A large new study conducted by scientists from the University of Surrey in collaboration with colleagues from Oxford and Reading has found that very low levels of vitamin D in the blood are associated with a higher likelihood of hospitalization due to respiratory tract infections such as bronchitis and pneumonia.

The results of the study were published in The American Journal of Clinical Nutrition.

The researchers analyzed data from 36,258 adult participants from the UK Biobank database, comparing vitamin D levels with rates of hospitalization for respiratory diseases.

In the study, “severe vitamin D deficiency” was defined as a concentration below 15 nmol/L, while a “sufficient level” was defined as 75 nmol/L or higher. People with such profound deficiency had an approximately 33% higher risk of being hospitalized due to a respiratory infection compared with those who had higher vitamin D levels. In addition, the researchers found that each increase in vitamin D concentration of 10 nmol/L was associated with an approximately 4% reduction in the risk of hospitalization.

The authors emphasize that these findings are based on real-world healthcare data and reflect a broad association between vitamin D and respiratory health, particularly among middle-aged and older adults, who are at increased risk of severe infections. In people over the age of 50, lower respiratory tract infections such as bronchitis and pneumonia rank among the top ten leading causes of death worldwide.

Although the study does not provide definitive proof of a cause-and-effect relationship, it strengthens the hypothesis that adequate vitamin D levels may play an important role in supporting immune function and reducing the severity of respiratory diseases.

The researchers suggest that vitamin D supplementation, as well as increasing intake through diet and sun exposure—especially during the winter months, when vitamin D levels naturally decline—may help reduce the risk of severe infections and ease the burden on healthcare systems.