Vitamin D supplementation as therapy for influenza and respiratory diseases

It is no coincidence that many people are affected by influenza and other respiratory diseases during the winter months, when vitamin D deficiency is more widespread. In a review article published in Nutrition Reviews, the authors describe how vitamin D plays a role in several parts of our highly developed immune system. In addition, vitamin D helps regulate inflammatory processes. This function is vital, as overactivation of the immune system in the form of hyperinflammation can make influenza and other respiratory infections complicated and, in the worst-case scenario, life-threatening. It is therefore important to ensure optimal vitamin D levels in the blood throughout the year.

Influenza is a global health problem that contributes to many days of illness, particularly during the winter months. Reduced resistance increases the risk of complications such as pneumonia, bronchitis, and sinus and middle ear infections. This occurs particularly because bacteria from the microflora of the mucous membranes (that are normally under control) are given an opportunity to spread.
In severe cases, influenza can develop into a life-threatening pulmonary complication called ARDS (acute respiratory distress syndrome), which is caused by the immune system overreacting with a cytokine storm and hyperinflammation in the lungs. This can trigger cascades of reactions in which immune cells attack healthy tissue, causing blood vessels to leak, blood to clot, and organs to fail. These are the same dysregulated immunological mechanisms that can make COVID-19 and sepsis life-threatening.
WHO (World Health Organization) estimates that between 290,000 and 650,000 people worldwide die from influenza each year, with older adults and people with chronic diseases being particularly vulnerable. This may also be because these vulnerable groups are more likely to have insufficient vitamin D levels in their blood.
The purpose of the review article was to examine the importance of vitamin D for the immune system and its therapeutic role in relation to influenza and other respiratory diseases. Using various databases (PubMed, Web of Science, Scopus, and Google Scholar) and relevant search terms, the researchers identified a number of controlled studies published within the past 12 years. These studies included both human and animal studies.
Most of the body's cells have receptors for vitamin D, which regulate a range of gene activities through on-off switches. All cells and tissues of the immune system also have vitamin D receptors, and the review article discusses in greater detail the importance of vitamin D for the following functions:

• The innate immune system
  Vitamin D boosts the production of antimicrobial peptides in the respiratory tract, which protect the lungs and mucous membranes against constant attacks from viruses, bacteria, and fungi. Vitamin D activates phagocytes (monocytes, macrophages, and granulocytes) when various pathogens need to be destroyed. The innate immune system fights most pathogens without causing symptoms. Macrophages also function as messengers, signaling an infection to the adaptive immune system when reinforcements are needed.
• The adaptive immune system
  This part of the immune system develops after birth. It consists of T cells and B cells. Activation of T cells provides the best protection and immunity against viral infections such as influenza. B cells are particularly important for the immune system's protection against bacteria and for the production of antibodies.
• Regulation of cytokines and inflammation
  White blood cells communicate by releasing various cytokines. Cytokines can also trigger fever. There are many different cytokines, some of which are pro-inflammatory and others anti-inflammatory. Cytokine production should be balanced so that we can effectively fight infections without developing chronic inflammation or a life-threatening cytokine storm and hyperinflammation.

In the review article, the researchers describe how vitamin D can boost antimicrobial peptides in the respiratory tract, strengthen the innate and adaptive immune systems, and, not least, regulate the production of pro-inflammatory and anti-inflammatory cytokines.
According to a number of randomized controlled trials (RCTs) and observational studies, there are also indications that optimizing blood vitamin D levels with vitamin D supplementation may help prevent influenza infection and various complications, particularly among vulnerable groups.

Vitamin D supplementation as a relevant therapy for influenza and many chronic diseases

Since exposure to the summer sun is the primary source of vitamin D, many people become deficient during the winter months. Lack of summer sun exposure, aging processes, dark skin, overweight, and chronic diseases contribute to widespread deficiency.
For optimal effects of vitamin D, blood levels of the vitamin must be higher than the official minimum value of 50 nmol/L. This is stated in an earlier review article published in Nutrients. Here, the researchers refer to a number of population studies in which higher concentrations of vitamin D in the blood reduce the risk of respiratory infections and a number of chronic diseases.
Blood vitamin D concentrations should therefore be above 75 nmol/L, which can generally be achieved by taking a daily vitamin D supplement of 50 micrograms. Vulnerable groups and individuals who, for genetic reasons, utilize the vitamin poorly may need more. EFSA (the European Food Safety Authority) has established an upper safe intake level of 100 micrograms per day.

References:

Achyut Pandey et al. Vitamin D and Influenza: Immunological and Therapeutic Potential for Respiratory Health. Nutrition Reviews 2026

William B Grant et al. Vitamin D: Evidence-Based Health Benefits and Recommendations for Population Guidelines. Nutrients 2025

Immunomodulatory Effects of Vitamin D in Respiratory Tract Infections and COVID-19 in Children. Nutrients 2023