Toxic Effects of Fish Oils

To the digestive System

  • The fish Oils in the form of concentrated supplements may cause Diarrhea in some people.
  • The oils in the form of concentrated supplements may cause temporary Gastrointestinal
    discomfort in some people.

To metabolism

  • The fish Oils may increase LDL Cholesterol ( bad cholesterol) levels after consumption of 12
    grams of Fish Oils per day. This causes an average increase in LDL Cholesterol levels of 8.5%.
    This increase can be negated by consuming garlic concurrently with the oils
    This is presently believed to be the only detrimental effect of Fish Oils and is not a reason to avoid
    consumption of Fish Oils as Fish Oils possess numerous other health benefits.

Fish Oils may Interfere with these Substances

  • The oils when taken regularly may cause the depletion of vitamin E. It is therefore advisable to
    consume additional vitamin E to counteract this depletion

Contamination of Fish Oils

There have been concerns raised that the fish oils do contain contaminated substances such as mercury
and organochlorines. The most contaminated fish include king mackerel, swordfish, shark and tilefish.
Studies have shown that the oils generally do not contain these contaminants as feared.

According to the World Health Organization (WHO) and the Food and Drug Administration (FDA),
concentrations of methylmercury (MeHg) in various species of fish cover a wide range, from less than
0.01 parts per meter (ppm) to over 3.0 ppm fresh weight, depending on factors such as pH, the
redox potential of the water, and the species, age and size of the fish. Human hair analysis for mercury
is a useful biomarker for determining long-term exposure to mercury from fish and non-fish food


In a study done by Foran, and his colleagues (The measurement of mercury levels in concentrated
over-the-counter fish oil preparations: is fish oil healthier than fish?) They found out that the levels of
mercury in the 5 different brands of fish oil ranged from nondetectable (<6 microg/L) to negligible (10-
12 microg/L). They also concluded that the mercury content of fish oil was that same as that found in
human blood. It was discussed that fish such as swordfish and shark are also a source of exposure to
the heavy metal toxin, mercury. The fish oil brands examined in this manuscript have negligible
amounts of mercury and may provide a safer alternative to fish consumption.

References: Arch Pathol Lab Med. 127(12):1603-1605, 2003.
Division of Laboratory Medicine, Department of Pathology, Massachusetts General Hospital and
Harvard Medical School, Boston, Mass, USA.

Other studies were done by Melanson and colleagues
Ref: Arch Pathol Lab Med. 129(1):74-77, 2005.
Clinical Laboratories Division, Department of Pathology, Brigham and W omen’s Hospital and
Harvard Medical School, Boston, MA, USA.