Cod liver oil has been used as a prevention against acute respiratory tract infections (ARTI) and for symptom relief for rheumatism, for decades if not centuries (1). It is likely useful in the prevention of COVID-19 and/or alleviating its symptoms.
Cod liver oil (CLO) is a natural source of vitamin D, vitamin A, and Omega-3 fatty acids.
Vitamin D (which is a prohormone, rather than a vitamin) is essential in the body for multiple reasons, including bone health. Low levels of vitamin D are associated with increased vulnerability to ARTI ((2), (3), (4)). Surprisingly, vitamin D supplementation does not noticeably increase the odds against ARTI among adults ((3), (5), (6)). In 2006, a high impact paper (9) demonstrated that ultraviolet radiation and cod liver oil reduce the incidence of viral respiratory infections. However this effect is not achieved solely through an increase in vitamin D levels. It seems that for increased protection against ARTI, vitamin D must either be produced naturally, by exposure to sunlight, or obtained from food.
Recommended levels of sunlight exposure depend on location and natural skin color. Doctors can research and give individualized recommendations about exposure duration and optimal hours of the day (11). (Caution: excessive exposure (erythema) suppresses immunity.) Exposure of a larger area of skin for a short time seems preferable to exposure of a smaller area of skin for a long time. For example, in Southern California’s summer, light skinned individuals will probably get sufficient exposure during their regular daily routines. Dark skinned individuals probably require more intentional sunbathing for a fraction of an hour daily. Of course none of this is possible if people are told to stay home all day.
Fish is the only practical food source of vitamin D for most people. Industrially synthesized Vitamin D (found in supplements and fortified food) provides other health benefits but does not enhance immunity against ARTI.
A lesser-known property of vitamin D is its anti-inflammatory effects. In the context of COVID-19, it is suggested that vitamin D can decrease the cytokine response, thus reducing the risk of ARDS in severe COVID-19 patients (7). For this effect to take place, the vitamin D source probably does not matter.
Omega-3 fatty acids also have anti-inflammatory properties. Recently, Omega-3 has been shown to help relieve symptoms of auto-immune diseases, such as rheumatoid arthritis and lupus. Sound familiar? Hydroxychloroquine, the pharmaceutical drug used to relieve rheumatoid arthritis and lupus symptoms, is also being used to treat COVID-19 infections.
Vitamin A also has immunity enhancing and anti-inflammation properties (8). Thus the Omega-3 fatty acids and vitamins D & A found in cod liver oil might have multiple synergistic effects.
Fish is another good source of Vitamin D, Vitamin A, and Omega-3 fatty acids. Wild caught salmon, for example, is one of the best sources. Nutritional values of many other kinds of fish are widely available.
Five studies from the 1930’s, involving more than 7,000 participants, show that cod liver oil reduced respiratory infections. Two of the studies were controlled. One of them found that cod liver oil reduced colds by 50%; another one found that cod liver oil reduced the number of missed workdays, due to colds and respiratory illness, by 30% ((9), (10)). These results were attributed to either vitamin D or A, apparently prematurely. The importance of Omega-3 fatty acids was only understood in the 1980’s, and research into Omega-3 anti-viral properties is recent.
Published in 2006, (9) pointed out that Norway has one of the highest vitamin D levels (measured as concentration of 25(OH)D in serum) in Europe, and compares it favorably with Great Britain, which has a low level of vitamin D. Also, many Norwegians frequently take CLO (see comments). COVID-19 mortality per M: Norway – 43, GB – 513 (12). (Irrational rejection of Hydroxychloroquine-based treatment in Britain is another important factor.)
Iceland has the highest fish consumption in the world, and it consumes wild catch fatty fish from the North Atlantic – probably equivalent to the cod liver oil. It has only 29 deaths per M (i.e., 10 deaths). Spain (593), Italy (522), and France (433) – large European countries with the highest COVID-19 mortality per M – also have the highest incidence severe vitamin D deficiency (6).
Multiple pre-prints point to correlation between low levels of vitamin D and COVID-19 morbidity and mortality: Daneshkhah at al. , De Smet et al. , Lau et al. , Davies et al. This correlation seems to be stronger than correlation between low level of vitamin D and incidence or severity of common colds and flu.
Computer modeling suggests that vitamin D might directly inhibit CoV-2 virus by interfering with approx 70% of functions of CoV-2 proteins (Glinsky, a preprint on chemrxiv).
The role of vitamin D in the prevention of coronavirus disease 2019 infection and mortality. Ilie, P.C., Stefanescu, S. & Smith, L., Aging Clin Exp Res (May 6, 2020), Springer
Negative correlations between mean levels of vitamin D (average 56 mmol/L, STDEV 10.61) in each country and the number of COVID-19 cases/1 M (mean 295.95, STDEV 298.7, and mortality/1 M (mean 5.96, STDEV 15.13) were observed. Vitamin D levels are severely low in the aging population especially in Spain, Italy and Switzerland. This is also the most vulnerable group of the population in relation to COVID-19.
Annotation of Some Sources
(3) is a review and meta-analysis of several studies on the subject. It reported that while observational studies have shown a correlation, interventions with vitamin D supplements had mixed results.
(5) is the most popular meta-analysis on the subject. While it reported the benefits of vitamin D supplementation, it mostly covered its effects in children. Only a few small adult studies were included, and those had, at best, mixed results.
(2) is the latest meta-analysis of observational studies. It quantified that people with a severe vitamin D deficiency are almost twice as likely to get ARTI, compared to people with vitamin D sufficiency.
(4) reported that 42% of the general population is deficient in vitamin D. Individuals with naturally darker skin are more likely to have a vitamin D shortage; 82% of African Americans have vitamin D deficiency.
I declare no competing interests.
This article discusses a supplement (cod liver oil) and fish as a component of healthy diet. It is aimed at researchers, doctors, and health care policy makers and is not written as medical advice.
This article is not peer reviewed.
WARNING: do not take mega-doses of vitamin D (5,000 IU or more) without consulting your doctor. It might decrease protection against ARTI, possibly through a dysregulation of the associated body functions.
Thanks to A.O. for contribution to this article.
- Mother Was Right About Cod Liver Oil. Griffing, George T. 2008, The Medscape Journal of Medicine.
- Hydroxyvitamin D Concentration: A Systematic Review and Meta-Analysis. Pham et al. 2019, International Journal of Environmental Research and Public Health.
- Vitamin D in the prevention of acute respiratory infection: Systematic review of clinical studies. Jolliffe, David A., Martineau, Adrian R. and Griffiths, Christopher J. 2012, The Journal of Steroid Biochemistry and Molecular Biology.
- Prevalence and correlates of vitamin D deficiency in US adults. Forrest , Kimberly Y.Z. and Stuhldreher, Wendy L. s.l. : Elsevier, 2011, Nutrition Research.
- Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data. Martineau et al. 2017, BMJ.
- Does vitamin D status impact mortality from SARS-CoV-2 infection? Marik, Paul E, Kory, Pierre and Varon, Joseph. s.l. : Elsevier, April 29, 2020, Medicine in Drug Discovery.
- Panarese, Alba and Shahini, Endrit . Letter: Covid-19, and vitamin D. s.l. : Wiley, 2020.
- Role of Vitamin A in the Immune System. Huang, Zhiyi et al. s.l. : MDPI, 2018, Journal of Clinical Medicine.
- Epidemic influenza and Vitamin D. Cannel, J J, Vieth, R and Umhau, J et al. 2006, Epidemiology and Infection.
- Warfighter Nutrition: Advanced Technologies and Opportunities. Umhau, John. 2008. Uniformed Services University of the Health Sciences, Health Affairs and DARPA.
- Ultraviolet-B radiation increases serum 25-hydroxyvitamin D levels: The effect of UVB dose and skin color. Armas et al. 2007, Journal of the American Academy of Dermatology.
- Worldometers. Coronavirus Statistics. Worldometers.info. [Online] [Cited: May 18, 2020.] https://archive.is/mBtUn.
Originally published on May 16, 2020. Updated on May 17-18, 29.