When we talk about vitamin D, we usually think “bones and teeth.” But vitamin D also plays a role in immune function — and low levels are common in the UK (especially in winter).

This study (published in The American Journal of Clinical Nutrition) looked at whether blood vitamin D levels were linked to respiratory tract infections (RTIs) serious enough to require hospital admission in the UK.

Study at a glance

What the researchers found

• Each 10 nmol/L higher 25(OH)D was associated with a 4% lower hazard of RTI hospitalisation (HR 0.96; 95% CI 0.94–0.99).
• Compared with ≥75 nmol/L, having <15 nmol/L was linked to a 33% higher hazard of RTI hospitalisation (HR 1.33; 95% CI 1.05–1.67).
• Intermediate categories (15–74 nmol/L) were not statistically significant versus ≥75 nmol/L.
• The authors tested whether the vitamin D–RTI relationship differed by ethnicity, and the interaction was not statistically significant.

Why this matters (and what it doesn’t prove)

The outcome here is important: hospital admissions (not self-reported “colds”). That makes the findings clinically meaningful — but it’s still an observational study, so it can’t prove that low vitamin D causes RTIs.

What it does suggest is a practical “watch-out”: severe deficiency may be a risk marker (and potentially a modifiable one). The authors also call for further studies to validate the findings and explore mechanisms across groups.

What can parents do in the UK?

While this specific study analysed UK adults, the practical UK takeaway remains relevant for families: vitamin D deficiency is common, and NHS guidance recommends a daily vitamin D supplement during autumn/winter for most people (and year-round for some groups).

Simple foundations that help in real life:

• Encourage outdoor play / daylight where possible
• Build vitamin D-supportive habits into your routine (especially October–March)
• If supplementing, follow NHS guidance for your child’s age group