More people are taking fish oil pills than ever before, yet very strong data show that fish oil pills do not reduce risk for heart disease. Ten large randomized studies following 77,917 people for an average of 4.4 years showed that taking fish oil pills does not protect against heart attacks, heart disease or major clotting events such as strokes (JAMA Cardiology, Jan 31, 2018). These data include studies on healthy people, those with prior history of heart attacks, diabetes or high cholesterol, and those taking statin drugs.
An earlier review of 21 studies showed that fish oil pills did not prevent or treat any form of heart disease (Curr Cardiol Rep, Jun 2017;19(6):47). Other studies have shown that neither fish oil pills nor eating fish reduce the risk for heart attacks (American Journal of Preventive Medicine, September 16, 2016; Am J Clin Nutr, Sept 14, 2016;104(4):951-952). From 2005 to 2012, at least 24 rigorous studies of fish oil pills were published and 22 showed no benefit at all in preventing heart attacks or strokes in high-risk populations: those who were obese, did not exercise, ate meat daily, smoked, had a history of heart disease, or had high cholesterol, high blood pressure or Type 2 diabetes (JAMA Intern Med, 2014;174(3):460-462). A clinical trial of 12,000 people found that fish oil pills did not reduce the rate of death from heart attacks and strokes (N Engl J Med, May 9, 2013; 368:1800-1808).
Why Do So Many People Believe That Eating Fish Prevents Heart Attacks?
Several studies have shown that eating fish is associated with reduced risk for heart attacks, but this does not mean that eating fish prevents heart attacks. It could be that people who eat lots of fish eat less red meat which is a known heart attack risk factor.
In the 1970s, Danish scientists Dr. Hans Olaf Bang and Dr. Jorn Dyerberg reported that the Inuits living in Greenland ate a lot of fish and had low rates of heart attacks. Later a University of Ottawa cardiologist, George Fodor, showed that the Inuits had the same heart disease rate as other populations. That didn't stop the American Heart Association from listing fish oils as a way to reduce heart attack risk. In 1999, 1.3 percent of Americans took fish oil pills, and in 2012 that number had gone up to 12 percent (JAMA, Oct 11, 2016;316(14):1464-1474). North Americans now spend more than $1.1 billion on fish oil pills per year.
Most fish oil pills contain two omega-3 fatty acids, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), that help to prevent clotting. Clots are the primary cause of heart attacks. First you suffer inflammation where your overactive immunity punches holes in the inner linings of your arteries. The holes bleed and clot, and plaques start to form in the healing clots lining your arteries. Having plaques does not cause a heart attack; a heart attack occurs after a plaque breaks off from the arterial wall. Then the damaged area bleeds and clots. The clot that forms in the place where the plaque broke off can block the blood flow to the heart. The part of the heart muscle that has had its blood supply shut off dies and you suffer a heart attack.
Since omega-3 fatty acids can reduce clotting, it is also possible that they could help to reduce inflammation, the process that punches holes in arteries to start the formation of plaques. The Food and Drug Administration has approved some fish oil pills to treat high triglycerides, another risk factor for heart attacks. However, in spite of these theories, no one has yet shown that fish oil pills reduce risk for heart attacks.
Oxidized Fish Oils Can Increase Risk for Heart Attacks
The reason that scientific studies have failed to show that fish oil pills prevent heart attacks may be that the harm from rancid fish oil pills offsets any benefits the pills may offer. The oil used in these pills is extracted from fish such as sardines, herring, mackerel, salmon or tuna. A study from Canada showed that during processing the oil is exposed to air to form oxidized (rancid) fish oil (J Nutr Sci, November 23, 2015;4:e36). This study found that 50 percent of Canadian brands of fish oil pills exceeded the voluntary limits for at least one measure of oxidation, and 39 percent exceeded the international voluntary safety recommendations for total oxidation. In other studies, 27 percent of fish oil products tested in South Africa were found to have more than twice the recommended levels of lipid peroxides (Cardiovasc J Afr, 2013;24:297–302), and more than 80 percent of supplements tested in New Zealand exceeded recommended levels (Sci Rep, 2015;5:7928).
Many foods that you eat and chemicals in your body are harmless, but when exposed to oxygen, they combine with oxygen to form harmful oxidized substances. That is why you hear so much about antioxidants that can help to get rid of the oxidized chemicals. An example of an oxidized substance that is harmful is the bad LDL cholesterol in your bloodstream that doctors use to predict susceptibility for a heart attack. Plain LDL cholesterol is harmless; it becomes harmful only after the fats in it are oxidized to form oxidized LDL cholesterol. See Cholesterol and Oxycholesterol
• I do not recommend taking fish oil pills. There is no proven benefit.
• If you decide to take fish oil pills, test each bottle to see if the oil is rancid. Oxidized fish oil smells like stale fish, so if you cut open a fish oil capsule and it smells or tastes fishy, it is rancid and you should throw the bottle away or return it to the store. Even if you buy a brand that you trust, check each bottle because one batch can be fresh while the next batch may be rancid.
• If your doctor recommends that you take fish oil, prescription pills are preferable to those sold over the counter because the omega-3 fatty acids are more concentrated and there is a higher level of quality control for the prescription-grade oils. However, even prescription fish oil pills should be checked to make sure they are not rancid. See Check those Fish Oil Pills
• Some studies show benefit from eating fish once or twice a week, and eating fish has not been shown to be harmful (even with possible issues of mercury or other contamination). However, no one has shown additional benefit from eating fish more often than twice a week.