Safety and Benefits of Fish Oil

OXFORD, UNITED KINGDOM. Animal studies have shown that an increase in fat intake can decrease the number of natural killer (NK) cells found in the blood and spleen. NK cells are an integral part of the natural immune response to virus infections and certain types of cancer. Researchers at Oxford University now report that fish oil significantly decreases NK cell activity in healthy human subjects.

Their clinical trial involved 48 men and women aged 55 to 75 years. The participants were randomized to receive one of six supplements for 12 weeks. The supplements were all provided in the form of capsules, three of which were to be taken with each meal. The nine capsules (daily intake) contained either a total of 2 g alpha-linolenic acid, 770 mg gamma-linolenic acid (from evening primrose oil), 680 mg arachidonic acid, 720 mg docosahexaenoic acid (DHA), 720 mg eicosapentaenoic acid (EPA)+ 280 mg DHA (fish oil) or a placebo (an 80:20 mix of palm and sunflower oils). All the participants had blood samples taken four weeks before start of supplementation, immediately before start of supplementation, and then every four weeks during the trial as well as after a four-week washout period. The researchers found no changes in killer cell activity except in the group taking fish oil. Here they observed an average decline of 20 per cent after 8 weeks and 48 per cent after 12 weeks. The decline was completely reversed after the washout period. The fact that no decline was observed with pure DHA strongly suggests that EPA was responsible. The researchers conclude that an excessive EPA intake could have adverse effects for people at risk of viral infections and some cancers.

Editor’s Note: The British researchers’ speculation about fish oils perhaps affecting the effectiveness of NK cells in killing cancer cells is at odds with the results of many other studies. There are at least a dozen studies that show a clear protective effect of fish or fish oil against breast, colon, and prostate cancer.
Note: This study was partly funded by Unilever. [54 references].

Thies, Frank, et al. Dietary supplementation with eicosapentaenoic acid, but not with other long-chain n-3 or n-6 polyunsaturated fatty acids, decreases natural killer cell activity in healthy subjects aged >55 years. American Journal of Clinical Nutrition, Vol. 73, March 2001, pp. 539-48

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