Every year, as the leaves change and temperatures drop, a familiar visitor returns: flu season. We shrug, we snuffle, we reach for tissues and hot tea—but what if part of the reason so many of us fall ill in the winter isn’t just the virus, but something missing from our diet and environment? A landmark review published in Epidemiology & Infection proposes a bold idea: vitamin D might help explain influenza’s seasonality and could be a key to stronger immune defenses. Those of you who frequent our Boston Chiropractor office, know that we are big advocates for getting vitamin d from supplements and the sun.
From Mystery of Seasonality to Sunshine Hormone
Influenza doesn’t strike evenly throughout the year. It peaks in winter, declines in summer, and reliably returns like clockwork. Early 20th‑century physician R. Edgar Hope‑Simpson coined the term “seasonal stimulus” to describe whatever environmental factor was driving this pattern. What could it be? Indoor crowding? Chilly air? Not entirely. The strongest contender may be something simpler and closer to home: sunlight and the vitamin D it helps our bodies produce.
Vitamin D isn’t just a nutrient for bones. Once activated in the body (as 1,25(OH)₂D), it behaves like a steroid hormone with powerful effects on the immune system. It modulates inflammation, enhances the activity of macrophages (immune cells that gobble up pathogens), and especially ramps up production of antimicrobial peptides—small proteins that patrol the respiratory tract and neutralize invading viruses.
Wintertime Vulnerability: More Than Just Cold Noses
The conventional wisdom has long been that people get sick in the winter because they’re indoors more and that’s where viruses spread. But there’s a problem with that explanation: people are clustered indoors year‑round—in offices, schools, public transit—yet influenza still shows striking seasonality. Vitamin D deficiency, however, does follow a seasonal pattern. During fall and winter, when UVB radiation from sunlight is weakest, our skin produces far less vitamin D. Consequently, our immune system may be operating in a more suppressed state just when influenza is circulating.
Historical evidence adds fuel to the theory. Before antibiotics and antivirals, cod liver oil—rich in vitamin D—was used as an “anti‑infective,” and studies in the early 20th century linked it to reductions in respiratory infections and workplace absenteeism.
How Vitamin D Might Help Fight Flu
Here’s how vitamin D works behind the scenes:
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Immune modulation: It helps prevent overactive inflammatory responses, which can damage tissues during infections.
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Enhanced pathogen defense: Activated vitamin D boosts antimicrobial peptides in immune cells and respiratory tract lining cells, acting like your body’s own antiviral defense force.
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Reduced winter infections: Observational studies show that giving vitamin D supplements or UVB exposure reduces the incidence of viral respiratory infections in children and adults.
This doesn’t just make intuitive sense—it fits with decades of observational and clinical data. Volunteers given live attenuated influenza virus in winter develop more symptoms than in summer, likely due to lower vitamin D status during colder months. Likewise, children with vitamin D deficiency are more prone to respiratory infections.
So Should You Take Vitamin D for Flu Prevention?
There have been many study’s that have made it clear that you should take vitamin D for both flu protection, immunity strength and cancer prevention.
However, they do emphasize that vitamin D deficiency is widespread—especially in higher latitudes during winter (like Boston) —and is linked not only to infections but also to a host of other chronic conditions. Adequately diagnosing and treating deficiency (aiming for blood levels similar to those seen in summertime sun exposure) may be a prudent public health strategy even independent of its effects on the flu. It is my experience that 10,000 IU per day is needed to raise your levels. During the summer or when you go to the Caribbean or Florida during the winter, I always use strong sunscreen on my face and lie out for 15 minutes on each side in my bathing suit to make sure I get enough vitamin D. Research has also noted that your magnesium levels need to be adequate for your body to produce and process vitamin D.
Final Thoughts: Sun, Supplements, and Seasonal Strategies
This visionary review gives us a fresh lens for viewing an age‑old problem. Instead of only thinking about how viruses spread, it invites us to consider why our defenses are particularly weak at certain times of year, and whether something as simple as vitamin D could help explain—and potentially mitigate—those patterns.
As with all good science, this isn’t the final chapter—but it’s a compelling one. Whether through sensible sun exposure, diet, or supplementation under medical guidance, supporting your vitamin D status may be a smart move for your immune system. And who knows? Next winter, your best flu strategy might not be just a scarf and tea—but also a nutrient your body makes when the sun shines. In our office, Charles Street Family Chiropractic, we have vitamin D blood tests. They cost $99 and can give you insight into how your vitamin D levels are faring. Call us today at 617-720-1992 to get an appointment to get one.