The Lowdown on Fish Oil Supplements: 5 Questions and Answers

Walk into any drugstore or natural foods emporium and you'll see jars of fish oil capsules on the shelves. The thought of swallowing fish oil may be unappetizing, but these supplements may offer some big health benefits, including a lower risk of heart disease.

But fish oil supplements raise some questions: Can you take them instead of eating fish? Do they increase your risk of mercury contamination? And, finally, how do you know if the fish oil supplement you've chosen is living up to its health promises? Here are answers to five of the most frequently asked questions about this popular supplement.

1. Why Is Fish Oil Important?

Fish contain two particularly beneficial fatty acids, docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA). DHA and EPA, part of a group of polyunsaturated fatty acids better known as omega-3 fatty acids, can significantly slash the risk of cardiovascular [heart and blood vessel] disease, according to Amitava Dasgupta, PhD, professor of pathology and laboratory medicine at the University of Texas Health Science Center at Houston. "[They] reduce the risk of a blood clot," he explains. "When you have a heart attack, those arteries [the tubes that pump blood from the heart] are blocked. If you can reduce the clotting of the blood, you reduce the risk."

Fish oil supplements, which contain varying amounts of DHA, EPA, and other omega-3s, have been shown to lower triglyceride (a type of "bad" cholesterol) levels by 20 to 50 percent. The supplements also may help

  • Lower blood pressure.
  • Relieve the pain of rheumatoid arthritis and menstrual cramps.
  • Lower the risk of stroke.
  • Slow the rate of bone loss.
  • Prevent full-blown psychosis in people with mild psychotic symptoms.
  • Prevent miscarriage.
  • Play a preventive role in a host of other conditions.

Fish oil supplements should contain about 200 to 800 milligrams (mg) of EPA and DHA combined.

2. How Do You Choose the Right Fish Oil Supplement?

The key to making sure your fish oil supplement is up to par is to choose one that hasn't been sitting on a shelf for too long. That's because the older the fish oil, the greater the likelihood that it has begun to oxidize, or become exposed to air. Oxidation changes the structure of DHA and EPA; what exactly this does to fish oil capsules is unknown, Dasgupta says, but it could render them less potent or even harmful. Dasgupta recommends you avoid any fish oil supplements with a manufacture date that exceeds six months prior. He also suggests buying the capsules in the smallest quantities possible to ensure freshness. Check each individual bottle for storage recommendations.

3. Can Fish Oil Supplements Replace Fish?

Not completely. "Nutrients from real food are always preferred over supplement form," says Lori Chong, RD, of The Ohio State University Wexner Medical Center in Columbus. According to the American Heart Association, people should consume fish at least twice a week and aim for a portion size of about 3.5 ounces cooked. Baking or broiling is recommended as a healthful alternative to frying.

Another reason to choose fish over fish oil capsules is that fish are able to prevent their own fatty acids from oxidizing. It's also worth mentioning that eating fish provides you with protein and other nutrients. That said, fish oil capsules are a worthy substitute if you just can't stand eating fish.

4. Who Should Not Take Fish Oil Supplements?

Fish oil is considered safe for most people, but there are exceptions. "If somebody has [blood] clotting problems or is on blood thinners, they probably should not take fish oil," maintains Nina Eng, RD, chief clinical dietitian at Plainview Hospital in Plainview, NY. "Fish oil is a thinner as well." Even for people without blood clotting disorders, it's important not to take too much fish oil without consulting a doctor. The recommended dosage is about one gram per day (or about 1,000mg, the unit of measurement generally used on fish oil labels), with amounts up to three grams considered safe.

5. Can you Get Mercury Poisoning From Fish Oil Supplements?

One form of the element mercury, methyl mercury, can be absorbed by fish. When ingested, mercury can build up in the bloodstream and interfere with the brain and nervous system. The Environmental Protection Agency recommends that children and women who may become pregnant avoid high-mercury fish including shark, swordfish, king mackerel, and tilefish.

But mercury poisoning from fish oil supplements is unlikely, according to Eng, who says, "Mercury is usually bound to the protein of the fish rather than the lipid [fat]." You may want to call the manufacturer to find out if the supplement’s ingredients have been tested by a third party, however. (The non-profit US Pharmacopeial Convention, USP, is one such third party.)

Amitava Dasgupta, PhD, reviewed this story.


Amitava Dasgupta, PhD, professor of pathology and laboratory medicine at the University of Texas Health Science Center at Houston. Phone call, October 19, 2014. 

Nina Eng, chief clinical dietitian at Plainview Hospital, Plainview, NY. Phone call with source, October 20, 2014.

"Fish Oil." National Institutes of Health. Accessed October 21, 2014. 

"What You Need to Know About Mercury in Fish and Shellfish." Environmental Protection Agency. Last updated November 20, 2013. 

"Mercury Contamination in Fish." Natural Resources Defense Council. Page accessed October 28, 2014. 

"Time to Talk Tips: 7 Things to Know About Omega-3 Fatty Acids." National Institutes of Health/National Center for Complementary and Alternative Medicine (NCCAM). Page last modified October 24, 2013.