STOCKHOLM, SWEDEN. Several test tube (in vitro) and animal experiments have clearly shown that the long-chain omega-3 polyunsaturated fatty acids (PUFAs) eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), the main components of fish oil, help inhibit the promotion and progression of cancer. Their beneficial effect is particularly pronounced in hormone-dependent cancers such as breast and prostate cancer. Some, but not all, epidemiologic studies have also found a beneficial effect.
Researchers at Sweden's famous Karolinska Institutet have just published a comprehensive review of the current knowledge regarding the role of PUFAs in carcinogenesis. They conclude that omega-3 PUFAs are protective against cancer progression, while omega-6 PUFAs, notably arachidonic acid and its derivatives, help promote the growth of cancer. They believe the n-3 PUFAs exert their beneficial effects in several different ways:
Free radicals and reactive oxygen species produced in cells may attack PUFAs resulting in the formation of
more free radicals, specifically hydroperoxides. The hydroperoxides, in turn, may damage DNA ultimately
leading to cancer. These effects have indeed been observed in some in vitro experiments, but not in
actual human beings. Many studies have shown that fish oils actually retard aging and suppress so-called
free radical diseases such as atherosclerosis and cancer. Other studies have shown that a daily EPA +
DHA intake in excess of 2.3 grams decreases the production of superoxide, a potent cancer promoter.
At least one in vitro and one animal experiment have observed that EPA + DHA kill human breast
cancer cells via the formation of hydroperoxides, but that this effect is strongly inhibited by vitamin E. Thus,
at this point, it is not entirely clear whether EPA + DHA exert part of their beneficial effect through an
increase or a decrease in the production of free radicals and reactive oxygen species. The researchers
recommend more work in this area, but emphasize that the major benefits of fish oils probably are
associated with their ability to inhibit the synthesis of arachidonic acid-derived, pro-inflammatory
eicosanoids. The Swedish researchers also confirm that fatty, cold-water fish are the best sources of EPA
and DHA and that the conversion rate of alpha-linolenic acid (flaxseed oil) to EPA is very low, even in
healthy humans – probably in the order of 2-5%.
Editor's comment: There would appear to be a growing body of evidence to the effect that long- chain omega-3 fatty acids, in particular EPA and DHA, help prevent the promotion and progression of certain cancers, notably hormone-dependent ones. Some of the mechanisms involved in this protective effect are well understood. While others, notably the role of free-radical formation, clearly need more work. Of some concern is the uncertainty surrounding vitamin E. Both vitamin E and fish oils have been found to help prevent hormone-dependent cancers, so taking both for cancer prevention is probably desirable. The situation is much less clear when it comes to slowing down an existing cancer and preventing it from spreading. Should one just rely on vitamin E (particularly the succinate form) or place one's faith in fish oils, or is the combination of the two the best way to go? Clearly more research in this area is urgently required.