Omega-3 fatty acids have been linked to a wide-range of health benefits, including cardiovascular disease (CVD), good development of a baby during pregnancy, joint health, behaviour and mood, and certain cancers.
However, a recent meta-analysis claimed that there was no evidence linking omega-3 intake and improvements in heart health, conclusions that were later slammed by a UK-based fish group.
And the new cross-sectional cohort study, lead by Darius Mozaffarian from Harvard Medical School, extends previous research on fish oil and heart health by reporting that fish consumption is linked to improved electrical properties of heart cells (electrophysiology).
The Cardiovascular Health Study (CHS) recruited 5,096 men and women between 1989-1990 (average age 73, average BMI 26 kg per sq. m) in four US communities. Dietary intake was assessed using a picture-sort version of the National Cancer Institute food frequency questionnaire (FFQ).
Average consumption over one year was assessed by asking participants about intake of tuna, other fish (broiled or baked), and fried fish or fish burgers. Responses ranged from less than four portions per year to more than five portions per week.
The participants also underwent heart rhythm measurements using a standardized electrocardiograph (ECG).
A subset of 56 participants was selected to measure blood levels of omega-3s, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA).
People who ate five or more servings of tuna and/or other broiled or baked fish every week were found to have lower heart rates, averaging 3.2 beats per minute less than those who ate less than one portion per month.
A higher resting heart rate, said the researchers, has been linked by other studies to an increased risk of sudden death, and so lowering the heart rate is a significant health benefit.
High intake of both tuna and other broiled or baked fish was also associated with a lower likelihood of extended ventricular repolarisation - an electrical property of the ventricle part of the heart that needs to revert back to its original electrical state before the heart beats again. Abnormalities, such as prolonged time needed to repolarise the ventricle are important factors in developing arrythmias (abnormal beating of the heart).
The researchers also report that similar findings were observed when they took into account the omega-3 blood concentrations from the subset of 56 participants.
"A one gram per day higher intake [of EPA and DHA] was associated with 2.3 beats per minute lower heart rate and 46 per cent lower likelihood of prolonged ventricular repolarisation," wrote Mozaffarian in the Journal of the American College of Cardiology (Vol. 48, pp. 478-484).
"Thus although other mechanisms may also contribute to reductions in clinical risk, the observed differences in HR and ventricular repolarisation may, in part, account for the lower incidence of arrhythmic events seen with fish and fish oil intake," said the researchers.
The mechanism behind the the lower risk of prolonged repolarisation seen in this study, say the researchers, may be the effects of the omega-3 fatty acids on the flow of sodium and calcium in the ion channels associated with electrical signals in cells.
"In experimental studies, marine n-3 fatty acids inhibit the fast voltage-dependent sodium ion current and the L-type calcium ion current," they said.
The researchers called for more studies to confirm these findings, as well as to explore the potential implications for reduced arrhythmia risk.
The risk of pollutants from oily fish, such a methyl mercury, dioxins, and polychlorinated biphenols (PCBs) have led to some to advocate a reduction in fresh fish intake, despite others advising that the benefits of fish consumption outweigh the risks.
Such conflicting views on fish intake have seen the number of omega-3 enriched or fortified products on the market increase as consumers seek omega-3s from 'safer' sources. Most extracted fish oil are molecularly distilled and steam deodorised to remove contaminants.
But fears about dwindling fish stocks have pushed some industries to start extracting omega-3s from algae. Indeed, companies such as Martek Biosciences and Lonza are already offering algae-derived omega-3 DHA as a dietary supplement.