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. 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.
Vitamin D Status and Hospitalised Respiratory Infections
The study analysed 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 hospitalisation. 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.
Ethnicity, Vitamin D, and Respiratory Infection Risk
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 hospitalisation 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.
What This Means for Clinicians and Public Health
The authors emphasise 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.
