The Infection Connection: Vitamin D, Cold Exposure, and the Immune Response.

Why our flu is seasonal and what we can do about it

Why do we get sick in winter? Because our immune systems are compromised by Vitamin D deficiency.

by Thomas P Seager, PhD and A.J. Kay

Everyone knows that winter is “flu season,” and most people think we are predisposed to sickness because the outdoor temperatures get cold.

But cold temperatures are not directly responsible for the annual swath of infections. In fact, the opposite is true.

Exposure to cold is a huge boost to the human immune system.

A study conducted jointly by the US and Canadian Armies illustrates this phenomenon by describing how deliberate cold exposure can double the number of natural killer (NK) cells in the bloodstream (Figure 1, Castellani et al, 2002). Natural Killer cells are a critical type of white blood cell that act as our bodies’ ‘first responders’ to all kinds of cell damage, including that from viruses, bacterial infections, and even cancer.

Figure 1, Castellani et al, 2002

Given the immune system’s ramped-up response to cold exposure, the question remains ‘What is the underlying mechanism for the “seasonal stimulus” (Cannel et al. 2006) on the rates of influenza infection?’

According to Dr. John Campbell, a physician in the United Kingdom with a popular video blog about coronavirus, the answer is ‘Vitamin D deficiency’.

The best available science agrees with him.

Current dietary recommendations for Vitamin D were established back in the 1960s at levels just above those required to prevent rickets (Papadimitriou, 2017). At that time, when combined with the then-typical lifestyle rife with sun exposure and a diet of whole, unprocessed foods, the recommendation of a lower supplemental dose was sufficient for most people. Today, our lives are different. We live primarily indoors, wear sunscreen when we do go outside, and our highly processed diets are insufficient to provide us with dietary levels of Vitamin D that we actually need — especially in the winter.

And we do need them. Increased levels of Vitamin D confer a myriad of benefits (Holick, 2007) including:

- reversal of metabolic syndrome

- increased bone density

- prevention of Type 1 diabetes

- protection against common cancers, and

- reduced risk of infectious disease

The evidence with regard to infectious disease is particularly strong — and timely (Adams & Hewison, 2008).

For example, Figure 2 below highlights the inverse relationship between influenza deaths and levels of sun (UVB) exposure in Norway (Juzeniene et al., 2010). The more sun people get (and more Vitamin D is synthesized by their bodies), the fewer people die of the flu.

Juzeniene et al., 2010

Even more compelling is the fact that populations in tropical latitudes show no such seasonality in flu infections — because they don’t experience the same variance in levels of sunshine.

Figure 3: Temperate latitudes show seasonal variation in viral infection rates that correspond to changes in solar irradiance. Tropical latitudes do not, because the solar variation is minimal (Cannel et al 2006).

At Northern latitudes, when the sun is at its lowest angles in the sky and the days are shortest, the UVB solar radiation that reaches the surface of the Earth is insufficient to initiate Vitamin D synthesis in the skin. Consequently, most people in the United States suffer from a Vitamin D deficiency in their bloodstream, especially during winter months.

To make matters (immunologically) worse, the colder temperatures that prevail in winter cause people to cover more of their skin and stay indoors more often, which deprives them of the immune-boosting exposure to the cold winter weather. Our modern environments have created a comfortable existence away from the prophylactic powers of the natural environment by encouraging indoor habitation during the winter months.

In addition, clustering people in close quarters with warm, moist air — the conditions we contrive in order to maximize comfort and minimize exposure to the cold of the winter — also maximize the survivability and transmissibility of the influenza virus.

It’s no wonder we get sick in the winter given that we

1. are deprived of the Vitamin D that our immune systems depend on to stay active, and
2. limit our exposure to the cold that the winter months confer, and then
3. we enclose ourselves indoors, giving pathogens access to countless vulnerable hosts.

Unfortunately, our susceptibility extends to all kinds of pathogens, not just the flu.

Deaths from the flu are not primarily attributable to the flu itself, but from secondary infections like bronchitis or pneumonia or from systemic complications due to underlying disease processes. One of the great clinical advantages of cold exposure is that the immune-boosting benefits extend to the opportunistic infections that can accompany influenza infection (Gruber-Bzura 2018).

Given the simplicity and affordability of Vitamin D supplements and deliberate cold exposure, there’s no reason why we should have to experience a “flu season” at all.

Getting enough Vitamin D in the winter months, as well as practicing intentional and repeated cold exposure, can give your immune system the boost it needs to minimize your susceptibility to bacterial and viral infections — as well as other types of cell damage — year-round.