, vitamin D reduces numerous inflammatory cell signaling molecules.

This reduction appears to have real-world effects as a clinical trial of vitamin D combined with antibiotic treatment for tuberculosis resulted in a rapid drop of inflammation as compared with controls. Inflammation in tuberculosis is correlated with mortality, and the study authors suggest vitamin D might improve outcomes. This mechanism could be critical in other infections since an overaggressive inflammatory response is often responsible for tissue damage, which in extreme cases, can lead to death.

‌‌‌‌Vitamin D and Respiratory Tract Infections

The latest data on vitamin D and respiratory tract infections provide some of the strongest evidence yet of potential benefits on immune function. A recent meta-analysis evaluating the research to date on vitamin D levels and acute upper respiratory infections (colds and flu) shows worse outcomes in individuals that are deficient in the vitamin. 

Another recent meta-analysis looked at vitamin D supplementation for limiting colds and flu. The authors concluded that vitamin D may reduce upper respiratory infections by 70% in those that were most deficient in the vitamin. People that weren’t as deficient still saw a reduction of 25%. In addition, the authors noted that the evidence was of “high quality.” Interestingly, these benefits were not seen if vitamin D was given in large infrequent doses as compared to daily or weekly dosing.

‌‌‌‌Can You Take Too Much Vitamin D?

When supplementing with vitamin D, it is important to keep in mind that the vitamin is fat-soluble and can accumulate in tissues. As such, it is possible to get too much. To know if a person needs vitamin D, testing is typically necessary.

While up to 2000 international units (IU) are generally considered safe for adults, I’ve frequently encountered patients that needed higher doses to reach adequate levels. For anyone wishing to supplement, I highly recommend testing, both for initial levels and to verify changes over time. Blood levels come up slowly with supplementation taking about four to six months to plateau. Based on the research, I don’t recommend taking higher doses initially to try to raise levels more quickly.


The latest research suggests that vitamin D may have a potential place in helping balance some components related to immune function. While more research is necessary, based on the data, individuals with immune-based conditions might benefit from testing their vitamin D levels and treating deficiencies appropriately. 


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