"Reduced HRV predicts mortality and arrhythmic complications in patients who have had a heart attack, as well as those who are considered healthy," said lead scientist Dr Fernando Holguin of the Emory University School of Medicine in Atlanta.
"Taking a daily supplement of fish or soy oil may help reduce the risk of suffering an adverse cardiovascular event, such as arrhythmia or sudden death, specially in persons with known cardiovascular disease or at increased risk for it, such as those with lipid disorders, advanced age, hypertension, a history of smoking, and family history of heart disease."
In a randomized study involving 58 elderly nursing home residents, Holguin assessed the effect of omega-3 fatty acids on heart function, which is measured in high-frequency (HF) and low-frequency (LF) domain components and standard deviation of normal RR intervals (SDNN).
After a two-month supplement-free control period to establish a baseline HRV for each patient, participants received a daily 2g dose of either marine- or plant-derived omega-3 fatty acid (fish oil or soy oil respectively).
Over an eleven-week period their HRV was measured for six minutes every other day, while they were lying down.
The researchers found that supplementation "increased significantly" the average time- and frequency-domain parameters of HRV in both the fish and soy oil groups.
After adjustments were made for age and mean heart rate, supplementation with fish oil was associated with a "significant increase" in both HF and LF components and in SDNN.
In the soy oil group, only a "marginally significant" HF and LF components was seen, coupled with a "significant increase" in SDNN.
The fish oil supplements also delivered an increase in HRV in a shorter time period - within 2.7 weeks compared with 8.1 weeks for soy oil.
"Our findings contradict the current belief in the medical community that increasing the intake of omega-3 fatty acids produces only long-term cardiac benefits," said Holguin.