Increases in serum vitamin D were associated with a modest but statistically significant reduction in the risk of hospitalization for respiratory tract infections.

By Ada Enesco

RT’s Three Key Takeaways:

Severe vitamin D deficiency: Adults with very low vitamin D levels (<15 nmol/L) had a 33% higher risk of hospitalization for respiratory tract infections compared with those with sufficient levels.

Threshold effect: Moderate vitamin D levels were not linked to increased risk, suggesting that vulnerability is concentrated mainly among people with extreme deficiency.

Consistent across ethnicities: The association between very low vitamin D status and RTI hospitalization was similar across ethnic groups, indicating a broadly shared risk.

Very low vitamin D levels are associated with a higher risk of being admitted to hospital for respiratory tract infections (RTIs), according to a large analysis of UK Biobank data published in The American Journal of Clinical Nutrition.

RTIs include infections of the airways and lungs, such as bronchitis and pneumonia, which can range from mild illness to severe disease requiring hospital care.

Researchers examined whether blood levels of 25-hydroxyvitamin D, the main marker of vitamin D status, were linked to serious RTIs in adults from different ethnic backgrounds across the UK. The study analyzed data from 36,258 UK Biobank participants who had baseline serum 25-hydroxyvitamin D measurements and long-term hospital follow-up. Over a median of nearly 15 years, 8.5% experienced at least one RTI requiring hospital admission.

Each 10 nmol/L increase in serum vitamin D was associated with a modest but statistically significant reduction in the risk of RTI hospitalization. Overall, higher vitamin D status was linked to a 4% lower hazard of being admitted for a respiratory infection.

Very Low Vitamin D Levels Carry Higher Risk

When participants were grouped by vitamin D category, the increased risk was concentrated among those with the lowest levels. Adults with serum vitamin D below 15 nmol/L had a 33% higher risk of RTI-related hospital admission compared with those whose levels were at least 75 nmol/L.

Intermediate vitamin D categories (from 15–74 nmol/L) were not associated with a statistically significant increase in risk, suggesting a potential threshold effect rather than a simple linear relationship.

Vitamin D deficiency is known to be more common in some ethnic minority groups, particularly in the UK. The study population was ethnically diverse, including White, Asian, Black, mixed and other ethnicities. However, when researchers tested whether the association between vitamin D status and RTI hospitalization differed by ethnicity, they found no significant interaction. This suggests that very low vitamin D levels may be linked to increased RTI risk across ethnic groups, rather than in one group alone.

The authors emphasize that this was an observational study and does not prove that low vitamin D causes respiratory infections. Other factors linked to poor health may also contribute.

Nonetheless, the findings highlight severe vitamin D deficiency as a potential marker of vulnerability to serious RTIs. Further studies are needed to confirm these results and to explore biological mechanisms, as well as whether correcting very low vitamin D levels could help reduce hospital admissions for respiratory infections.

Reference

Bournot AR et al. Association between serum 25-hydroxyvitamin D status and respiratory tract infections requiring hospital admission: unmatched case-control analysis of ethnic groups from the United Kingdom Biobank cohort. Am J Clin Nutr. 2025; DOI:10.1016/j.ajcnut.2025.101179.

This article was originally published by EMJ and was made available under the terms of the Creative Commons Attribution-Non Commercial 4.0 License.