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Fish Oil versus Flax Seed Oil—Which Is Better?

2 March 2007
by Wyn Snow, Managing Editor

For strict vegans, the answer is clear: flax seed oil. For the rest of us, there are pros and cons on both sides.

Benefits of eating fish

Numerous authorities tout the health benefits of eating fish, especially fatty species which are rich sources of omega-3 fatty acids. Proven benefits range from lowering triglycerides and blood pressure and the risk of heart disease, to the more subtle effects of reducing chronic inflammation.

Some researchers are convinced that chronic inflammation at the cellular level is the underlying cause of most of today’s significant diseases, including diabetes and cancer as well as heart disease, high blood pressure, and strokes. Dr. Barry Sears of “Zone Diet” fame would add osteoarthritis and Alzheimer’s disease to that list.

Obtaining optimal levels of omega-3s, which are abundant in fatty fish, is especially important for children and women of childbearing years. Omega-3s are vital for brain development. Low levels during pregnancy and childhood can have detectable negative impacts on intelligence.

Risks of eating fish

Concerns have grown during the past several decades about toxic contaminants in fish. Recommendations from experts vary about how much fish to eat. Some claim that the benefits outweigh the risks. However, the FDA’s official advice is to eat fish twice a week, but not more often. This limit is especially important for young children and women who are pregnant or may become pregnant, because these toxins can impair neurological development.

Contaminants in fish can include mercury, other heavy metals including arsenic, industrial byproducts such as dioxins, industrial compounds such as PCBs, and pesticides such as DDT. Levels of various contaminants vary by species, by locale, and whether the fish are wild or farmed.

For example, mercury is lowest in salmon and highest in swordfish (see table below). In recent years, farmed salmon has been shown to contain more toxic chemicals than wild salmon, largely because of contaminants in the foods they are given. While some suppliers are taking steps to improve salmon farming conditions, it is not clear how widespread these changes are in the industry.

Average Mercury Concentration in Various Fish and Shellfish
Fish/Shellfish  (parts per million)
Swordfish   0.97
Albacore tuna   0.35
American lobster   0.31
Chunk light tuna   0.12
Cod   0.11
Pollock   0.06
Catfish   0.05
Salmon   0.01
Source: Northeast States for Coordinated Air Use Management; Food and Drug Administration.

How the body deals with contaminants

The human body can and does rid itself of contaminants. However, the process is slow for fat-soluble pollutants—which most of these are. It takes more than one year for mercury levels to drop significantly. For some pollutants, it can take as much as ten years to reduce the load by half. Typically, our intake rates of these substances are higher than our ability to get rid of them, so levels build up.

As many as 8% of US women of childbearing age have high mercury levels, which means approximately 300,000 babies born each year are at risk for various cognitive problems. Also, breast milk is one of the avenues the body uses to rid itself of organic pollutants. Women who breastfeed can transfer high levels of these chemicals to their infants along with the nutrition. Therefore, women who are breastfeeding or plan to do so need to be especially careful to minimize their exposure to mercury and other contaminants.

As a result of all these concerns, many people wonder: Is fish oil safe? And if so, is it safer to consume than fish itself?

Fish oil safety

Many omega-3 fish oil supplements do not contain these troublesome contaminants—as evidenced by ConsumerLab.com, Consumer Reports, and other independent testing organizations. However, some products have been found to be contaminated, especially from countries whose production standards are not rigorous.

If you are concerned about the quality of your favorite brand of fish oil, Dr. Barry Sears recently described a simple “toothpick poke test” that may reassure you. Puncture several capsules with a needle or pin, and squeeze the contents into a small cup-shaped container, such as a thimble. Set the thimble (or other container) in the freezer for 5 hours. If you can easily push the toothpick into the oil, Sears says that means it does not contain serious levels of contamination.

 NSF quality seal
NNFA - GMP quality seal

Another strategy is to look for quality seals on the label. Both the NSF and NNFA quality seals (shown at right) mean that the product does not contain heavy metals. Specifically, any lead, mercury, arsenic, cadmium and/or chromium that might be present must be at levels below the standards set by the World Health Organization. In order to quality for the NSF and NNFA quality seals, any claims on the label that the product does not contain other contaminants must be supported by independent laboratory tests. Also, supplement products consisting of 2% oil or more must pass two tests for the presence of oxidation and rancidity.

Some manufacturer labels that do not have these quality seals do claim their product does not contain PCBs, mercury, lead, or various other contaminants. They should have reports from independent, third-party labs that verify this claim. You can ask to see a copy of these reports. If they refuse to supply this documentation, you can complain to the Federal Trade Commission (FTC)—which is charged with making sure that companies have evidence to support their marketing claims.

Given these concerns about the quality of fish oils, many consumers are wondering whether they should continue taking fish oil or rely instead on flax seed oil.

Is fish oil “better” than flax seed oil?

In one respect, fish oil is definitely “better” than flax seed oil. Fish oil contains two omega-3s that are especially important: EPA and DHA. The body uses EPA to create many hormone-like substances that reduce inflammation and other “excited” states in the body, such as raised blood pressure. Also, eight percent of the brain is composed of EPA and DHA, and one wants to be sure this 8% stays healthy!

Taking fish oil can guarantee that the body gets enough of these two vital omega-3s.

However, Dr. Udo Erasmus, author of Fats that Heal, Fats that Kill, claims that the process manufacturers use to produce most vegetable cooking oils—a process often used to remove contaminants from fish oils as well—is itself destructive to the quality of the oil. According to Erasmus, oils that have undergone this refining, bleaching and deodorizing process “contain 0.5 to 1.0% damaged, highly toxic molecules.” On the other hand, Erasmus manufactures and sells a competing product, so such statements may be convenient marketing claims rather than independently verified scientific fact.

If you want to avoid oils that have been exposed to this refining, bleaching, and deodorizing process, look for either cold-pressed or unrefined on a product’s label. Both terms mean that a mechanical process was used to extract the oil rather than chemicals.

Benefits of flax oil

Flax seed oil contains an omega-3 called alpha-linolenic acid (ALA), which is one of two fatty acids that the body needs and cannot make for itself. Several other sources of ALA do exist, most notably walnuts and hemp seed. Omega-3s are needed by every cell in the body. Among other things, an ample supply helps ensure that cell membranes stay flexible so that cells can get nutrients easily.

The body can use ALA to make all the other omega-3 fatty acids that it needs, including both EPA and DHA. Thus, if you get enough ALA, you don’t need to eat any other sources of omega-3s.

Another advantage of getting one’s omega-3s from the ALA in flax oil is that the body does not create more EPA and DHA than it needs. Therefore, ingesting too much EPA/DHA is not an issue.

The human body uses a variety of omega-3s, not just EPA and DHA. To make the full range of these omega-3s, the body needs ALA from flax oil (or walnuts or other sources) in addition to EPA and DHA. Thus, one needs to consume some ALA even if fish and/or fish oil are plentiful in one’s diet.

Is flax oil “better” than fish oil?

Since one needs ALA anyway, and the body can make all the other omega-3s it needs from ALA, does that mean flax seed oil is a better source than fish oil for one’s omega-3s? Not necessarily.

The body uses various enzymes to convert ALA to other omega-3s, and the process is not very efficient, especially as one gets older. Estimates of the rate of conversion range from 5% to 25%. In order to make sufficient amounts of EPA and DHA, one needs to consume 5 or 6 times more ALA than if one relies on fish oil alone. Also, women convert ALA to the other omega-3s more efficiently than men, largely so they can meet the nutritional demands of their infants during pregnancy and breastfeeding.

Another consideration is that ALA competes metabolically with the other essential fatty acid that the body cannot make for itself. Linoleic acid (LA) plays the same role for omega-6 fatty acids that ALA does for omega-3s: The body uses LA to make all the other omega-6s that it needs. (To understand the difference between an omega-3 and omega-6 fatty acid, see our article Chemical and Physical Structure of Fatty Acids.)

By competes, we mean that when LA is oversupplied in the diet, the body makes more of the LA-derived fatty acids than it needs, and not enough of the ALA-derived ones. The “LA side” of these substances help us react to dangers and stress and are therefore crucial to our health and survival, but when they are oversupplied, the result is chronic high blood pressure, cellular inflammation, and other conditions leading to today’s panoply of degenerative diseases.

Unfortunately, the recent emphasis on vegetable oils has led to a 10:1 ratio between LA and ALA in the American diet. While there is no consensus yet on what an optimal ratio would be, estimates range from 4:1 to 2:1. Consuming smaller amounts of the omega-6 LA helps the body maintain a healthy balance between the “stimulating” LA substances and the “calming” ALA substances. One excellent method of improving this ratio is switching to monounsaturated oils like olive oil. While canola oil does contain some ALA, it also contains a higher level of LA, so is not a recommended method of improving one’s LA-to-ALA ratio.

How much do we need?

For healthy adults, the recommendation is 300-500 mg per day of EPA and DHA combined, plus an additional 800 to 1100 mg of ALA.

The EPA/DHA recommendation can usually be met with one softgel capsule of fish oil (with 1 gram or 1000 mg of fish oil) which usually contains 180 mg of EPA and 120 mg of DHA, totalling 300 mg of the two omega-3s. However, amounts do vary (some products are stronger, some weaker), so look at the amounts of EPA and DHA provided, and add them together to see if the product supplies 300 mg in one serving.

Dr. Barry Sears further recommends that people with diabetes, osteoarthritis, and heart disease take twice that amount of fish oil. He also recommends that people with cancer take four times that amount. However, people with congestive heart failure should not be taking large quantities of fish oil, see toxicity discussion below.

While cod liver oil is a potent source of EPA/DHA, containing as much as 1000-1200 mg in one tablespoon, it is also a concentrated source of vitamins A and D. Both vitamins A and D are fat-soluble, and become toxic at high dosage levels. As noted earlier in this article, the body does not easily rid itself of fat-soluble substances, so prudence is called for.

For vegans and other health-conscious consumers, another option is vegetarian DHA supplements derived from algae oil. Fish get their DHA by feeding on algae.

Flax seed oil contains 8 grams (8000 mg) of ALA per tablespoon.

ALA Content of Selected Foods
Food  ALA (grams)
Flaxseed oil, 1 Tbsp.   8.0
Hempseed oil, 1 Tbsp.   2.7
Canola oil, 1 Tbsp.   1.6
Soybean oil, 1 Tbsp.   1.0
Walnuts, 1 oz.   2.7
Flaxseeds, 1 Tbsp.   2.6
Soybeans, 1 cup cooked   1.1
Leafy greens, 1 cup raw   0.1
Wheat germ, 2 Tbsp.   0.1

Are there any toxicity concerns?

Ingesting too much of anything—including water!—can cause problems. Medline recommends that “Patients taking more than 3 grams of omega-3 fatty acids from supplements should do so only under a physician’s care.”

Concerning the potential impact of high dosages, the National Institutes of Health (NIH) say:

“Very large intakes of fish oil/omega-3 fatty acids (‘Eskimo’ amounts) may increase the risk of hemorrhagic (bleeding) stroke. High doses have also been associated with nosebleed and blood in the urine. Fish oils appear to decrease platelet aggregation and prolong bleeding time, increase fibrinolysis (breaking down of blood clots), and may reduce von Willebrand factor.”

As for the impact on type II diabetes, research indicates that taking fish oil might result in a slight increase to fasting blood glucose levels, and/or a slight decrease to blood sugar levels—presumably during the day! Thus, people taking insulin or other drugs for diabetes should be supervised by their doctor when changing their intake of fish oil or ALA.

People with congestive heart failure—or any other condition where the heart is receiving insufficient blood flow—should consult with their doctor and be cautious about consuming omega-3 fatty acids. In congestive heart failure, according to Wikipedia, “Cells that are only barely receiving enough blood flow become hyperexcitable,” which can lead to irregular heartbeats and sudden cardiac death.

It appears that omega-3s stabilize heart rhythm by removing these hyperexcitable cells from functioning. This is beneficial for people with plenty of heart cells to spare, but in congestive heart failure, removing hyperexcited cells can mean the heart no longer pumps enough blood for the patient to survive.

In conclusion

Both fish oil and flax seed oil have benefits and potential drawbacks. Fish oil is an excellent and usually uncontaminated source of EPA and DHA, which the body uses to make the “calming” omega-3 fatty acids and keep the brain healthy. Consuming them directly can ensure that one gets enough. Flax seed oil contains ALA, which the body can use to make all the omega-3s that it needs. The body needs ALA to make other omega-3s, even when it gets enough EPA and DHA from fish or fish oils.

As for drawbacks, some fish oil products are contaminated, and even those that are not may have undergone a cleaning process that creates a small percentage of toxic molecules. On the other hand, getting all one’s omega-3s from flax oil means that one needs to consume significantly more. Also, it is possible to ingest too much omega-3s, even though the greater health risk is of consuming too much omega-6 LA. Also, people with congestive heart failure should take omega-3s only with the full knowledge and active supervision of their physician.

In conclusion, why limit oneself to either/or when it’s better to have both/and? Eating a modest amount of fish or fish oil (or algae-based DHA supplements) ensures a direct supply of EPA and DHA, while adding flax seed oil to one’s diet ensures a healthy intake level of ALA. Every cell in your body will thank you for it.

A physician comments on the fish oil–flax seed oil debate

Epidemiologic evidence has pointed to the cardiovascular benefits of fish and fish oil for a very long time [1], gaining momentum in the 1970s with the observation that Greenland Inuits, with a very high omega-3 intake and very high fat diets had a very low risk of cardiovascular disease, but it was not until the large and well-conducted Italian GISSI Prevenzione [2] trial in 1999 which showed that a very modest 1gm supplementation of EPA & DHA significantly reduced mortality in post-heart attack patients that the medical establishment took widespread notice of fish oil.

As far as I know, there is no similar evidence base in favor of flax seed oil. Therefore, I would have a hard time saying that flax seed has nearly as much to offer as fish oil. We can extrapolate and theorize that flax seed oil should confer a similar benefit, based on its (much lower) omega-3 content, enough to recommend it as an alternative to fish oil for strict vegans, but I would call it clearly second best.

The reference to fish oil lowering von Willebrand factor [vWF] is probably correct, based on the 1987 study at Brown [3] which showed that vWF was significantly lowered by fish oil in type-1 diabetics, who as a group are at increased risk for stroke and heart attack, possibly because of increased vWF, which is known to be increased in first-time ischemic stroke victims, regardless of diabetes, and is a marker of endothelial stress. Perhaps more significantly, triglycerides, which are associated with cardiovascular disease, and which are notoriously elevated in type-2 diabetics, are lowered by fish oil. (This is the only FDA-approved indication for fish-oil, in the form of the eight to ten times more expensive prescription extract Omacor, for treating severe hypertriglyceridemia.)

While bleeding times are slightly increased and platelet activity is decreased by fish oil, there are no demonstrated adverse health outcomes linked to this bleeding. A much more potent anti-platelet drug, low-dose aspirin, increases bleeding much more, but is taken by millions of people with an elevated risk of cardiovascular disease because it is proven to significantly lower overall risk, including that associated with bleeding.

Dr. Peter Everett is a founder of the Dietary Supplement Quality Initiative, sponsor of supplementquality.com


1. Kromhout D, Bosschieter EB, de Lezenne Coulander C. The inverse relation between fish consumption and 20-year mortality from coronary heart disease. N Engl J Med, 1985 May 9;312(19):1205-9.

2. Gruppo Italiano per lo Studio della Sopravvivenza nell'Infarto miocardico. Dietary supplementation with n-3 polyunsaturated fatty acids and vitamin E after myocardial infarction: results of the GISSI-Prevenzione trial. Lancet, 1999 Aug 7;354(9177):447-55.

3. Miller ME, Anagnostou AA, Ley B, Marshall P, Steiner M. Effect of fish oil concentrates on hemorheological and hemostatic aspects of diabetes mellitus: a preliminary study. Thromb Res, 1987 Jul 15;47(2):201-14.


American Heart Association. "New Guidelines Focus on Fish, Fish Oil, Omega-3 Fatty Acids." Nutrition Perspectives, University of California at Davis, Volume 27, No. 6, November/December 2002.

Claudette Bethune, Ph.D. "Norwegian farmed salmon production raises global concern." EnvironmentalChemistry.com, May 31, 2006.

Debra Fulghum Bruce, PhD. "Are Fish Oil Supplements Safer Than Eating Fish?" LE Magazine, October 2005.

Karen Collins, R.D. "Fish isn't the only way to get ‘good’ fat omega-3. It's also found in seeds and nut oils, but is it as effective?" MSNBC.com: Nutrition Notes, September 8, 2006.

EnvironmentalDefense.org. "Farmed Salmon Purchasing Policy: What It Means for Consumers." EnvironmentalDefense.org, March 13, 2006.

Udo Erasmus, PhD. "Humans Turn ALA to EPA/DHA." UdoErasmus.com, [date not clear, includes references published in 2002].

FDA: Center for Food Safety & Applied Nutrition. "What You Need to Know About Mercury in Fish and Shellfish." FDA website, 2004.

Food and Nutrition Board of the Institute of Medicine of the National Academies of Science. "Consumers Need Better Guidance to Fully Weigh Possible Benefits and Risks When Making Seafood Choices." NAS website, October 17, 2006.

Vesanto Melina, MS, RD. "Omega-3s, EPA & DHA. Which fats do we need? What are our best sources? Fats: Brain Food and Much More." Nutrispeak.com,[no date].

Natural Standard Research Collaboration. "Omega-3 fatty acids, fish oil, alpha-linolenic acid." National Institutes of Health website, November 1, 2006.

NewsHour, PBS. "Salmon Scare." ScienceNews.org, January 26, 2004.

Julie J. Rehmeyer. "Salmon Safety." Science News, January 20, 2007.

Barry Sears, PhD. Private communication, presentation at Newton-Wellesley Hospital sponsored by the National Institute of Whole Health, December 2, 2006.

Minh-Hai Tran. "Fish oil supplements: finding the one that's right for you." Better Nutrition, March 2005.

David Wetzel. "Cod Liver Oil -- Notes on the Manufacture of Our Most Important Dietary Supplement." [no date].

Wikipedia. "Essential fatty acid." Wikipedia, [no date].

Wikipedia. "Oily fish." Wikipedia, [no date].

Wikipedia. "Omega-3 fatty acid." Wikipedia, [no date].end-of-story


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