Vitamin D for Influenza

By Dr. James Meschino

Every year the medical profession and government authorities encourage citizens in many developed countries to get immunized against the current form of influenza virus. This year is no different, particularly with the "dreaded" swine flu still on everyone's minds. While the decision to get vaccinated is an individual one with both pros and cons, consider that a natural approach, namely supplementation with specific nutrients such as vitamin D, can boost immune function and may play an important role in the prevention of respiratory tract infections.

Why Vitamin D?

In recent years, studies have shown that vitamin D is an important modulator of immune function. Some authorities suggest it has the potential to reduce the risk of life-threatening influenzas based on the initial observation that influenza normally strikes in countries during the colder (winter) months, when vitamin D production in the skin declines. This happens because the most generally available source of vitamin D is sunlight. Reduction in skin production of vitamin D due to reduced or no exposure to daily sunlight is accompanied by a decline in blood levels of vitamin D.

Some vitamin D experts suggest adults should supplement with 2,000 IU vitamin D per day (especially during the winter) as a means to maintain more optimal vitamin D status in general, strengthen immune function and help reduce the risk of influenza and its invasion into the lung cavity. Other experts suggest dark-skinned individuals should supplement with 5,000 IU of vitamin D per day during the winter months to help ensure they attain blood vitamin D levels (25-hydroxycholecalciferol) at or above 50 ng/ml. (Melanin, the pigment that makes the skin darker, acts as a sunscreen, reducing vitamin D production in the skin upon exposure to solar radiation.)

How Vitamin D Affects Immunity

Most immune cells contain vitamin D receptors which allow vitamin D to enter the cells and exerts its effects on immune cell behavior. In this capacity, vitamin D has been shown to dramatically stimulate the expression of potent antimicrobial peptides. These peptides exist in white blood cells such as neutrophils, monocytes and natural killer cells, and the epithelial cells that line the respiratory tract, where they play a significant role in protecting the lung from infection.

Vitamin D influences both innate and adaptive immunity. The cells of the innate system recognize and respond to pathogens (infectious agents or germs) in a generic way, and the adaptive immune cells have the ability to recognize and remember specific pathogens. They, in turn, generate immunity by mounting stronger attacks each time the same pathogen is encountered. In simpler terms, vitamin D appears to make immune cells better able to fulfill their primary function - defense.

Adaptive immunity involves lymphocytes (a specific type of white blood cell) that are able to express a vast number of specific antigen receptors on their cell surface. Should the pathogen be reintroduced at a later point in time, these receptors are activated and the lymphocytes launch an assault against the pathogen. In adaptive immunity, all of the offspring of the activated cells inherit genes, encoding the same receptor specificity. These cells include the memory B cells and memory T cells that are the keys to long-lived specific immunity.

Vitamin D receptors are expressed in monocytes and in activated macrophages, dendritic cells, natural killer cells, and T and B cells. Activation of these receptors by vitamin D has been shown to increase the activity of natural killer cells and enhance the phagocytic activity of macrophages. (Phagocytes are white blood cells that protect the body by, for lack of a better word, "eating" harmful foreign particles, bacteria, etc.) Active vitamin D hormone also increases the production of cathelicidin, an antimicrobial peptide that is produced in macrophages. The release of cathelicidin is triggered by the presence of bacteria, viruses and fungi.