Respiratory tract infections (RTIs) are widely prevalent and cause significant morbidity and mortality globally. RTIs are broadly divided into upper RTIs, such as influenza, sinusitis, and tonsillitis, or lower RTIs, such as bronchitis and pneumonia.
Influenza, although it predominantly causes upper RTI, can also affect the lower respiratory tract. In the UK, GlobalData epidemiologists forecast that diagnosed incident cases of influenza-like illness will be 1.44 million cases, whereas 50,000
cases will be hospitalised in 2026. Vitamin D is considered to have properties that protect against RTIs. Although numerous studies have already been published reporting the health benefits of vitamin D, robust studies quantifying the association
between vitamin D levels and RTI hospitalisation were lacking until recently. In a study published in January 2026 in The American Journal of Clinical Nutrition, Bournot and colleagues measured the association between serum vitamin D level and hospitalisation for RTIs. The study outcome was that lower vitamin D status is associated with higher RTIs, and the relationship was significant.
To observe the effects of vitamin D on RTI-related hospitalisation, a case-control study was conducted using data from the UK Biobank, with 36,258 adult participants. Serum vitamin D status and hospital admission from linked records were recorded. The survival analyses and binary logistic regression models were used to determine the link between serum vitamin D level and RTIs. Serum vitamin D status of under 15nmol/L was associated with 33% higher hazard risk for RTI hospitalisation in the UK adults, compared with 75nmol/L or more of vitamin D. The risk of RTI hospitalisation was linear, as even 10nmol/L increase in serum vitamins resulted in 4% lower risk of RTI-related hospitalisation.
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This study by Bournot and colleagues provides evidence that lower vitamin D levels in the body increase the risk of serious RTI requiring hospitalisation. Vitamin D, also known as the sunshine vitamin (since vitamin D is produced naturally by the body using sunlight), is also available in oily fish, red meat, and egg yolks. Vitamin D is important for bone health, but it is also needed for building immunity. In the UK, seasonal influenza-related hospitalisation spikes in the winter. Seasonal influenza is a contagious respiratory illness that causes mild to severe illness. Serious illness from this infection can result in hospitalisation or death. Coincidentally, in the UK, lower vitamin D levels are quite common in people due to a lack of sunshine in the winter months. According to the UK Government estimate, one in six people in the UK have vitamin D level that is below the government-recommended guidelines. The government recommends a daily supplement containing 10mcg of vitamin D during the winter months when there is insufficient sunlight. By avoiding vitamin D deficiency in the population, the UK can minimise the risk of hospitalisation caused by RTIs.
