Vitamin D is well known for its role in bone and muscle health. Vitamin D is also thought to help the immune system fight viruses and bacteria that might cause illness.
To test this, researchers examined the connection between vitamin D levels and respiratory infections such as colds, flu, pneumonia, and bronchitis. The study, published in February 2026 in the American Journal of Clinical Nutrition, analyzed health data from more than 36,000 adults ages 40 to 69 who were participants in a large, long-term health study. Among other health data, vitamin D levels were recorded at the start of that study. For the current analysis, the researchers placed participants into five groups based on their vitamin D level, from a low of less than 6 nanograms per milliliter (ng/ml) to at least 30 ng/ml, the lower level defined as optimal by the Endocrine Society. Over 15 or more years, people with the lowest vitamin D levels were 33% more likely to be admitted to the hospital for respiratory infections than those with the highest levels. For every 4 ng/ml increase in vitamin D, there were 4% fewer hospitalizations.
Good ways to maintain normal vitamin D levels include getting sun exposure, eating foods rich in vitamin D, and taking a daily multivitamin or 1,000 IU vitamin D supplement.
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