Low vitamin D tied to hospitalizations from respiratory tract infections

Respiratory tract infections remain a major cause of morbidity and healthcare utilization worldwide, particularly among older adults and those with underlying medical conditions.

A new study, which was published online late last year in the American Journal of Clinical Nutrition, showed that adults exhibiting very low vitamin D levels face a significantly higher risk of hospitalization for respiratory tract infections.

The study, led by Abigail R. Bournot, Ph.D., of the University of Surrey in the United Kingdom, provides more research feeding the active discussion about the role of vitamin D might play in boosting immune function, with knock-on effects for respiratory health.

Although vitamin D has been implicated in immune regulation and host defense, evidence linking vitamin D levels to clinically significant infection outcomes has been mixed.

To further explore the association, the study authors conducted an observational analysis using data from more than 36,000 adult participants in the UK Biobank with available baseline serum 25-hydroxyvitamin D (25[OH]D) measurements and linked hospital records. Serum 25(OH)D is the standard biomarker used to assess vitamin D status.

Participants were categorized according to vitamin D levels, with concentrations below 15 nanomoles per liter (nmol/L) defined as severe deficiency. The researchers then evaluated the risk of hospitalization for respiratory tract infections over the follow-up period, adjusting for potential confounders, including age, sex, ethnicity, body mass index, socioeconomic status and comorbidities.

The results show that those with low vitamin D levels had a significantly higher risk of hospitalization related to respiratory tract infection compared with those whose 25(OH)D levels were 75 nmol/L or higher. Specifically, levels below 15 nmol/L were associated with approximately a one-third increase in hospitalization risk.

The association appeared most pronounced at the lowest end of the vitamin D spectrum, with more modest differences in 25(OH)D linked to smaller increases in hospitalization risk, suggesting a potential threshold effect in which severe deficiency confers the greatest vulnerability.

In the paper, the authors noted that vitamin D plays a role in both innate and adaptive immune responses, including modulation of inflammatory pathways and enhancement of antimicrobial peptide production. These mechanisms provide biologic plausibility for the observed association between low vitamin D status and increased susceptibility to severe respiratory infections.

Bournot and her team also examined whether ethnicity modified the relationship between vitamin D levels and hospitalizations related to respiratory tract infections, given known disparities in vitamin D status across populations. Although baseline vitamin D concentrations varied among ethnic groups, interaction analyses did not demonstrate a statistically significant difference in the association between vitamin D deficiency and hospitalization risk.

Importantly, Bournot and her colleagues emphasized that the study’s observational design precludes conclusions about causality. Residual confounding remains possible despite statistical adjustments, and a single baseline measurement of 25(OH)D may not fully capture long-term vitamin D status. What’s more, seasonal variation in vitamin D levels was also not comprehensively accounted for.

Still, the large sample size and linkage to real-world hospitalization data are ingredients of a study with some probative value. By focusing on hospital admissions rather than self-reported infections, the study highlights potential implications for more severe respiratory outcomes.

Although some randomized trials and meta-analyses have suggested modest protective effects of supplementation, especially in people with low baseline levels, other studies have shown limited or no benefit.

Based on their findings, the authors concluded that further research, including well-designed randomized controlled trials, is needed to determine whether correcting severe vitamin D deficiency can reduce the risk of hospitalization for respiratory infections. Identifying subgroups most likely to benefit from supplementation will be an important focus of future investigation, they said.

For clinicians, the study underscores the potential importance of recognizing and addressing severe vitamin D deficiency, particularly in patients at higher baseline risk for respiratory complications. Although routine supplementation for infection prevention remains controversial, ensuring that people’s vitamin D levels are adequate, at the very least, may represent a modifiable factor in overall respiratory health.