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As one of the most common sources of omega-3 fatty acids, fish oil has been well researched, and has been shown to aid heart and brain health. But you can't just pop a pill every day and expect to see benefits. Depending on the type of fish used, you might have to take several pills per day to reach recommended doses. The omega-3s in fish oil and cod liver oil may prevent clots from forming, so if you have a bleeding disorder or liver disease or take blood thinners -- such as Coumadin (warfarin), Plavix (clopidogrel), Lovenox (enoxaparin) or aspirin -- you should check with your health care provider before using them.

Just as the type of fish used in fish oil can vary, so can the amount of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). That's why you may have to take as many as 10 capsules a day to obtain your desired dose. It's important to read the label and the ingredients because supplements from different manufacturers aren't identical. There's also potential for an allergic reaction depending on the individual and the type of fish used to make the oil.

Cod liver oil has similar health benefits to fish oil, but it can contain high levels of vitamins A and D. When taken above the upper limits -- 10,000 IU for vitamin A and 4,000 IU for vitamin D for adults -- these fat-soluble vitamins can cause health problems.

The most common side effects of supplementing with fish oil and cod liver oil are burping, nausea and a fishy taste. These can be reduced by taking an enteric-coated capsule that releases its contents after passing through the stomach or by taking the supplement with a meal. Contaminants are usually negligible to undetectable since fish lower on the food chain are often used to make the oil.

Multiple fish oil and cod liver oil supplements have been verified by the United States Pharmacopeia (USP). For more on USP verification, see What to Look For.

Fish oil: Reading the label

Fish oils, especially cod liver oil, have been used for centuries and make up the majority of omega-3 fatty acid supplements. The most common types of fish used to manufacture fish oil are anchovy, halibut, herring, mackerel, sardine, salmon and tuna; cod liver oil, of course, comes from cod. While fish oil and cod liver oil may contain small amounts of alpha-linolenic acid (ALA), they're primarily composed of EPA and DHA.

It's the EPA and DHA that provide health benefits, and their ratio to one another is one way to determine if your supplement is high-quality: Look for a ratio of either three parts EPA to two parts DHA or vice versa. For instance, a supplement containing 650 mg EPA and 450 mg DHA fits this ratio. Checking the label for the type of fish used may help. Menhaden (a type of herring), for example, has 1.5 times as much EPA as DHA while salmon has more DHA than EPA.

If you have any known allergies to fish or shellfish, reading the supplement label is especially important. While a product can be labeled as either fish oil or krill oil, closer examination may find that it contains both. This can be dangerous for those with shellfish allergies because krill are shrimplike crustaceans. While allergic reactions usually occur in response to the protein in the fish, they can also occur when only the oil has been consumed.

In addition to EPA and DHA, fish oil supplements may also contain small amounts of ALA, calcium, iron and vitamins. Traditionally, a small amount of vitamin E is used to help stabilize the oil and prevent it from becoming rancid. Cod liver oil or any fish liver oil will have more vitamins and at higher levels -- particularly vitamins A and D -- than regular fish oil. Even though water-soluble vitamins such as vitamin C can usually be taken in excess without concern, the fat-soluble vitamins A, D, E and K can be stored in bodyfat, and excessive amounts may create health problems and even birth defects. If you choose cod liver oil, check the amount of vitamins in each serving and avoid supplements that list Daily Value percentages of more than 75 percent. The upper tolerance limit for vitamin A is 3,000 IU daily and 4,000 IU daily.

Contamination of fish oil supplements by methyl mercury, polychlorinated biphenyls (PCBs) and dioxins is usually at trace levels due to product processing. Supplements that are verified by the USP and by ConsumerLab.com will include data as to the exact amount of each contaminant. For more on contamination, see our What to Look For page.

Forms of fish oil supplements

Fish oil and cod liver oil supplements come in three forms: softgels, liquid and enteric-coated capsules. All are equally effective, but the amount you need to take and the side effects may vary. For example, one study found that the median amounts of EPA and DHA in a softgel was 216 mg and 200 mg, respectively; in contrast, the median amounts in a serving of liquid fish oil are 460 mg and 400 mg, respectively. For the same benefits, you could swallow fewer pills and pay less. However, the complaint of a fishy aftertaste occurs more frequently with the oil form than with softgels. Enteric-coated capsules are designed to not release their contents until after stomach digestion and closer to the small intestine, where the body absorbs omega-3 fatty acids. In addition, enteric-coated capsules may reduce the common side effects of burping, nausea and fishy taste.

Fish oil brands

According to the Natural Medicines Comprehensive Database, there are thousands of fish oils on the market but only 11 are verified by the USP. They include products made by Sunmark, Berkley & Jensen, Equaline, Nutri Plus, Kirkland Signature and Nature Made. Of the hundreds of cod liver oil supplements lining store shelves, five have been verified -- Nature Made, Sunmark and Your Life products. For more on USP verification and to see if your supplement is USP-verified, see USP Dietary Supplements.

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