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food for thought

New research is questioning if Omega-3 fatty acids in fish reduce inflammation, lower blood triglycerides (fats) and make the blood less likely to clot.LINDA XIAO/The New York Times News Service

We’ve been told for years to eat fish, especially oily fish, twice a week to guard against heart disease. Omega-3 fatty acids in fish have been shown to reduce inflammation, lower blood triglycerides (fats) and make the blood less likely to clot.

Now, new research questions that advice.

According to the study, published March 8th in JAMA Internal Medicine, eating two servings of fish each week did not offer protection from heart disease in healthy people. Among individuals with existing heart disease or diabetes, however, regularly eating fish did provide modest cardiovascular benefits.

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About the study

Researchers from McMaster University in Hamilton analyzed data from 191,558 participants enrolled in the Prospective Urban Rural Epidemiology (PURE) study.

Participants, average age 54, did not have cardiovascular disease and provided complete information about their usual diets. They were followed for nine years.

Compared with eating little or no fish (less than two ounces a month), eating at least 12 ounces a week was not associated with the risk of a major cardiovascular event (e.g., heart attack, stroke, sudden cardiac death, congestive heart failure) or total mortality.

The researchers also pooled data from three previous randomized controlled trials conducted in 40 countries involving 43,413 participants who had cardiovascular disease or diabetes.

The risk of a major cardiovascular event and death was the lowest among people who ate six to twelve ounces of fish a week, compared with those who consumed little or no fish. Eating more than 12 ounces of fish a week was not associated with further heart benefits.

Fish higher in omega-3 fatty acids were more strongly tied to protection.

For both analyses, the researchers accounted for other cardiovascular risk factors including age, sex, smoking status, physical activity, alcohol intake and fruit and vegetable intake.

Limitations

These new findings support the heart health benefits of eating two servings of fish a week for people who have cardiovascular disease or diabetes.

The fact, however, that the researchers didn’t find benefits in healthy people shouldn’t stop you from following current dietary advice to consume fish twice a week. This analysis has limitations.

The type fish eaten and how fish was cooked were not assessed in the PURE study. It’s not known, for example, if higher intakes of fish low in omega-3 fats and/or deep-fried fish may have explained the lack of a protective effect.

As well, dietary information in the PURE study was self-reported and, as a result, can be prone to error.

Bottom line: This one study doesn’t alter the existing large body of evidence that supports the cardiac benefits of fish consumption in healthy populations.

Fish intake advice

The Heart and Stroke Foundation recommends eating at least two servings of fish each week; a serving is typically three ounces of cooked fish.

The protective effects of fish are largely attributed to its two omega-3 fatty acids, EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid). Good sources include salmon, herring, mackerel, sardines, trout, Arctic char, anchovies and albacore tuna.

To minimize mercury exposure, women of childbearing age, especially women who are pregnant and breastfeeding, and children younger than 11, should avoid eating high-mercury fish, which include swordfish, shark, marlin, orange roughy, king mackerel and escolar.

Albacore tuna may also be high in mercury depending on where it is from. If it’s fished in the waters of the Pacific Northwest, it is low in mercury. When buying canned albacore tuna, look for “product of Canada” on the label.

Canned light tuna, which contains skipjack, yellowfin and tongol species, is relatively low in mercury.

Enjoy fish baked, poached, steamed or grilled, cooking methods that cause minimal loss of omega-3 fatty acids. Deep-frying and pan-frying fish at high temperatures can destroy these beneficial fats.

Fish delivers other nutrients too. B vitamins, vitamin D, selenium, iodine, potassium and certain proteins in seafood may also confer health benefits.

If you don’t eat fish, consider taking a fish oil capsule. While not conclusive, evidence from the VITAL trial, published in 2018, suggests that doing so may lower the risk of heart attack.

DHA supplements made from algae are available for people who follow a vegan diet.

Leslie Beck, a Toronto-based private practice dietitian, is director of food and nutrition at Medcan. Follow her on Twitter @LeslieBeckRD

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