What Is Cod Liver Oil?
Cod liver oil is a natural supplement extracted from the livers of cod fish that contains the omega-3 fatty acids, EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid), along with vitamins A and D. Because it’s sourced from the liver, cod liver oil contains high amounts of vitamins A and D, unlike regular fish oil. It was even used to treat and eradicate rickets (a disease caused by a vitamin D deficiency) in the 19th and 20th centuries [1, 2].
Active Components of Cod Liver Oil
Omega-3 Fatty Acids
Omega-3 and omega-6 fatty acids are considered essential because they can’t be produced by the body. Modern diets often have a ratio of approximately 15 to 1 of omega-6 to omega-3. Omega-3 supplements are important to restore this imbalance. The ideal ratio of omega-6 to omega-3 is somewhere between 2 and 5 to 1, which varies depending on whether you’re healthy or have a specific disease [3, 4].
1 tsp/5 mL of cod liver oil contains approximately 850-4,000 IU of vitamin A, equivalent to 255 – 1200 mcg of retinol/RE. This is quite high, as the daily intake requirement is 1000 mcg RE for men, 800 mcg RE for non-pregnant as well as pregnant women and 1,300 mcg RE for lactating women [12, 13].
Vitamin A protects the eyes and skin from UV damage and helps control stress. It is an essential nutrient (can’t be produced by the body), critical for memory, maintaining the circadian rhythm, and controlling blood sugar levels [14, 15, 16, 17, 18, 19, 20, 21].
One serving (1 tsp/5 mL) of cod liver oil contains approximately 400 IU of vitamin D. Vitamin D reduces the risk of high blood pressure, heart disease, autoimmune disorders, infections, and cancer while reducing inflammation [22, 23, 24, 25, 26].
How It Works
Cod liver oil works by providing the body with adequate levels of omega-3 fatty acids, vitamins A and D. Both vitamin A and omega-3 fatty acids can’t be produced by the body. The vitamins and the omega-3 fatty acids have a synergistic effect on the body – they work best when all taken together [27, 28, 29, 30, 31].
Cod liver oil acts to lower cholesterol and inflammation while boosting immune and brain function.
Lowering Cholesterol Levels + Heart Health
The vitamin D in cod liver oil reduces inflammatory agents (TNF-alpha and interleukin 1beta) produced by white blood cells. Vitamin D deficiency is associated with multiple autoimmune diseases (multiple sclerosis, inflammatory bowel diseases, rheumatoid arthritis) [33, 34].
Vitamin A and D together could combat the prostate cancer cells, by increasing a protein that causes cell death (apoptosis regulator BAX) and decreasing a protein that controls cell growth (cyclin D1) .
Both vitamin A and omega-3 fatty acids – especially the DHA found in cod liver oil – are critical for brain function. DHA is found in large quantities in the eyes and brains of mammals, as it helps to make neurotransmitters. Vitamin A reduces the effect of stress hormones on the part of the memory-forming part of the brain (hippocampus) [9, 5, 37].
Health Benefits of Cod Liver Oil
1) May Reduce Cholesterol and Triglycerides
Cod liver oil increased HDL cholesterol levels and decreased bad cholesterol (VLDL) and triglycerides levels in 18 patients with diabetes and kidney problems (albuminuria). LDL levels were unaffected by cod liver oil .
In a study of 34 subjects, men and women responded differently to 8 weeks of 25 mL daily cod liver oil. In women, levels of LDL cholesterol decreased, but in men, they increased. There was no effect on HDL .
20 mL cod liver oil increased omega-3 fatty acid levels, decreased omega-6 fatty acid levels, and reduced triglyceride levels in a study of 21 male heart attack patients. It had no effect on other types of fat (total cholesterol, good cholesterol (HDL), apolipoproteins A1 and B) .
However, a study of 120 healthy subjects with high cholesterol failed to find any effect of daily 15ml cod liver oil for a year on total cholesterol levels .
2) Helps With Rheumatoid Arthritis
In a pilot study of 30 patients with rheumatoid arthritis, cod liver oil (five 300 mg capsules, 2x/day) reduced the use of arthritis medications (Diclofenac Sodium) and had fewer side effects .
In a follow-up study of 58 patients with rheumatoid arthritis, 10 g cod liver oil (with 2.2 g of omega 3s) over 9 months reduced the use of painkillers (diclofenac, naproxen, and ibuprofen) .
In a pilot study of 43 patients with rheumatoid arthritis, cod liver oil (1 g/day for 3 months) decreased morning stiffness, painful and swollen joints, and pain intensity .
3) Prevents and Fights Infections
In a study of 94 infants and children (6 months to 5 years old) given cod liver oil had fewer pediatric visits for upper respiratory tract infections (1 tsp cod liver oil/day). Children given cod liver oil and a multivitamin also had fewer visits for sinus issues (4 male children with a history of sinus issues) [28, 44].
Cod liver oil supplements helped mice fight a pneumonia infection .
In mice with a vitamin E deficiency, cod liver oil was extremely effective against multiple parasites (P Plasmodium yoelii, Babesia rodhaini, Trypanosoma congolense, and Trypanosoma vivax). Cod liver oil might be converted to a compound toxic to the parasites (an effect that the vitamin E cancels out with its antioxidant properties) [46, 47, 48].
This means that cod liver oil would help fight malaria and other parasites only during vitamin E or antioxidant deficiency. A similar effect was also seen in chicken (against the parasite Plasmodium gallinaceum) .
4) Improves Bone Health
In 3113 postmenopausal women living at high latitudes, using either vitamin D from cod liver oil or from multivitamins slowed down bone breakdown and loss .
Taking vitamin D from cod liver oil or from multivitamins also lowered the risk of fractures in another study with 1246 older women .
In rats whose ovaries were removed to mimic the osteoporosis that occurs after menopause, cod liver oil improved bone mineralization .
5) May Fight Cancer
In a study that interviewed 2107 subjects, women who took the most cod liver oil between ages 10-19 were least likely to develop breast cancer. However, the likelihood was not reduced as much with cod liver oil taken between ages 20-29, and not at all with cod liver oil taken between ages 45-54 .
In a questionnaire involving 68,518 subjects, daily use of cod liver oil for a year was associated with a lower risk of death in patients with solid tumors and lung cancer .
In rats, a combination of cod liver oil with a potential anti-cancer compound, DIM (3,3ʹ-Diindolylmethane), and vitamin E slowed the growth rate of tumor cells .
6) Is Beneficial During Pregnancy
The essential fatty acid, DHA, is important for healthy brain development. It can’t be made in the body and must be consumed in the diet. Infants of mothers who consumed 10 mL of cod liver oil during pregnancy and breastfeeding had higher IQ scores at 4 years of age in a study of 76 infants .
In a study of 435 pregnant women, intake of cod liver oil during pregnancy was associated with an increased weight of infants at birth .
Beneficial nutrients from cod liver oil pass into breast milk. Cod liver oil increased levels of DHA in breast milk in a study of 22 women. In two studies of 77 nursing mothers, cod liver oil and fish intake increased the amount of healthy fatty acids (DHA, EPA, and DPA) as well as fat-soluble vitamins (A, D, and E) in breast milk [58, 59, 60].
7) May Protect Against Multiple Sclerosis
Cod liver oil reduced the risk of multiple sclerosis among those who spent less time outside during the summer in a study of 554 children (Retrospective recall questionnaire study) .
In a study of 2670 Norwegians, the use of cod liver oil across the ages 13-18 was associated with a reduced risk of multiple sclerosis .
8) May Prevent and Help with Type-1 Diabetes
Consumption of cod liver oil during the first year of life reduced the risk of developing type-1 diabetes during childhood (study with 545 children) .
9) May Improve Heart Health
Cod liver oil users had much lower blood pressure (diastolic) in a study of 9,732 subjects .
In rats with diabetes, cod liver oil improved the swelling of blood vessels and helped lower triglycerides, cholesterol, and blood sugar levels. Altogether, this suggests it’s able to protect the heart and blood vessels .
However, cod liver oil users were not less likely to develop heart disease in one study (56,718 adults).
10) May Reduce Depression and Anxiety
11) Reduces Inflammation
Cod liver oil reduced inflammation of the middle ear in 8 children undergoing ear surgery (1 tsp of cod liver oil, pilot study) .
12) May Improve Asthma
In a study of 16,187 subjects, weekly intake of cod liver oil was associated with fewer symptoms of asthma. However, both no intake and more frequent, daily, intake were linked to more symptoms .
13) May Help with Ulcers
In rats, cod liver oil prevented the development of ulcers in the small intestine. It also hastened to heal existing stomach ulcers .
Rats fed cod liver oil had half the amount of stomach ulcers caused by stress compared to rats fed corn oil .
14) May Protect the Kidneys and Liver
In mice, cod liver oil reduced kidney damaged caused by a cancer drug (Daunomycin) .
However, another study found that cod liver oil prevented damage to the liver, but not the kidney, of rats exposed to a toxic substance (carbon tetrachloride) .
Another rat study found that cod liver oil reduced liver damage (caused by nitrates) by acting as an antioxidant .
15) May Help Heal Wounds
16) May Boost Memory
In rats, long-term use of cod liver oil prevented memory loss caused by chronic stress .
17) May Protect the Brain
In newborn rats, cod liver oil protected against a brain toxin by decreasing brain cell death and preventing a drop in neurotransmitters .
18) May Help with Glaucoma
The vitamin A in cod liver oil has antioxidant properties that slow glaucoma progression and maintains the health of different parts of the eye. The omega-3 fatty acids can decrease pressure in the eye and increase blood flow to the eye. Cod liver oil may be beneficial for people suffering from glaucoma [81, 82, 83].
Cod liver oil reduced eye pressure in rabbits, which is the main symptom and risk factor in glaucoma .
Limitations and Caveats
A lot of the studies done on cod liver oil are large, observational studies. Thus, conclusions drawn from these studies are only correlational, meaning there we can’t draw conclusions about the cause and effect.
The typical cod liver oil/supplement user in these studies differs from non-supplement users. People who consume supplements were more likely to live a healthier lifestyle (higher physical activity levels, diets with more fruits, vegetables, fatty fish, and vitamin D, and non-smoking) and a better socioeconomic status (higher education, higher income) compared to non-supplement users. Thus, the effect of cod liver oil on health is harder to isolate [85, 86, 87].
Some studies have only been done on animals (mice and rats).
Side Effects & Precautions
1) Can Cause a Vitamin A and D Ratio Imbalance
Some researchers oppose the use of cod liver oil due to a risk of vitamin A overdose with vitamin D deficiency. While this ratio imbalance may explain the discrepancy between data on cod liver oil’s effect on bone mineral density, the vitamin A content in cod liver oil was recently reduced by 75% in Norway .
2) May Increase the Risk of Thyroid Cancer
A study of 60,000 Norwegians found that consumption of fish and cod liver oil may increase the risk of thyroid cancer. Since thyroid follicle cells are sensitive to vitamin D, the improper ratio of vitamin A to D is a possible factor [89, 90].
3) May Increase Bleeding Time
Cod liver oil increased bleeding time in several studies. This effect was not seen in other studies (such as a study with 17 patients with a family history of high cholesterol, and a study with dogs) [91, 92, 93, 94, 95].
4) Can Weaken Bones
A study of 3,052 women in Norway found that childhood cod liver oil intake was associated with lower bone mineral density. The authors speculated a decrease in vitamin A levels in cod liver oil may prevent this effect, as high vitamin A level increases the risk of bone fractures [96, 50, 97, 98].
However, multiple studies found the opposite to be true. This might be due to a recent decrease in vitamin A levels in cod liver oil, which helps to restore the balance between vitamin A and vitamin D [50, 51, 52].
5) May Cause High Blood Pressure In Pregnancy
A study of 549 pregnant women found that slightly higher intakes of the omega-3 fatty acids found in cod liver oil (more than 0.87 g/day or 1 tsp/5 mL of cod liver oil) in the early stages of pregnancy increased incidence of high blood pressure. The dosage associated with the lowest risk was 0.10 – 0.87 g/day (or 0.6-5.0 mL of cod liver oil) .
6) Increases Asthma Rates
Consuming cod liver oil supplements with high vitamin A was associated with an increased rate of adult-onset asthma among 25,616 Norwegian adults .
- Blood thinners: Cod liver oil may increase bleeding time so it might be dangerous to consume with blood-thinning medications [91, 92, 93].
- Vitamin E: Excess levels of cod liver oil can cause vitamin E deficiency .
Supplementing With Cod Liver Oil
There are no strict guidelines on the recommended dosage of cod liver oil. However, because high doses of vitamin A can be harmful, it’s important not to take too high a dose.
A 1 tsp/5 mL serving of cod liver oil contains approximately 850-4,000 IU of vitamin A. The recommended daily dose of vitamin A is around 2,300 IU for adult women and 3,000 IU for adult men. A safe upper limit for vitamin A is 13,000-20,000 IU/kg body weight, which amounts to the maximum safe amount that can be taken. Vitamin A toxicity occurs at 500,000 IU [101, 102, 103].
- 500 mg/day of DHA and EPA, equivalent to 0.5 tsp or 2.5 mL of cod liver oil.
A higher dosage recommended for people with heart problems:
- 1,000-4,000 mg DHA and EPA, equivalent to 1 tsp/5 mL – 4 tsp/20 mL cod liver oil.
The standard serving of cod liver oil contains approximately 400 IU of vitamin D. The recommended daily dose of vitamin D is approximately 600 IU .
Cod liver oil capsules have variable vitamin A and D levels compared to a similar amount of liquid cod liver oil. The content will also depend on the manufacturer.
Cod liver oil is available in a variety of forms:
- Extra virgin cod liver oil
Fermented vs. Extra Virgin Cod Liver Oil
Fermented and extra virgin both refer to the method of extracting the oil from the cod livers. Fermented cod liver oil is extracted by fermenting cod livers and collecting the extruded oil. Extra virgin cod liver oil is extracted from raw cod liver without using heat.
While many tout fermented cod liver oil as superior to non-fermented, there have been others that claim fermented cod liver is not fermented but decomposed (and is thus rancid). Neither of these claims has been substantiated. Fermented cod liver oil has not been extensively studied, unlike non-fermented cod liver oil.
There is no consensus on whether fermentation of oil can occur, whether fermented cod liver oil is rancid, and how to test for rancidity of fermented cod liver oil.
Genetics of Omega-3 Fatty Acids
People with genes linked to a higher risk for certain diseases might be affected differently by cod liver oil. For example, people with a gene variation for the 5-lipoxygenase protein are more likely to have clogged arteries. Cod liver oil helps decrease the likelihood of developing heart disease in this group [106, 107, 108].