Severe vitamin D deficiency linked to 33% higher hospitalization risk for respiratory infections

Researchers at the University of Surrey have documented a stark connection between vitamin D deficiency and the risk of hospitalization for respiratory illness. The study, published in The American Journal of Clinical Nutrition, examined health records from 36,258 adults aged 40 to 69 drawn from the UK Biobank—a database that includes both vitamin D measurements and hospital admission records. What they found was straightforward and concerning: people whose vitamin D levels fell below 15 nmol/L, classified as severely deficient, were 33% more likely to require hospitalization for respiratory infections like pneumonia and bronchitis than those with adequate levels of at least 75 nmol/L.

The relationship between vitamin D and infection risk followed a clear dose-response pattern. For every 10-unit increase in vitamin D concentration, hospitalization risk for respiratory infections dropped by approximately 4%. This wasn't a threshold effect where deficiency suddenly mattered—it was a gradient, suggesting that more vitamin D meant better protection across the board. Among the 36,258 participants studied, just over half were women, nearly three-quarters were between 40 and 60 years old, and about 30% had a body mass index of 30 or higher. The prevalence of severe deficiency was notable: roughly 11% of the group had vitamin D levels below that critical 15 nmol/L threshold, while only 5.4% had levels at or above 75 nmol/L.

The researchers found that vitamin D levels varied by ethnicity, with white participants showing the highest median levels and Asian participants the lowest. Yet despite these differences in baseline vitamin D status, the protective effect of adequate vitamin D held steady across all ethnic groups. The association between vitamin D and respiratory infection risk did not differ meaningfully by ethnicity, suggesting that the biological mechanism operates similarly regardless of background. Other factors that increased hospitalization risk included obesity, male sex, age over 60, lower income, and the use of statin medications—a reminder that vitamin D deficiency exists within a broader landscape of health vulnerabilities.

Abigail Bournot, the lead researcher, emphasized that vitamin D's antibacterial and antiviral properties may be the mechanism at work. "Vitamin D is vital to our physical well-being," she said. "Its antibacterial and antiviral properties may help reduce the risk of respiratory tract infections that can lead to hospitalization. Supplementation, especially during the winter months when sunlight exposure is limited, is an effective way to maintain adequate levels." Her colleague Andrea Darling, a postdoctoral researcher and senior author, framed the findings in terms of public health burden: respiratory infections not only sicken individuals but strain healthcare systems. The NHS, already under pressure, could see admissions reduced if vitamin D intake increased across the population.

The researchers acknowledged limitations in their work. They relied on a single baseline vitamin D measurement, meaning they could not track how levels changed over time or account for seasonal variation. The study population, drawn from the UK Biobank, may not represent younger adults or populations outside the UK. The U.S. National Academies' Food and Nutrition Board considers vitamin D levels of at least 50 nmol/L sufficient for most people, with deficiency risk rising below 30 nmol/L. Yet many people fail to meet the UK government's recommended daily intake of 10 micrograms, a shortfall that may compound their vulnerability to severe respiratory infections.

Vitamin D can be obtained through several routes: sunlight exposure, which is the most effective and safest method; fatty fish such as salmon and mackerel; egg yolks; and mushrooms exposed to sunlight. Supplements and fortified foods offer another pathway, particularly valuable during winter months when daylight is scarce. The researchers suggest that increasing vitamin D intake through these means could reduce hospital admissions for respiratory infections and ease pressure on healthcare services. For middle-aged and older adults, who face particular vulnerability to pneumonia and bronchitis—conditions that rank among the leading causes of death globally for those over 50—the implications are significant. A relatively simple intervention, sustained over time, might prevent serious illness and hospitalization.