Vitamin D may play an important role in fighting the influenza virus by helping control the inflammatory response and reducing the amount of virus in the lungs. Researchers at Pennsylvania State University reached this conclusion after studying the course of infection in mice.

The findings were published in the journal Frontiers in Immunology. The scientists examined the role of vitamin D receptors (VDR) and parathyroid hormone in fighting H1N1, a subtype of influenza A.

Mice that lacked vitamin D receptors and were therefore unable to use the vitamin normally developed more severe symptoms and cleared the virus more slowly. The mortality rate in this group was 21%. The animals also showed increased immune activity in the lungs, which may have contributed to excessive inflammation and tissue damage.

“Vitamin D appears to be important in helping the immune system return to normal after a flu infection. If the immune system continues working at full capacity, inflammation increases, potentially causing even more lung damage,” explained the study’s senior author, Margherita Cantorna, a professor of molecular immunology at Pennsylvania State University.

The researchers studied several groups of mice with disruptions in different mechanisms related to vitamin D. In particular, they monitored animals with elevated levels of parathyroid hormone—a substance that regulates calcium levels in the blood—as well as mice lacking vitamin D receptors exclusively in the immune system’s myeloid cells. These include macrophages and other cells involved in the early immune response to infection.

In all three models, the scientists found more severe symptoms, higher viral loads and greater inflammation than in the control group of healthy mice. This suggests that vitamin D receptors, parathyroid hormone levels and the presence of receptors in myeloid cells may independently influence the course of influenza.

According to the study’s lead author, Mengzhu Tang, a graduate student in the Department of Pathobiology at the University of Pennsylvania, the findings may have implications for people with vitamin D deficiency. When this vitamin is lacking, the body increases its production of parathyroid hormone in an attempt to maintain normal blood calcium levels. Correcting vitamin D deficiency helps regulate the immune response and normalize hormone levels.

However, the researchers emphasize that these are preliminary findings from animal studies. Further research is needed to determine whether similar mechanisms operate in humans.

The scientists also advise against increasing vitamin D doses on your own when you have the flu.