ScienceWeek

MEDICAL BIOLOGY: OMEGA-3 FATTY ACIDS AND CANCER RISK

The following points are made by C.H. MacLean et al (J. Am. Med. Assoc. 2006 295:403):

1) Studies show that tissue levels of arachadonic acid and eicosopentaenoic acid (EPA)-derived eicosanoids influence many physiological processes, including calcium transport across cell membranes, angiogenesis, apoptosis, cell proliferation, and immune cell function.[1-4] These processes are integral to the immune system and hence the pathogenesis of autoimmune diseases such as arthritis, systemic lupus erythematosus, and asthma, as well as cancer. Epidemiological studies have suggested that groups of people who consume diets high in omega-3 fatty acids may experience a lower prevalence of some types of cancer,[5] and many small trials have attempted to assess the effects of omega-3 fatty acids on cancer treatment by adding omega-3 fatty acid to the diet either as omega-3 fatty acid rich foods or as dietary supplements. In addition, dietary omega-3 fatty acids have been found to modulate mammary tumor formation and proliferation in rodents.

2( In response to this evidence, a number of omega-3 fatty acid containing dietary supplements have appeared on the market claiming to protect against the development of a variety of conditions including cancer. To assess the validity of claims that omega-3 fatty acids prevent cancer, the authors systematically reviewed the literature for studies that evaluated the effect of omega-3 fatty acids on the incidence of cancer.

3) Articles published from 1966 to October 2005 were identified through MEDLINE, PREMEDLINE, EMBASE, Cochrane Central Register of Controlled Trials, and CAB Health; unpublished literature sought through letters to experts in the neutraceutical industry. A total of 38 articles with a description of effects of consumption of omega-3 fatty acids on tumor incidence, prospective cohort study design, human study population; and description of effect of omega-3 among groups with different levels of exposure in the cohort were included. Two reviewers independently reviewed articles using structured abstraction forms; disagreements were resolved by consensus.

4) The authors conclude: A large body of literature spanning numerous cohorts from many countries and with different demographic characteristics does not provide evidence to suggest a significant association between omega-3 fatty acids and cancer incidence. Dietary supplementation with omega-3 fatty acids is unlikely to prevent cancer.

References (abridged):

1. Baronzio G, Freitas I, Griffini P, et al. Omega-3 fatty acids can improve radioresponse modifying tumor interstitial pressure, blood rheology and membrane peroxidability. Anticancer Res. 1994;14:1145-1154

2. Avula CPR, Lawrence RA, Jolly CA, Fernandes G. Role of n-3 polyunsaturated fatty acids (PUFA) in autoimmunity, inflammation, carcinogenesis, and apoptosis. Recent Res Develop Lipids. 2000;4:303-319

3. Troyer D, Fernandes G. Nutrition and apoptosis. Nutr Res. 1996;16:1959-1987

4. Johnson IT. Anticarcinogenic effects of diet-related apoptosis in the colorectal mucosa. Food Chem Toxicol. 2002;40:1171-1178

5. Kato I, Akhmedkhanov A, Koenig K, Toniolo PG, Shore RE, Riboli E. Prospective study of diet and female colorectal cancer: the New York University Women's Health Study. Nutr Cancer. 1997;28:276-281

J. Am. Med. Assoc. http://www.jama.com

--------------------------------

Related Material:

ON FISH FATTY ACIDS AND THE RISK OF SUDDEN DEATH

The following points are made by C.M. Albert et al (New Engl. J. Med. 2002 346:1113):

1) It has been reported that fish consumption is associated with a reduced risk of sudden death from cardiac causes, but not a reduced risk of myocardial infarction, and it has been hypothesized that the long-chain n–3 polyunsaturated fatty acids found in fish, primarily eicosapentaenoic acid and docosahexaenoic acid, may be responsible for this association. Experimental data from studies in animals and at the cellular level suggest that these n–3 fatty acids have antiarrhythmic properties, and a recent randomized trial testing supplements of these n–3 fatty acids in survivors of myocardial infarction found a statistically significant 45 percent reduction in the risk of sudden death, with no effect on nonfatal myocardial infarction. However, prospective data on blood levels of long-chain n–3 fatty acids and sudden death from cardiac causes are sparse, and there have been no randomized trials of the effects of long-chain n–3 fatty acids in the diet or as supplements among persons without a history of cardiovascular disease, who represent over half of all cases of sudden death from cardiac causes.

2) The authors report a prospective, nested case–control analysis among apparently healthy men who were followed for up to 17 years. The fatty-acid composition of previously collected blood was analyzed by gas–liquid chromatography for 94 men in whom sudden death occurred as the first manifestation of cardiovascular disease and for 184 controls matched with them for age and smoking status. The authors report that base-line blood levels of long-chain n–3 fatty acids were inversely related to the risk of sudden death both before adjustment for potential confounders and after such adjustment. As compared with men whose blood levels of long-chain n–3 fatty acids were in the lowest quartile, the relative risk of sudden death was significantly lower among men with levels in the third quartile and the fourth quartile. The authors conclude that the n–3 fatty acids found in fish are strongly associated with a reduced risk of sudden death among men without evidence of prior cardiovascular disease.

References (abridged):

1. Albert CM, Hennekens CH, O'Donnell CJ, et al. Fish consumption and risk of sudden cardiac death. JAMA 1998;279:23-28.

2. Kang JX, Leaf A. Antiarrhythmic effects of polyunsaturated fatty acids: recent studies. Circulation 1996;94:1774-1780.

3. Billman GE, Kang JX, Leaf A. Prevention of sudden cardiac death by dietary pure omega-3 polyunsaturated fatty acids in dogs. Circulation 1999;99:2452-2457.

4. GISSI-Prevenzione Investigators (Gruppo Italiano per lo Studio della Sopravvivenza nell'Infarto Miocardico). Dietary supplementation with n-3 polyunsaturated fatty acids and vitamin E after myocardial infarction: results of the GISSI-Prevenzione trial. Lancet 1999;354:447-455. [Erratum, Lancet 2000;357:642.]

5. Kannel WB, Schatzkin A. Sudden death: lessons from subsets in population studies. J Am Coll Cardiol 1985;5:Suppl B:141B-149B.

New Engl. J. Med. http://www.nejm.org

ScienceWeek http://scienceweek.com