A review of 79 randomized and controlled studies of more than 110,000 men and women, with or without heart disease, showed that omega-3 fats in fish oil or in cod liver oil pills, taken for one to six years, do not prevent heart attacks, strokes or deaths in general (Cochrane Review, July 18, 2018). Fish oil pills did lower triglycerides which may be healthful, but they also lowered blood levels of the good HDL cholesterol, which may be harmful.
These findings are similar to those of a study from the University of Oxford that reviewed 10 different trials of fish oil supplements on 77,917 older adults who were at increased risk for heart attacks. The fish oil pills, at doses ranging from 226 milligrams to 1,800 milligrams per day of omega-3 fatty acids, did not prevent heart attacks in people with previous heart disease or diabetes (JAMA Cardiol, Mar 2018;3(3):225–233).
Eating Fish May Help to Prevent Heart Attacks
The studies on fish oil pills did not evaluate the effects of eating fish on heart disease. Many studies have shown that eating fish lowers high triglyceride levels, and may reduce irregular heartbeats, lower high blood pressure, lower high heart rates and relax constricted blood vessels. However, several studies show that there is no evidence that eating fish more often than twice a week increases protection against heart attacks and death from heart attacks. Early reports that Eskimos who eat a lot of fish do not suffer heart attacks were seriously flawed; Eskimos actually suffer a very high rate of heart attacks.
There is no good evidence that the types of fish that do not contain omega-3 fatty acids help to prevent heart attacks. Fish that are rich in omega-3s tend to be cold-water fish such as salmon, sardines and mackerel, those that eat plankton in sea water or other fish that contain omega-3s. Commercially farmed fish that can be raised on corn, such as catfish and tilapia, do not contain omega-3s. Farmed fish that are fed the more expensive fish meal are good sources of omega-3s.
It is possible that eating fish helps to prevent heart attacks because the fish replaces eating red meat or processed meats, which are associated with increased risk for heart attacks. Taking fish oil pills does not provide this benefit.
Potential Harm from Fish Oil Pills
Omega-3 fatty acids are highly susceptible to oxidation that can convert them into oxidized fatty acids that increase risk for inflammation, arterial damage, heart attacks and some cancers. That could be the reason why the studies quoted above failed to show health benefits from fish oil pills. The oil used in pills is extracted from oily fish such as sardines, herring, mackerel, salmon or tuna. A study from Canada showed that during processing the oil can be 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 it 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 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 Oxycholesterol and Cholesterol
• I do not recommend taking fish oil pills. There is no proven benefit and there is potential for harm.
• 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