Possible Role of Cod Liver Oil or Fish Rich Diet in COVID19 Prevention

In the US and Northern Europe, cod liver oil was used for prevention of acute respiratory tract infections (ARTI) and relieving symptoms of rheumatism for decades, if not centuries (1). Can it be taken to help to prevent COVID-19?

Cod liver oil (CLO) is a natural source of vitamin D, vitamin A, and Omega-3 fatty acids.

Vitamin D (which is a pro-hormone, rather than a vitamin) is necessary for human body for multiple reasons, including bones health. In the context of COVID-19, two of them are important. Low levels of vitamin D are associated with increased vulnerability to ARTI ((2), (3), (4)). Surprisingly, vitamin D supplementation does not noticeably increase odds against ARTI among adults ((3), (5)).

It is likely that for increased protection against ARTI we need to either produce vitamin D naturally, by exposure to sunlight, or to get it with food containing another component. Fish is the only practical food source of vitamin D for most of us. Vitamin D in supplements and fortified food is industrially synthesized. It provides other health benefits, but not increased protection against ARTI.

A less known property of vitamin D is that it is anti-inflammatory. In the context of COVID-19, it is suggested that vitamin D can decrease cytokine response, thus reducing risk of ARDS in severe COVID-19 patients (6). It probably does not matter for this effect how vitamin D was obtained.

Omega-3 fatty acids have anti-inflammatory properties, too. Recently, Omega-3 has been shown to help to relive symptoms of auto-immune diseases, such as rheumatoid arthritis and lupus. Sounds familiar? Hydroxychloroquine is a pharmaceutical drug used to relieve rheumatoid arthritis and lupus symptoms.

There are more tasty alternatives for cod liver oil. Wild caught salmon is one of the best source of vitamin D and Omega-3. Many other kinds of fish, down to canned tuna, provide them, although at lower concentration and/or with compromises. Their nutritional values are widely available.

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. It is not a medical advice.

WARNING: do not take mega-doses of vitamin D (>=5,000 IU) without consulting your doctor. It might decrease protection against ARTI, possibly through a dysregulation of the associated body functions.

Annotation of References

(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 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.

References

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.
Panarese, Alba and Shahini, Endrit . Letter: Covid-19, and vitamin D. s.l. : Wiley, 2020.