As temperatures drop, rates of respiratory infections — the common cold and influenza, primarily — increase exponentially. Many believe this has to do with the drop in temperature, but cold exposure actually ramps up your immune system, making you less prone to infection. 

According to a 2002 study^1,2 by the U.S. and Canadian armies, cold exposure can double the number of natural killer (NK) cells in your body, which are part of your first line of defense against pathogenic infiltration and other types of cell damage.

As detailed by retired nurse and academic teacher John Campbell in the video above, a scientific review³ published in 2006 concluded that epidemic seasonal influenza is most likely related to the prevalence of vitamin D deficiency during winter months. According to the authors:⁴

“In 1981, R. Edgar Hope-Simpson proposed that a ‘seasonal stimulus’ intimately associated with solar radiation explained the remarkable seasonality of epidemic influenza. 

Solar radiation triggers robust seasonal vitamin D production in the skin; vitamin D deficiency is common in the winter, and activated vitamin D, 1,25(OH)2D, a steroid hormone, has profound effects on human immunity. 

1,25(OH)2D acts as an immune system modulator, preventing excessiveexpression of inflammatory cytokines and increasing the ‘oxidative burst’ potential of macrophages. 

Perhaps most importantly, it dramatically stimulates the expression of potent anti-microbial peptides, which exist in neutrophils, monocytes, natural killer cells, and in epithelial cells lining the respiratory tract where they play a major role in protecting the lung from infection.”

Inverse Relationship Between Flu Deaths and UVB Exposure

While vitamin D has been linked to many health benefits, the relationship between vitamin D and infectious disease is particularly robust. For example, a 2010 study⁶ by Norwegian researchers found there’s an inverse relationship between UVB sun exposure — which is how your body synthesizes vitamin D naturally — and influenza deaths. According to the authors:⁷

“Non-pandemic influenzas mostly occur in the winter season in temperate regions. UVB calculations show that at high latitudes very little, if any, vitamin D is produced in the skin during the winter. 

Even at 26°N (Okinawa) there is about four times more UVB during the summer than during the winter. In tropical regions there are two minor peaks in vitamin D photosynthesis, and practically no seasonality of influenza. 

Pandemics may start with a wave in an arbitrary season, while secondary waves often occur the following winter. Thus, it appears that a low vitamin D status may play a significant role in most influenzas The data support the hypothesis that high fluences of UVB radiation (vitamin D level), as occur in the summer, act in a protective manner with respect to influenza.”

Vitamin D Protects Against Fatal Lung Disease

Other studies^8,9,10 have confirmed the long-held belief that vitamin D protects against tuberculosis, a fatal lung disease that kills an estimated 1.8 million people around the world each year.¹¹ This is largely related to vitamin D stimulating antimicrobial peptides (AMPs) like cathelicidin (LL37).

In the past, tuberculosis was treated by making sure patients got plenty of sun exposure. In fact Finsen was given the Nobel Prize in 1903 for this determination. Around the turn of the 20th century regular sun exposure was the most effective clinical strategy for the treatment of tuberculosis, but was eventually phased out with the development of antibiotics.

A 2011 study in Science Translational Medicine examined the mechanisms responsible for your immune system’s ability to ward against tuberculosis, concluding that T cells play a central role. They release a protein called interferon-g, which in turn activates the release of AMPs so your immune cells can mount an effective attack against the tuberculosis bacteria. 

However, in order for this activation to occur, you have to have sufficient levels of vitamin D. In patients with low vitamin D levels, this immune response was not activated. Meanwhile, among those with adequate levels, there was an 85% reduction of colony-forming tuberculosis bacteria. As reported by UCLA:¹²

“The team noted that vitamin D may help both innate and adaptive immunity, two systems that work synergistically together to fight infections. Previous research by the team found that vitamin D played a key role in the production of a molecule called cathelicidin, which helps the innate immune system kill the tuberculosis bacteria. 

Humans are born with innate immunity, which is the preprogrammed part of the immune system. The current research findings demonstrate that vitamin D is also critical for the action of T cells, key players in adaptive immunity, a highly specialized system that humans acquire over time as they encounter different pathogens.”