Cod liver oil is a popular type of fish oil supplement available in liquid and capsule form. The oil is rich in vitamin A, vitamin D, and omega-3 fatty acids. It's used to treat a wide range of conditions ranging from high blood pressure to depression and certain types of infection. Some people also use cod liver on the skin to aid in wound healing.
Some, but not all, of these uses are supported by scientific evidence. Cod liver oil products are widely available and the oil is also found in many natural products marketed as omega-3 supplements. You can also get the oil by eating the liver of a codfish—a common fish that lives near the bottom of the sea.
In the 1800s, cod liver oil became popular as a dietary supplement for children whose lack of sunlight exposure increased their risk for rickets, a disease caused by vitamin D deficiency. Today cod liver oil is a popular supplement used for a wide range of conditions.
Cod liver oil is one type of fish oil. If you buy a common fish oil supplement, the oil contained in the product usually comes from cold-water oily fish, such as salmon, herring, or sardines. Codfish oil is a slightly different type of oil, specifically sourced from the liver of the Pacific or Atlantic codfish. Each type of oil provides healthy omega-3 fatty acids.
Omega-3 fatty acids have been widely studied for their health benefits. According to the National Institutes of Health (NIH), the omega-3s available in different types of fish oil are probably effective for the prevention of heart disease. In addition, the NIH states that fish oil may help:
- Decrease blood pressure
- Lower cholesterol levels
- Ease asthma symptoms
- Manage rheumatoid arthritis
- Reduce menstrual pain
- Reduce stroke risk
- Protect against osteoporosis
- Slow the progression of atherosclerosis
However, the NIH does not specify whether cod liver oil in particular—rather than fish oil sourced from other types of fish—might be what produces these health benefits. Although research on the specific health effects of cod liver oil is somewhat limited, some studies show that cod liver oil may help with certain medical conditions.
Upper Respiratory Tract Infections
Cod liver oil may help prevent upper respiratory tract infections, according to a 2004 study of 94 young children. Children who received the cod liver oil and the multivitamin-mineral experienced a significant decrease in the number of upper respiratory tract infections (such as the common cold), while the control group showed no change in the typical number of infections. The patients in the treatment group also reduced the number of doctor's office visits for such conditions.
Fish oil, in general, is not associated with a decreased risk for insulin sensitivity or type 2 diabetes. However, there have been some small studies specifically looking at the association between cod liver oil and type 1 diabetes.
For people with rheumatoid arthritis, taking cod liver oil may reduce the need for non-steroidal anti-inflammatory drugs (NSAIDs). In an earlier study from Advances in Therapy, scientists discovered that taking cod liver oil may help alleviate stiffness, pain, and swelling in people with rheumatoid arthritis.
Vitamin D Deficiency
Increasingly, studies are showing that the vitamin plays a role in the prevention of many diseases and conditions that are more common as you age. Vitamin D is also essential for absorbing calcium and building bones. There have been mixed results when researchers have examined whether or not taking a cod liver oil supplement may help boost vitamin D levels, particularly in post- menopausal women. One population study showed that increased daily intake of a cod liver oil supplement is associated with higher serum levels of vitamin D. And another study on postmenopausal women showed that those taking a daily cod liver oil supplement are less likely to be vitamin D deficient during the winter and spring than those not taking a supplement. However, other research showed that the increased vitamin D levels gained through supplementation may not be enough to provide a benefit.
Possible Side Effects
Fish oil may cause a number of side effects including bad breath, heartburn, nausea, and belching. When overused, cod liver oil may lead to toxic levels of vitamins A and D. Additionally, high doses of fish oil may prevent blood clotting, damage the immune system, and increase blood levels of LDL ("bad") cholesterol. Cod liver oil may also interfere with the treatment of certain conditions. If you take medications or herbs for a health concern, especially for hypertension or to manage blood clotting, speak to your healthcare provider before taking this supplement. Lastly, it's important to note that avoiding standard medical care and self-treating a chronic condition with cod liver oil (or any other type of alternative medicine) may have serious health consequences.
Dosage and Preparation
There is no standard dose for fish oil or cod liver oil. Different amounts have been studied in research. For example, in a study investigating hypertension, test subjects consumed 21 milliliters (mL) orally daily for eight weeks. In another study, subjects consumed 5 mL orally with meals four times daily for six weeks. In a study investigating rheumatoid arthritis, test participants took one gram of cod liver oil daily for three months. A 10-gram mixture of cod liver oil and fish oil was used daily for 36 weeks in a different study. The right dose for you may depend on your age, sex, and health. Speak to your healthcare provider to get personalized advice.
What to Look For
If you would like to try cod liver itself, some specialty markets sell it canned, though it may be hard to find. Many say that the liver is mild and tastes nothing like the oil. Most, however, opt to consume cod liver as a supplement; it is sold in liquid and capsule forms. When choosing a product, it's worth reading the label to see if there are claims regarding purity or safety. Cod liver oil may contain other unwanted substances such as polychlorinated biphenyls (PCBs)—industrial products or chemicals that may negatively affect your health. Some agencies, including International Fish Oil Standards (IFOS), test fish oil supplements for the presence of toxins such as PCBs. Products that meet IFOS's standards are permitted to display the organization's certification mark on their labels.
You can also look for a recognized certification by organizations like ConsumerLabs, U.S. Pharmacopeia (USP), or NSF International. These organizations don't guarantee that a product is 100% safe or effective, but they do provide a certain level of testing for quality.
What's the difference between cod liver oil and other types of fish oil?
Cod liver oil contains a higher concentration of vitamin D than fish oil from other sources. For that reason, some medical experts have recommended that those with vitamin D deficiency choose cod liver oil rather than other types of fish oil supplements.
However, since some species of cod are thought to be endangered, getting your omega-3s from other types of fish oil (such as salmon) may be more ecologically responsible. If you're vegetarian/vegan, you can obtain omega-3 fatty acids from plant sources like flaxseed.
Is fermented cod liver oil better than non-fermented?
Proponents of fermented cod liver oil claim that this form of cod liver oil is more pure and higher in vitamin A, vitamin D, and omega-3 fatty acids than heat-processed forms of cod liver oil. However, there is currently a lack of scientific evidence to support this claim.