Three hundred days of sunshine is the state slogan, and yet vitamin D deficiency is one of the most consistent findings on our panels. Sun exposure in winter at this latitude produces very little vitamin D, and most people spend the short daylight hours indoors anyway.
Why vitamin D matters for immunity
Vitamin D functions more like a hormone than a vitamin. It regulates immune cell behavior, helps control inflammation, and supports the barrier tissue in your airways and gut that keeps pathogens out. Low levels correlate with more frequent and longer respiratory illness.
What we test when someone is sick constantly
- 25-OH vitamin D with a target range well above the bare minimum used to prevent bone disease.
- Zinc and copper ratio - zinc is essential for immune cell function and is easily depleted.
- Ferritin and full iron panel - both deficiency and overload impair immunity.
- CBC with differential - lymphocyte and neutrophil patterns tell a story most people never have explained to them.
- hs-CRP - chronic background inflammation consumes immune capacity.
- Thyroid panel - low thyroid function slows immune response and recovery.
- Gut markers including secretory IgA - the majority of your immune system lives in the gut.
The gut connection people miss
If secretory IgA is depleted, your first line of defense at every mucosal surface is weakened. That is why patients who resolve gut overgrowth and food allergies often report their first winter in years without three rounds of illness.
A practical winter plan
- Test vitamin D and dose to your actual level rather than a guess, then retest in 90 days.
- Protect sleep. Immune function drops measurably below seven hours.
- Keep protein and colorful produce high through the darker months.
- Get outdoor daylight early in the day for circadian and mood support.
- Address the gut, because immune resilience is built there.
Getting sick five times a winter is common. It is not normal, and it is usually explainable.