People with very low vitamin D levels were more likely to be hospitalized for serious respiratory infections, according to a large new study that adds clarity to a topic often surrounded by mixed messages.
Researchers analyzed health data from more than 36,000 adults and found that those with severe vitamin D deficiency were 33% more likely to be admitted to the hospital for respiratory tract infections such as bronchitis or pneumonia compared with people who had adequate vitamin D levels. The study was led by scientists at University of Surrey and published in the American Journal of Clinical Nutrition.
The analysis drew on records from the UK Biobank, one of the world’s largest health databases, and focused primarily on middle-aged and older adults. These age groups already face higher risks from respiratory infections, which can become serious enough to require hospital care.
Vitamin D levels appeared to matter along a spectrum. For every 10 nmol/L increase in vitamin D, hospitalization rates for respiratory infections fell by about 4%. The strongest association was seen among people with very low levels, below 15 nmol/L, rather than among those with modest shortfalls.
“Vitamin D is vital to our physical wellbeing,” said lead author Abi Bournot. “Despite its importance to our overall health, many people are deficient and do not meet the government’s recommended intake.”
The researchers stressed that the study does not prove vitamin D prevents respiratory infections or that raising vitamin D levels will directly reduce hospital admissions. Because the research was observational, it can identify patterns but cannot establish cause and effect. Other factors linked to low vitamin D, such as overall health, mobility or chronic illness, may also influence hospitalization risk.
That distinction is important given how often vitamin D research is translated into blanket supplement advice. The findings point more clearly to the risks associated with severe deficiency than to benefits of pushing vitamin D levels beyond what is considered sufficient.
Respiratory tract infections remain a major public health concern, particularly for older adults. Pneumonia and bronchitis are among the leading causes of illness and death worldwide in people over 50. Vitamin D deficiency is also common, especially in regions with limited winter sunlight, making nutrient adequacy a recurring challenge.
Unlike many nutrients, vitamin D comes from a mix of sunlight exposure and food, including fortified products, making levels especially sensitive to season, geography and lifestyle.
The authors say the results support further research into whether improving vitamin D status through diet, fortified foods, sunlight exposure or supplementation could reduce hospitalization risk in the future. For now, the study adds evidence that avoiding clear deficiency may be one piece of maintaining immune health as people age, rather than a case for aggressive supplementation.
This work was supported by in-house funds from the University of Surrey for payment of UK Biobank access fees. UK Biobank was established by the Wellcome Trust, the Medical Research Council, the Department of Health, the Scottish Government and the Northwest Regional Development Agency, with additional funding from the Welsh Assembly Government and the British Heart Foundation. UK Biobank is hosted by the University of Manchester.