Many people swear by the benefits of cod liver oil, which is known to contain heart-healthy omega-3 fatty acids. Cod liver oil may also help boost immune and brain function, reduce anxiety and depression, reduce inflammation, and may even protect against diseases like multiple sclerosis. Read on to learn more about the benefits, with side effects.
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 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 Triglyceride 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
Possibly Effective for:
1) Lowering Triglycerides
As discussed above, cod liver oil’s omega-3 fatty acids reduce blood triglyceride levels by decreasing the production of these fatty molecules .
In a clinical trial on 34 healthy people, dietary cod liver oil reduced triglyceride levels in both men and women .
In a clinical trial on 21 men recovering from a heart attack, cod liver oil reduced blood triglyceride (but not cholesterol) levels and increased the omega-3 to omega-6 ratio in cell membranes .
You may try cod liver oil to lower your triglycerides if your doctor determines that it may help you.
2) Improving Bone Health
In a clinical trial on over 3000 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 of over 1200 older women .
In rats whose ovaries were removed to mimic the osteoporosis that occurs after menopause, cod liver oil improved bone mineralization .
To sum up, the evidence suggests that cod liver oil protects from bone loss and fractures. Discuss with your doctor if it may be helpful in your case.
3) Benefits 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 [45, 46, 47].
The results are promising but the evidence is still limited. Further clinical trials are needed to shed more light on the potential benefits of cod liver oil during pregnancy.
4) Preventing 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 over 500 children .
In a study of almost 2700 Norwegians, the use of cod liver oil across the ages 13-18 was also associated with a reduced risk of multiple sclerosis .
The evidence suggests that cod liver oil helps prevent multiple sclerosis but cannot conclude it for certain because only cohort studies have been conducted.
5) Preventing Depression
In two studies on over 23,000 people, those who took cod liver oil were less likely to suffer from depression. Again, the evidence is only based on cohort studies. It suggests that cod liver oil helps prevent depression, but further research is needed [49, 50].
6) Preventing Type-1 Diabetes
Consumption of cod liver oil during the first year of life was associated with a reduced risk of developing type-1 diabetes during childhood in a study of over 500 children .
Once again, the only clinical research is an observational study that cannot establish cod liver oil intake as the cause of the low type-1 diabetes incidence observed. Clinical trials are needed.
Insufficient Evidence for:
1) Rheumatoid Arthritis
In another trial on 30 people with this condition, 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) .
Because only three small clinical trials have been conducted, the evidence is insufficient to claim that cod liver oil improves rheumatoid arthritis. These results should be replicated in larger, more robust studies.
Cod liver oil reduced inflammation of the middle ear in 8 children undergoing ear surgery .
However, two small clinical trials and a rat study are not sufficient evidence to back the anti-inflammatory effect of cod liver oil. Larger, more robust clinical trials are needed.
3) Preventing and Fighting Infections
In a study of 94 infants and children (6 months to 5 years old), those 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 in a small study on 4 male children [28, 57].
Cod liver oil supplements helped mice fight pneumonia .
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). This suggests that the oil helps fight parasites only during vitamin E or antioxidant deficiency [59, 60, 61].
A similar effect was also seen in chicken (against the parasite Plasmodium gallinaceum) .
To sum up, the evidence supporting the use of cod liver oil in the prevention of respiratory infections is promising but limited. Because the studies testing this oil to fight parasites have only been tested in animals, there is no evidence that its effects will be the same in humans.
4) Lowering Blood Pressure
The oil also reduced blood pressure in a trial on 18 people with diabetes and kidney problems (albuminuria) .
However, the role of cod liver oil in preventing high blood pressure is unclear. A study on almost 10,000 people associated its intake with lower blood pressure but another one on almost 57,000 failed to find this link [65, 66].
All in all, two small clinical trials and two studies in rats cannot be considered sufficient evidence that cod liver oil lowers blood pressure and the evidence of its preventive role is contradictory. Further clinical research is needed.
5) Cancer Prevention
In a study that interviewed over 2000 women, those 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 over 68,000 people, daily use of cod liver oil for a year was associated with a lower risk of death in those with solid tumors and lung cancer .
Note, however, that these two studies associated high cod liver oil intake with a reduced incidence and mortality of these cancer types but didn’t establish its role at preventing them. Other genetic and environmental factors could have influenced cancer frequency.
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. Further research in humans is needed before cod liver oil can be considered in anticancer therapy .
Animal Research (Lack of Evidence)
No clinical evidence supports the use of cod liver oil for any of the conditions listed in this section. Below is a summary of the existing animal research, which should guide further investigational efforts. However, the studies listed should not be interpreted as supportive of any health benefit.
Ulcers and Wounds
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 .
Protecting the Kidneys and Liver
In mice, cod liver oil reduced kidney damage 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 .
In rats, long-term use of cod liver oil prevented memory loss caused by chronic stress .
Protecting the Brain
In newborn rats, cod liver oil protected against a brain toxin by decreasing brain cell death and preventing a drop in neurotransmitters .
Cod liver oil reduced eye pressure in rabbits, which is the main symptom and risk factor in 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 [83, 84, 85].
Possibly Ineffective for:
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 improved triglyceride and omega-3 to omega-6 ratio but had no effect on other types of fat (total cholesterol, good cholesterol (HDL), apolipoproteins A1 and B) in a study of 21 male heart attack patients .
Similarly, 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 .
Taken together, none of the studies found any effects of cod liver oil on total cholesterol. Regarding cholesterol bound to transport proteins, the results were mixed but most studies found the oil ineffective at increasing HDL.
In a clinical trial on 86 people with osteoarthritis, supplementation with cod liver oil offered no health benefit when compared with placebo (olive oil). Although limited, the evidence suggests that cod liver oil is ineffective to relieve osteoarthritis symptoms .
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 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 [88, 89, 90].
Some studies have only been done on animals (mice and rats).
Side Effects & Precautions
High intake of cod liver oil has been associated with some health risks. Because the link was often found in cohort studies, this doesn’t necessarily mean that cod liver oil caused these conditions.
1) 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) Thyroid Cancer
A study of 60,000 Norwegians associated the consumption of fish and cod liver oil with an increased incidence of thyroid cancer. Since thyroid follicle cells are sensitive to vitamin D, the improper ratio of vitamin A to D is a possible factor [92, 93].
3) High Blood Pressure During Pregnancy
A study of 549 pregnant women associated 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 with an 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) .
4) Bone Strength
A study of over 3,000 women in Norway associated cod liver oil intake during childhood with lower bone mineral density. The authors speculated that 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 [94, 40, 95, 96].
However, multiple studies came to opposite findings. This might be due to a recent reduction in vitamin A levels in cod liver oil, which may help restore the balance between vitamin A and vitamin D [40, 41, 42].
5) Bleeding Time
To help avoid interactions, your doctor should manage all of your medications carefully. Be sure to tell your doctor about all medications, vitamins, or herbs you’re taking. Talk to your healthcare provider to find out how cod liver oil might interact with something else you are taking.
Cod liver oil could interact with the following drugs and supplements:
- Blood thinners: Cod liver oil may increase bleeding time so it might be dangerous to consume with blood-thinning medications [64, 97, 98].
- Vitamin E: Excess levels of cod liver oil can cause vitamin E deficiency .
Supplementing With Cod Liver Oil
Although cod liver oil supplements are commercially available, they have not been approved by the FDA for medical use due to the lack of solid clinical research. Regulations set manufacturing standards for supplements but don’t guarantee that they’re safe or effective. Speak with your doctor before supplementing with cod liver oil.
Because cod liver oil is not approved by the FDA for any conditions, there are no strict guidelines on its recommended dosage. 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 [102, 103, 104].
- 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 others 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 [107, 108, 109].