Although my family doesn't generally take any supplements or vitamins, there is one supplement we take regularly during cold and flu season for its beneficial effect on the immune system: extra-virgin cod liver oil.
Why Cod Liver Oil Boosts the Immune System
Vitamins A and D are especially important for immune system support . Deficiency in Vitamin A is known to lead to an increase in illnesses, and it has been "well established that widespread immune alterations, anemia, and increased infectious disease morbidity and mortality occur during vitamin A deficiency ." Likewise, Vitamin D is known to boost the immune system , and "recent epidemiological evidence indicates a significant association between vitamin D deficiency and an increased incidence of several infectious diseases ."
Modern diets are generally deficient in vitamins and nutrients as compared to the diets of traditional peoples. Weston A. Price's studies  showed that the diets of traditional people contained ten times the amounts of Vitamin A and D present in modern diets. Price found that the higher nutrient-content of traditional diets led to much greater immunity to infectious diseases such as then-prevalent tuberculosis.
Cod liver oil is an excellent natural source of Vitamins A and D. (It also provides Omega 3's, DHA, and EPA, which are so important for brain and heart health .) With today's widespread nutrient-deficiencies, cod liver oil can be a safeguard against frequent illnesses during cold and flu season.
Which Type of Cod Liver Oil Is Best?
There are many inferior pseudo cod liver oils on the market. For instance, fermented cod liver oil (FCLO) has been shown to be rancid, and DNA testing indicated that it is actually made from pollock liver (which has a different nutrient profile than cod liver oil) . My own family stopped taking FCLO back in 2012 after having negative reactions to it. Many other cod liver oils have been over-processed to the point where they have little naturally-occurring vitamins, and are instead filled with synthetic vitamins. In my own family, Rosita's extra-virgin cod liver oil (EVCLO) is the cod liver oil that has worked best. Rosita's cod liver oil is superior to all others because the ancient-Viking harvesting process uses no heat, pressure, or chemicals. This minimal processing means that EVCLO is rich in naturally-occurring Vitamins A and D, which are so critical for the immune system. EVCLO's mild flavor and superb nutrition have made it a win-win in our household.
How Much Cod Liver Oil Do We Take?
We don't take extra-virgin cod liver oil daily, or even year-round. Rather, we let our bodies guide us in knowing when we would benefit from some cod liver oil. Here in southern New Mexico, we have abundant sunshine and, in the warm months, my kids and I get so much sunshine on our skin that our bodies seem able to make sufficient Vitamin D for our needs. During those times, we don't even think twice about taking any cod liver oil because it doesn't even sound appealing (and we store our cod liver oil in the freezer during those months to maintain freshness).
However, in the cold months, our sun exposure is greatly lessened, and we all start to crave extra-virgin cod liver oil. My kids and I tend to crave a teaspoon of EVCLO about 2-4x/week in the winter months. The kids and I take it straight off a spoon, but my husband prefers to take his EVCLO in capsules. Since Weston Price found that cod liver oil worked synergistically with high-vitamin butter , we try to make sure to take our cod liver oil at a time when we are also consuming nutrient-dense butter.
A Powerful Ally
Extra-virgin cod liver oil is a healthy, natural way to boost immunity. It's nutrient profile is rich in Vitamins A and D, which are critical for a strong immune system. Cod liver oil is a powerful ally against frequent illnesses, and it may be especially beneficial in the winter months when most people do not obtain enough sunlight for their bodies to produce adequate Vitamin D.
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 Price, W. Nutrition and Physical Degeneration. Lemon Grove, CA, Price-Pottenger; 1939.
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 Daniel, K. Hook, Line, and Stinker! The Truth About Fermented Cod Liver Oil. 2015. http://drkaayladaniel.com/are-you-hooked-on-fermented-cod-liver-oil/
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