When is Cold and Flu Season, What triggers the “Cold and Flu Season” and what can I do to protect myself from the Flu?

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This is a great question and one that functional medicine practitioners often highlight when discussing why flu season hits harder in the winter.

Cold and flu season typically occurs in the fall and winter, with flu activity peaking between December and February. While the viruses are present year-round, the highest activity is during these colder months. But there is one thing that is less in abundant that causes this.

In the winter sunlight intensity drops, especially in northern latitudes (above 37°N). Days are shorter, skies are cloudier, and people spend far more time indoors. Reduced sunlight exposure means reduced natural Vitamin D production. Vitamin D3 is primarily synthesized in the skin through UVB exposure from sunlight. Even if you go outside, the Sun’s angle is too low in the sky for UVB rays to effectively penetrate the atmosphere and trigger Vitamin D synthesis in the skin.

From a functional medicine lens, this means natural Vitamin D production is essentially shut off for several months—right when immune function needs to be strongest.

There is a clear seasonal connection as Vitamin D is essential for activating the immune response, especially the production of antimicrobial peptides in the respiratory tract. When levels drop in winter, the immune system’s ability to fight off respiratory viruses (like the flu) is weakened—making you more susceptible to getting sick and experiencing more severe symptoms.

Why is Vitamin D3 Crucial for immune modulation?

Vitamin D3 is considered crucial for immune modulation because it plays a multifaceted role in both activating and regulating the immune response which is essential for keeping the body balanced between fighting infections and avoiding overreactions (like autoimmunity or chronic inflammation). Vitamin D3 is one of the key switches that turns on or off various immune-related genes. It supports innate immunity (Your First Line of Defense) and enhances the pathogen-fighting effects of  the white blood cells that seek out and destroy invaders like viruses (including the flu).  Vitamin D3 stimulates the production of antimicrobial peptides such as cathelicidin and defensins, which act like natural antibiotics that make the epithelial barriers (like the lungs and gut lining) more resistant to infections.

Vitamin D3 Helps Regulate Your Long-Term Immune Memory.

Vitamin D tempers the response of the adaptive immune system which prevents overactivation of the immune system, which could lead to chronic inflammation or autoimmune reactions. It promotes the development of regulatory T cells (Tregs), which help the immune system distinguish between self and non-self, preventing immune overreactions. This balancing act is key to being strong enough to fight infection, but regulated enough to avoid damaging healthy tissue.

In infections like the flu, the body can release too many inflammatory molecules (cytokines), leading to what’s known as a cytokine storm—a harmful immune overreaction.  Vitamin D helps suppress this and supports a more controlled, anti-inflammatory immune response, which reduces symptom severity and potential complications.

Vitamin D3 Deficiency Is Linked to Higher Risk of Infection.

Multiple studies (which functional medicine often integrates with clinical experience) show that people with low Vitamin D levels are more susceptible to respiratory infections, including influenza and COVID-like viruses. Supplementation has been shown to reduce incidence, severity, and duration of these infections, especially in those who are deficient.

Focus on Optimization, Not Just Deficiency.

Conventional medicine may say you’re “fine” if your Vitamin D level is above 30 ng/mL, but functional medicine often recommends optimal levels between 50–80 ng/mL for full immune benefit. That means testing regularly, supplementing seasonally, and personalizing the dose based on genetics, body composition, sunlight exposure and  current immune status.

In Summary.

From a functional medicine view, Vitamin D3 is a regulatory powerhouse. It strengthens innate immunity, keeps adaptive immunity in check, prevents excessive inflammation, and helps maintain immune balance, all of which are critical in preventing and managing infections like the flu.

Get Vitamin D3 that supports healthy teeth, bones, and muscles, as well as cardiovascular and immune function directly:
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