A new study will make people think twice about taking fish-oil capsules—or eating fish, for that matter. The American Journal of Epidemiology reports that men with higher levels of DHA, one of the omega-3 fatty acids found in fish oil, were at increased risk of developing prostate cancer. Researchers from Seattle’s Fred Hutchinson Cancer Research Center looked at 3,461 participants in the Prostate Cancer Prevention Trial and found that men with the most DHA in their bloodstreams were two-and-a-half times more likely to have an aggressive form of prostate cancer. Similar results were found in the European Prospective Investigation into Cancer and Nutrition study, where men who had the highest omega-3 levels had the highest risk for prostate cancer. In many recent studies, fish oil has not lived up to its marketing claims. Specifically, it is no help for heart patients, does not forestall Alzheimer’s disease, does not prevent depression, and—so far at least—does not make babies smarter. Back in 2005, a Journal of the American Medical Association report showed that fish oil may actually increase the risk of cardiac arrhythmia in some patients. In the same year, JAMA also reported that fish oil does not prevent cancer. The following year, the British Medical Journal reported that omega-3 fatty acids have no heart-health benefit. Among nearly 4,000 heart attack patients, no difference was seen between those who consumed omega-3 supplements and those who took placebo pills. That conclusion was echoed in 2009, when researchers found that consuming fish does not reduce the risk of heart failure. Then in 2010, the New England Journal of Medicine reported similarly dismal results with heart patients given omega-3 fatty acids in addition to standard drug therapy. They had no reduction in cardiovascular events. Surprisingly, Harvard linked fish and omega-3 fats to type 2 diabetes. Following 195,204 adults for 14 to 18 years, researchers reported in 2009 that they had found that the more fish or long-chain omega-3 fatty acids participants consumed, the higher their risk of developing diabetes. Meanwhile, fish oil manufacturers pinned their hopes on brain function. Maybe fish oil will make you smarter, they reasoned. But last year, what researchers found dashed those hopes, too. A group of 867 elderly people were randomly assigned to either a fish-oil supplement or placebo. After two years of supplementation, elderly adults showed no benefit at all in tests for reaction time, spatial memory, and processing speed measurements. A later JAMA report showed that omega-3 supplements do not slow mental decline in Alzheimer’s patients. And at the other end of the age spectrum, babies get no benefit either. A JAMA report showed that consumption of fish oil during pregnancy does not benefit babies’ cognitive development. In these reports, fish oil is starting to look a lot like snake oil. The new findings linking higher DHA levels to cancer add yet another reason to skip fish and fish oil supplements. Are you a health professional? Learn more about diet and cancer for continuing education credits at NutritionCME.org. In case you’re interested in the references for these studies, here they are:
Brasky TM, Till C, White E, et al. Serum phospholipid fatty acids and prostate cancer risk: results from the Prostate Cancer Prevention Trial. Am J Epidemiol. Published ahead of print April 24, 2011. doi: 10.1093/aje/kwr027.
Raitt MH, Connor WE, Morris C, et al. Fish oil supplementation and risk of ventricular tachycardia and ventricular fibrillation in patients with implantable defibrillators: a randomized controlled trial. JAMA. 2005;293:2884-2891.
MacLean CH, Newberry SJ, Mojica WA, et al. Effects of omega-3 fatty acids on cancer risk: a systematic review. JAMA. 2005;295:403-415.
Hooper L, Thompson RL, Harrison RA, et al. Risks and benefits of omega-3 fats for mortality, cardiovascular disease, and cancer: systematic review. BMJ. 2006;332:752-760.
Dijkstra SC, Brouwer IA, van Rooij FJA, Hofman A, Witteman JCM, Geleijnse JM. Intake of very long chain n-3 fatty acids from fish and the incidence of heart failure: the Rotterdam Study. Eur J Heart Fail. 2009;11:922-928.
Kromhout D, Giltay EJ, Geleijnse JM. n-3 fatty acids and cardiovascular events after myocardial infarction. N Engl J Med. 2010;363:2015-2026.
Kaushik M, Mozaffarian D, Spiegelman D, Manson JE, Willett WC, Hu FB. Long-chain omega-3 fatty acids, fish intake, and the risk of type 2 diabetes mellitus. Am J Clin Nutr. 2009;90:613-620.
Dangour AD, Allen E, Elbourne D, et al. Effect of 2-y n23 long-chain polyunsaturated fatty acid supplementation on cognitive function in older people: a randomized, double-blind, controlled trial. Am J Clin Nutr. 2010;91:1725-1732.
Quinn JF, Rama R, Thomas RG, et al. Docosahexaenoic acid supplementation and cognitive decline in Alzheimer disease. JAMA. 2010;304:1903-1911.
Makrides M, Gibson RA, McPhee AJ, et al. Effect of DHA Supplementation During Pregnancy on Maternal Depression and Neurodevelopment of Young Children. JAMA. 2010;304:1675-1683.
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