Dr. Mercola: What You Need to Know About Cod Liver Oil

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What You Need to Know About Cod Liver Oil

cod liver oil
Story at-a-glance -

    Cod liver oil is derived from the Atlantic cod, and is usually sold as a supplement in health stores and websites
    Studies conducted on cod liver oil suggest that it can aid in addressing rheumatoid arthritis, speed up wound healing and lower the risk for depressive symptoms
    There are some side effects that may occur after taking cod liver oil, such as nausea, bad breath and loose stools
    While cod liver oil does have positive qualities, you’re better off using krill oil, another type of marine oil with more benefits

Table of Contents

    What Is Cod Liver Oil?
    Food Sources of Cod Liver Oil
    What's Fermented Cod Liver Oil?
    Cod Liver Oil Versus Fish Oil
    Cod Liver Oil Studies
    Cod Liver Oil's Side Effects
    Why You Should Avoid Products With Cod Liver Oil
    Opt for Krill Oil Products Instead of Cod Liver Oil
    Frequently Asked Questions About Cod Liver Oil

The use of fish oil supplements to address health concerns isn’t entirely new. According to the National Center for Complementary and Integrative Health (NIH), 7.8 percent of adults and 1.1 percent of children in the U.S. were taking fish oil supplements in 2012, making them the most popular natural health supplement for that year.1

For decades, researchers have been conducting studies on a type of fish oil called cod liver oil to see if it can deliver additional benefits. If you’ve encountered cod liver oil supplements and want to know more about their effects, continue reading this guide. 
What Is Cod Liver Oil?

Cod liver oil is extracted from the liver of the Atlantic cod (Gadus morhua),2 and is typically sold as easy-to-swallow liquid capsules.3 Cod liver oil has been gaining popularity because of its high vitamin A, vitamin D and omega-3 fatty acid content,4 and its potential to address health issues like:5

    High cholesterol/triglyceride levels
    High blood pressure levels
    Kidney disease among diabetics
    Osteoarthritis and rheumatoid arthritis
    Heart disease

    Depression
    Systemic lupus erythematosus (SLE)
    Glaucoma
    Middle ear infections (otitis media)

Cod liver oil has also been suggested to work in reducing your risk for respiratory infections and macular degeneration, while topical application may promote faster healing of wounds, burns and rashes.6,7

According to the book, “Green Chemistry and Technologies,” cod liver oil production involves a multistep process. After initial extraction of the oil from the fish, the resulting thick mixture is filtered to eliminate impurities and then undergoes refining via acid enzymatic degumming, deacidification, decolorization and deodorization. The final product is a substance that may be used or added to capsules or supplements.8

Food Sources of Cod Liver Oil

The liver of the Atlantic cod is the primary source of this product, although in some instances other members of the Gadidae or cod family may be used as well.9 This is why not all cod liver oil supplements contain the same quantities of nutrients, since the amounts depend on the species from which the oil was extracted.10

What’s Fermented Cod Liver Oil?

You may also encounter products labeled “fermented cod liver oil.” This type of oil is made by following a natural fermentation process, which uses old-fashioned techniques from Iceland and Ireland.11

It takes around six months to a year to produce fermented cod liver oil, and it’s not the most pleasant-smelling substance to exist. Because there is very limited research regarding fermented cod liver oil’s benefits, you must exercise caution and consult your doctor to know more about its possible effects.

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Cod Liver Oil Versus Other Fish Oils: What’s the Difference?

In terms of nutrition content, cod liver oil is different from other fish oils extracted from various marine creatures or body parts, such as mackerel, herring, tuna, salmon, whale blubber, seal blubber and cod liver.12 Cod liver oil possesses less omega-3 fatty acids, but has higher amounts of vitamins A and D compared to other fish oils like salmon oil and herring oil.13,14,15,16 However, note that cod liver oil is not my preferred form of marine oil supplement — more on this later.

Cod Liver Oil: What Do Studies Say About It?

As early as 1919, researchers have suggested that cod liver oil and other fat-soluble vitamin A-rich substances had potential in reducing animals’ risk against rickets, which is caused by vitamin D deficiency.17 A separate article published in 1923 revealed that this substance was also used by fishermen in Northern Europe to help boost their health.18

Fast forward decades later, and you’ll find more published studies have touched on the potential benefits of cod liver oil against different health issues, such as:

• Rheumatoid arthritis (RA) — Results highlighted that cod liver oil supplements were considered a top remedy against RA.19 Another study from 2002 supported this claim, as people with RA who took a cod liver oil capsule daily for three months experienced less pain and improved symptoms.20

• Atherogenesis — Cod liver oil helped combat atherogenesis, which is a multistep disease21 that may cause atherosclerotic plaque to form on artery walls,22 although study authors noted that another substance made from sardines was able to produce a stronger effect.23

• Type 1 diabetes — A study proposed that cod liver oil’s vitamin D or EPA and DHA content (or both) assisted in lowering the Type 1 diabetes risk of babies when their mothers took this supplement during pregnancy.24

• Wounds — Researchers noted that when a 25 percent cod liver oil ointment was applied topically, it assisted in speeding up wound healing compared to saline. This result was linked to the high amounts of vitamin A in cod liver oil.25

• Depression — People who used cod liver oil had a reduced risk of experiencing depression-related symptoms, according to a 2007 study.26

• Cognitive problems — Another 2011 animal study emphasized that cod liver oil administration helped prevent negative impacts of chronic restraint stress toward the cognitive functions of rats.27

Cod Liver Oil’s Side Effects

WebMD notes that cod liver oil may cause side effects like burping or belching, nausea, bad breath, heartburn and loose stools, although taking the oil alongside meals may reduce these effects. Very high amounts of cod liver oil can also raise your bleeding risk, inhibit blood clotting and cause your vitamin A and D levels to become excessively high.

Significantly reduced blood pressure levels have also been attributed to cod liver oil consumption, especially when taken alongside drugs used to address high blood pressure levels. Cod liver oil and other fish oils were reported to increase blood sugar levels among diabetics, but there isn’t enough strong evidence to back this claim. Excessively high amounts also may be unsafe for pregnant and breastfeeding women.28

Because cod liver oil contains some amounts of omega-3 fatty acids, it’s possible that it can oxidize, especially if stored improperly, and cause the substance to be highly toxic and not be tolerated well.29

Why You Should Be Cautious of Cod Liver Oil and Other Fish Oil Supplements

I no longer recommend taking cod liver oil because it can skewer your vitamin A-to-D ratio. Most cod liver oil supplements have extremely high amounts of vitamin A, which may counteract the benefits that the vitamin D can offer. Studies have noted that increased cod liver oil intake led to low bone mineral density30 and a higher risk for adult asthma.31,32,33

In addition, most fish oils are heavily processed, and the final product typically sold in health stores or websites tends to be drastically different, at least in terms of composition, from the natural oils originally derived from the whole fish. You can read more about it in my article, “Are Many Fish Oils Synthetic?”

Environmental toxins like mercury, polychlorinated biphenyls and dioxins that have now contaminated the world’s fish supply can be transferred to fish oils and negatively affect their quality. When such toxins are ingested because of capsule or supplement intake, they can trigger adverse health effects.34,35,36,37

Lastly, the demand for fish oils (and other fish products) has generated disastrous environmental impacts, such as overfishing, which has now affected 87 percent of the world’s oceans.38,39

Opt for Krill Oil Instead of Cod Liver Oil

If one of your goals is to optimize your body’s omega-3 levels, it’s best to obtain these healthy fats from your diet first. Wild-caught Alaskan salmon and small fatty fish like sardines, anchovies, mackerel and herring are good options. Supplements like krill oil are viable too, and it’s actually my main preference for omega-3 supplementation because of the following factors:

• Superior absorbability40

• Vital health-boosting nutrients — Krill oil is more potent because it contains phospholipids and an antioxidant called astaxanthin,41 both of which may help boost your health. Research has indicated that krill oil tends to be less prone to oxidation,42 contains little to no amounts of contaminants,43 promotes significant effects toward metabolism,44 and possesses health benefits like:45

◦ Improving cardiovascular and gastric health

◦ Promoting anti-inflammatory capabilities

◦ Lowering arthritis risk

◦ Boosting neurological growth among newborns

◦ Alleviating PMS symptoms

• Better sustainability — Krill harvesting is one of the most carefully regulated industries, and it has to be, given krill is the planet’s largest biomass.

Most of the Earth’s krill is concentrated in the Antarctic, although they’re found in all oceans too. Strict policies on krill harvesting, such as catch limit regulations, have been set in place, and are constantly reviewed and reassessed from time to time. Overall, such measures may help reduce the risk of overharvesting.

If you want to know more about krill oil’s potential health benefits and how you can choose high-quality products that’ll positively impact your well-being, read "Make This Omega-3 Supplement Part of Your Health Regimen."

Frequently Asked Questions (FAQ) About Cod Liver Oil

Q: Is cod liver oil good for you?

A: Many believe that cod liver oil can be good for the body because it possesses vitamins A and D, and omega-3 fatty acids.46 However, the excessively high amount of vitamin A in this supplement may counteract the health benefits vitamin D can offer, and may even increase your risk for health issues like adult asthma47,48 and bone-related problems.49,50

Q: Is cod liver oil good for your joints?

A: Initial research showed that people suffering from joint problems like rheumatoid arthritis may use cod liver oil to alleviate some of the disease’s common symptoms.51,52 However, because of the possible drawbacks of frequent cod liver oil use, you’re better off consuming notable anti-inflammatory foods and utilizing inflammation-reducing strategies to combat this painful condition.

Q: Can cod liver oil help you lose weight?

A: Results of a 2010 study in the journal Nutrients revealed that mice fed cod liver oil had better quantities of long-chain omega-3 polyunsaturated fats and less amounts of omega-6 fatty acids.53 While this result is promising, further research is still needed to fully confirm the link between cod liver oil and weight loss. If one of your health goals is to lose weight, there are other ways you can fulfill those goals, such as by incorporating a ketogenic diet.

Q: Is cod liver oil the same as fish oil?

A: No. While cod liver oil is extracted from the Atlantic cod (Gadus morhua),54 and is considered a type of fish oil, the terms must not be interchanged with each other. This is because fish oil can also refer to substances that come from mackerel, herring, tuna, salmon, whale blubber and seal blubber.55

Q: Where can you buy cod liver oil?

A: Cod liver oil products can be found in various health stores and websites.

Q: Is it safe to take cod liver oil daily?

A: Increased intake of cod liver oil isn’t advisable because it has been linked to side effects like loose stools, nausea and bad breath, higher blood sugar levels and significantly reduced blood pressure levels.56 Frequent cod liver oil consumption may also expose you to toxins like mercury, polychlorinated biphenyls and dioxins that can be a precursor for numerous health problems.57,58,59,60

+ Sources and References
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