‘Impressive’: DHA-rich fish oil raises Omega-3 Index & slows resting heart rate in healthy adults

A daily DHA dose of 540 mg, equivalent to two servings of fish per week, was associated with significant increases in the Omega-3 Index (EPA + DHA) from about 5.3% at the start of the study to over 7.5% after eight weeks. In contrast, the O3I for the control group did not change significantly.  

“This current study demonstrated that a low supplemental intake of fish oil results in an increased O3I which is reflective of the preferential incorporation of DHA into myocardial membranes,” ​wrote researchers from the University of Wollongong in the Journal of the American College of Nutrition.​

“In young and healthy adults who were free from CVD, resting HR [heart rate] was slowed, yet no appreciable modification of HRV [heart rate variability] responsiveness was evident in either the resting bradycardia or during cardiovascular reflex challenges (parasympathetic or sympathetic dominant) meaning that there was no change to the homeostatic process requiring rapid HR perturbations. ​

“This provides some support for the direct influence of long-chain ω-3 PUFA [omega-3 polyunsaturated fatty acids] on cardiac intrinsic beat rate independent of the autonomic nervous system in this group of healthy adults.”​

Cardiovascular health

The study adds to a large body of evidence supporting the potential cardiovascular benefits of omega-3s, first reported by Dr Jorn Dyerberg, Hans Olaf Bang, and Aase Brøndum Nielsen in landmark papers on the topic in the The Lancet​​ in 1971 The American Journal of Clinical Nutrition​​ in 1975.

To date, the polyunsaturated fatty acids (PUFAs) have been linked to a range of cardiovascular benefits, from improving in blood lipid levels to reducing the tendency of thrombosis, and from improving blood pressure and heart rate to reducing the risk of coronary heart disease (CHD) and cardiac death.

Commenting independently on the study’s findings, Harry Rice, PhD, VP of regulatory & scientific affairs for the Global Organization for EPA and DHA Omega-3s (GOED), told us: “Given the low baseline heart rate values, the effect of the high DHA fish oil supplement (versus placebo) on resting heart rate was particularly impressive. ​

“Past studies have demonstrated positive associations between omega-3s and heart rate variability, so it seems odd that there was no benefit demonstrated in the current study. At the same time, it could be due to any number of reasons, including the low number of subjects,”​ added Dr Rice.  

Study details​

For the new study, the Wollongong-based scientists recruited 20 young (average age 27) and healthy adults to participate in their study. The volunteers were randomly assigned to receive control supplements (soy oil) or DHA-rich tuna fish oil (providing 560 mg per day of DHA and 140 mg per day of EPA) for eight weeks.

In addition to the increases in the Omega-3 Index, the researchers also found that whole blood levels of EPA and DHA increased significantly in the DHA group by an average of 2.05%. There were no significant changes in this measure in the control group.

Resting heart rate decreased in the DHA group by an average of five beats per minute, reported the researchers.

“Elevation of the O3I in the fish oil group was associated with a slowing of resting HR, similar to the effects observed with exercise training,” ​they wrote. “This finding is consistent with previous observations which have demonstrated fish oil supplementation to slow resting HR in generally healthy but untrained humans and during cardiac induced stress from exercise protocols. ​

“However, … studies demonstrating slowed resting or exercise HR have predominantly supplemented with fish oil doses in the therapeutic range [greater than 2 grams per day]. The efficacy of the low dose fish oil supplement in slowing resting HR in the current study may be in part attributable to its high DHA content,” ​they added.

In contract to the researchers’ expectations, there was no impact on heart rate variability (HRV) in the study participants.

“Noting that elevated resting HR and low blood levels of long chain ω-3 PUFA (especially DHA) are independently associated with elevated risk of sudden cardiac death, these findings emphasize the value of nutritional interventions aimed at increasing fish or fish oil containing DHA even in healthy young adults and provides further support for the recommendation to establish a dietary reference intake,” ​concluded the researchers.

Source: Journal of the American College of Nutrition​
Published online ahead of print, doi: 10.1080/07315724.2021.1953417​
“DHA-Rich Fish Oil Increases the Omega-3 Index in Healthy Adults and Slows Resting Heart Rate without Altering Cardiac Autonomic Reflex Modulation”​
Authors: M.J. Macartney et al.