Study: Low Vitamin D Levels Associated with More Respiratory Tract Infections

A study by A.R. Bournot et al titled “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” published on 29 December 2025 shows that those with lower vitamin D levels have a higher risk of respiratory tract infections (RTIs).

This is a case-control study using UK data:

Of the 36,258 participants included in the analysis, 34% were White, 28% Asian, 19% Black, 11% other, and 7% of mixed ethnicity.

In short, those with <15 nmol/L have 33% higher risk of hospitalization due to RTIs. This is hardly a surprise as vitamin D aids the immune system, even if the mechanisms are unclear.

When stratifying for serum 25(OH)D, compared to those with ≥75 nmol/L (reference), those with <15 nmol/L had a higher HR for RTI hospitalization (HR: 1.33, 95% CI: 1.05, 1.67).

In the categories between 15 nmol/L and 74 nmol/L, there was no statistical significance.

As a sidebar, <50 nmol/L is typically considered low so <15 nmol/L is ridiculous anyway. One wonders what other health problems those individuals have.

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