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How much fish oil is too much?

MICHIGAN STATE (US) — Fish oil—long encouraged by doctors as a supplement to support heart and joint health—induces severe colitis and colon cancer in mice in new research.

“We found that mice developed deadly, late-stage colon cancer when given high doses of fish oil,” says Jenifer Fenton, a food science and human nutrition researcher at Michigan State University.

“More importantly, with the increased inflammation, it only took four weeks for the tumors to develop.”

The study, published this month in the journal Cancer Research, supports establishing a dose limit for docosahexaenoic acid (DHA), one of the omega-3 fatty acids present in fish oil, particularly in people suffering from chronic conditions such as inflammatory bowel diseases.

Specifically, the research team found an increase in the severity of the cancer and an aggressive progression of the cancer in not only the mice receiving the highest doses of DHA but those receiving lower doses as well.

The mice used in the study were prone to inflammatory-like bowel disease—an important risk factor for many types of cancers, including colon cancer.

“Our findings support a growing body of literature implicating harmful effects of high doses of fish oil consumption in relation to certain diseases,” Fenton says.

“Currently, there is a call by academics and the food industry to establish dietary guidelines for omega-3 consumption. This is primarily motivated by the fact that most Americans are deficient in omega-3 fatty acids, and there is substantial evidence supporting the beneficial effects of the consumption.”

The findings were surprising, specifically because DHA has been shown to have some anti-inflammatory properties, Fenton says.

“We hypothesized that feeding fish oil enriched with DHA to mice would decrease the cancer risk; we actually found the opposite. These mice were less equipped to mount a successful immune response to bacteria that increased colon tumors.”

Fenton cautions people may not need to avoid fish oil; what the research shows is needed are guidelines on dosing. With any nutrient, there is a “bell curve” effect. On the left of the curve are those deficient in a nutrient; on the right are those in excess.

People already receiving enough omega-3 fatty acids through their normal diet and foods have no need for added supplementation.

“With fish oil, we don’t yet know how much is appropriate,” says Fenton. “There are many examples of taking supplements, nutrients, or chemicals in excess that can promote cancer (for example, beta-carotene supplementation in smokers). Supplementation is most useful when the person taking them is deficient in that specific nutrient.”

The research team’s findings could have an important preventive health impact, specifically in light of the high rates of colon cancer in the United States. Individuals with inflammatory bowel disease have an increased risk of developing colon cancer, and when the cancer metastasizes it can be fatal.

The next step, Fenton says, is to test omega-3 levels on people with inflammatory bowel disease. “To help develop guidelines, we need to see how these findings correlate to human populations.”

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chat13 Comments


  1. Mark Zulewski

    An important thing to also look out for when choosing fish oil products is the presence of heavy metals such as mercury. You often get what you pay for … so look for “Heavy metal-free” or other similar products, –although they cost more, it is a small price to pay.
    “Mark Zulewski”

  2. Mario De La Torre

    Omega-3s are some of the most polyunsaturated fats. Like all PUFAs, they are less stable chemically and are most prone to rancidity and damage by oxygen and temperature.

    Omega-3 are no magic bullet as PUFAs require mediation by means of the body’s antioxidants. Else the double bonds can cross-link into harmful oxidants. The balance of omega-6 to omega-3 is likely key to cancer and inflammation due to their cascade effect in eicosanoid production.

  3. Dr. O'

    A point to consider is the genetics of the person taking fish oil. Someone who has no family history of inflammatory bowel disease probably could tolerate higher levels of supplementation than someone with multiple ancestors with colon cancer. As Mario noted, the questions of chemical instability and the ratio of omega 3 to omega 6 are also important. Many questions and many cross references to evaluate.

  4. Ronald W Gumbs

    I prefer to get my fish oil directly from fish; wild salmon, blue fish, sardines, rather than from supplements. I have a problem with supplements. So, I eat Brazil nuts for selenium because this metal is very toxic as a supplement. I love making cabbage soup and broth as a vehicle for other nutritious vegetables, some of which contain compounds that kill human cancer cells in vivo. If food contributes to the death of cancer cells in the human body, I am very much interested in its nutritional data.

    Let food be thy medicine, and you will not need supplements or pharmaceuticals, pushed by physicians and peddled by snake oil salesmen.

    There is a National Center for Complementary and Alternative Medicine that is grossly underfunded. It is responsible for conducting clinical studies in this area. With increased funding, more clinical studies with larger populations of human subjects, rather than rats, will help to reduce the number of deaths from chronic diseases here in the US and abroad. It is a shame that it only receives $125 million per year.

  5. Tony - KS

    As has been said, “the dose is the poison.”

    Which is why variety in the diet is also good, and perhaps variety in the genealogy….

  6. Lin and Ray Tetrault

    We are vegetarians, so we take Flaxseed oil instead of Fish oil. We’d like to see some data on that.
    Also, we agree that the Center for Complementary and Alternative Medicine needs much more funding.

  7. Greenojolly

    Ive been reading that you will be better off getting omega 3 from supplements then fish alone because of the high level of toxic in them. But i think farm raise fish are a lot less toxic. Food we consume are less nutrient these days because of the soil we all grow them on so this is why we have supplements. Less quality food….

  8. Paulette Chitwood

    Everywhere I look, “Warning! Too much fish oil is highly dangerous even though medical community agrees it has benefits for.heart health.” Nobody, however will publish how much, saying they don’t KNOW how much is too much. So much scientific double talk.

  9. emc2

    I’ve read that omega 3 contains both DHA and EPA, and that it may be the DHA that is causing problems while the EPA may be the good stuff that kills cancer and decreases inflammation. Cardiologists have found that DHA raises Low Density Lipids, which is bad, and they’re finding that EPA alone leads to the best results. Other researchers have found EPA alone cures a kind of chronic lukemia in mice. My question is, how much of this problem described above is due to DHA rather than omega 3/fish oil as a whole? OK, so I’m still hoping for a miracle supplement.

  10. mehray

    How much FISH oil is too much?

  11. E

    emc2 –
    Then why is DHA proposed to be given to babies as they develop?

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  13. LEH

    “The mice used in the study were prone to inflammatory-like bowel disease—an important risk factor for many types of cancers, including colon cancer.”

    Why were they using cancer prone mice in this study? What would the results be if they didn’t use cancer prone mice?

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