Eating fish, nuts, seeds and plants with omega-3 fatty acids may significantly lower your risk of dying from a heart attack, according to the most thorough study to date on this contested nutritional topic.

Previous research on fish oil supplements and other sources of omega-3 fatty acids has shown mixed results, with some studies revealing heart-healthy benefits and others finding no benefit at all.

The latest research, reported today (June 27) in the journal JAMA Internal Medicine, was the largest of its kind to measure the actual levels of omega-3 fatty acids in the participants' blood, as opposed to relying on questionnaires in which people report what they eat. Results based on diet questionnaires are prone to error as a result of people's faulty memory or exaggeration. [Top 10 Cancer-Fighting Foods]

The new study — from an international consortium comprising 19 studies from 16 countries with more than 45,000 participants — found that higher circulating blood levels of omega-3 fatty acids were associated with a nearly 10 percent lower risk of a fatal heart attack, on average, compared with lower levels. The participants with the highest level of omega-3s in their blood had the greatest risk reduction — a more than 25 percent lower risk of having a fatal heart attack, the study found.

One in four of all deaths in the United States are due to heart disease, according to the Centers for Disease Control and Prevention. Thus, the study's findings imply that if the average person in the U.S. were to eat several servings of foods with omega-3 fatty acids per week, that number could be reduced to one in five or one in six, the study results suggest.

"For the leading cause of death in the world, lowering the risk [of a fatal heart attack] by about 25 percent is quite meaningful," said senior author Dr. Dariush Mozaffarian, dean of the Friedman School of Nutrition Science and Policy at Tufts University in Boston. "Although our findings are observational using biomarkers, the observed risk reduction is about the same size effect as statins have on fatal heart disease," Mozaffarian told Live Science.

Fish is the major food source of omega-3 fatty acids, particularly the polyunsaturated fatty acids known as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Fatty fish such as salmon, trout, anchovies, sardines and herring contain the highest amounts of omega-3 fatty acids, according to the U.S. Department of Agriculture. Some seaweed has it, too.

Flaxseed, walnuts, a succulent green called purslane (which some people consider a weed) and certain other leafy greens, seeds and nut oils are sources of a plant-based omega-3 fatty acid called alpha-linolenic acid (ALA).

Editor's Recommendations

Previous studies have found that EPA and DHA are superior to ALA for heart health, because the body must convert ALA into EPA and DHA in order for it to be effective. Yet the new study finds circulating ALA blood levels to be almost as strongly linked with a reduced risk of heart attack as the other omega-3s.

"This is one of the new findings of our investigation — that when we combined the results of multiple studies from around the world, we observed lower risk of fatal coronary heart disease with ALA, as well," Mozaffarian said. "It was not quite as robust as seen for DHA, but still suggests benefits of eating plant omega-3s."

In contrast, higher levels of omega-3 fatty acids were generally not associated with a reduced risk of nonfatal heart attacks — a finding that remains a bit of a mystery, the researchers wrote. [Beyond Vegetables and Exercise: 5 Ways to be Heart Healthy]

Scientists have hypothesized that statin or aspirin use might reduce the benefits of omega-3 fatty acids, but the new research found little evidence for this.

Also, the new study could not assess the usefulness of taking fish oil supplements, as opposed to eating fish, because so few participants in the study took those supplements.

"These new results, [which include] many studies which previously had not reported their findings, provide the most comprehensive picture to date of how omega-3s may influence heart disease," said first author Liana Del Gobbo, a postdoctoral research fellow at the Stanford University School of Medicine, who conducted this study as part of her postdoctoral work with Mozaffarian. "Across these diverse studies, findings were also consistent by age, sex, race, presence or absence of diabetes, and use of aspirin or cholesterol-lowering medications."

The new research included results from studies conducted in the United States, Europe, Israel, Singapore and Costa Rica. The research project is called the Cohorts for Heart and Aging Research in Genomic Epidemiology (CHARGE) Fatty Acids and Outcomes Research Consortium (FORCe).

Copyright 2016 LiveScience, a Purch company. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.