A second concern has been raised as to the potential carcinogencitiy of long-term fish oil use, perhaps through the formation of oxidation products of the polyunsaturated fatty acids in the fish oil. However, no increase in the cancer rate among Eskimos has been reported, and some animal studies have suggested that fish oil may in fact have antineoplastic properties.- Furthermore, the antioxidant vitamin E is rountinely added to fish oil preparations currently available for clinical and experimental use as well as in the preparations now being made available to the public.
Fish oils may also have an immunosuppressive or anti-flammatory effect through its actions on monocytes, granulocytes, or other cells.- Suppression by fish oil of the rheumatologic symptoms in patients with rheumatoid arthritis has recently been reported. To date, there have been no reports, to our knowledge, of an increase in infections associated with the use of fish oil in animals or humans. in detail
Another potential problem with the use of fish oil is weight gain as a consequence of the increased caloric (9 cal/g) intake, increased cholesterol ingestion when some fish oils are used, and possible vitamin A or D toxicity due to the large amounts of these vitamins found in some fish oils such as cod liver oil. Several of these problems are now being addressed by the manufacture of fish oils that lack vitamin A and D and contain high concentrations of the omega-3 fatty acids with very low amounts of cholesterol. Another unwanted metabolic effect of fish oil that may occur in patients with type II diabetes mellitus is a fall in plasma insulin levels, leading to an increase in blood sugar concentrations.
Last, it is possible that the fish that are used as a source of the fish oil may ingest and concentrate heavy metals and organic chemicals that are a consequence of toxic waste disposal into the waterways or the seas of the world. Most fish oils on the market today have been carefully screened by their manufacturers before they are released for use and are free of these contaminants.
Clearly, continued monitoring is necessary to determine long-term effects or potential toxicities in the use of fish oil preparations in animals or humans. It should be noted that fish oils have been considered to be food products by the FDA, and therefore have not undergone the rigorous evaluation of safety and efficacy required of pharmaceutic agents in this country.
To date, the short-term use of smaller doses of fish oils in clinical studies has not produced significant toxicity. There has also been no evidence that the consumption of fish in the diet several times a week has been associated with long-term adverse effects.