This year’s flu season hit harder and earlier than expected — and it’s not done yet. A new strain, known as subclade K, helped drive more than 19 million flu illnesses, 250,000 hospitalizations, and 10,000 deaths nationwide, according to federal surveillance data.
After three weeks of mild decline, flu activity remains higher than normal across much of the country. Physicians continue to recommend the basics — vaccination, handwashing, and avoiding close contact with those who are sick — but a new study points to another possible layer of protection: vitamin D.
Vitamin D, often called the body’s primary nutrient for sunlight-driven immune signaling, plays a central role in how the immune system responds to viruses. It supports the production of antimicrobial peptides, helps regulate inflammation, and influences the body’s defenses in the respiratory tract.
Researchers at the University of Surrey in the UK conducted one of the largest population analyses yet on vitamin D status and respiratory infections. The team reviewed data from 36,200 adults, tracking vitamin D levels and hospitalizations for illnesses such as influenza, bronchitis, and pneumonia.
Participants with severe vitamin D deficiency — blood levels below 15 nmol/L — were 33 percent more likely to require hospitalization for respiratory infections compared with individuals who maintained levels at or above 75 nmol/L. The researchers also found that hospitalization risk dropped by about 4 percent for every 10 nmol/L increase in vitamin D levels.
Vitamin D deficiency becomes more common with age. Older adults absorb less sunlight, produce less vitamin D in the skin, and often have conditions that affect nutrient metabolism, which may help explain their significantly higher rates of influenza complications each winter.
The recommended daily intake for most adults is 600 IU (15 mcg), rising to 800 IU (20 mcg) after age 70. Levels can be easily checked with a blood test, and supplements remain widely available for just a few cents per dose.
Food sources include fatty fish such as salmon, mackerel, and tuna; fish liver oils; egg yolks; fortified dairy; and fortified cereals. Sunlight remains the primary natural driver of vitamin D synthesis for most people, though seasonal changes make wintertime production far less reliable.
Researchers emphasize that supplements do not replace flu vaccination, but the findings suggest vitamin D sufficiency may play a meaningful supporting role in reducing severe respiratory illness — especially in older adults or people with known deficiency.
The study appears in the American Journal of Clinical Nutrition.
Medical Disclaimer
This article is for informational and educational purposes only and does not constitute medical advice. Always consult a qualified healthcare professional with questions about health, nutrition, or medical conditions.
