The effects of fish oil on heart rate variability in humans remain unclear.

A meta-analysis of randomized controlled trials was performed to investigate the influence of fish oil on heart rate variability.

Human intervention studies were identified by systematic search of PubMed, Embase, The Cochrane Library, and references of related reviews and studies. A random-effects model was applied to estimate the pooled results.

Fifteen studies were included. Results of the meta-analysis showed that the SD of normal-to-normal interval [standardized mean difference (SMD) = 0.10, P = 0.35] and square of successive differences (SMD = 0.05, P = 0.35), 2 of the time-domain parameters of heart rate variability, were not significantly influenced by fish-oil supplementation. For the frequency-domain parameters, the high-frequency power (HF), a surrogate of vagal function, was significantly increased by fish-oil supplementation (SMD = 0.30, P = 0.005), the low-frequency power (LF) was not significantly affected (SMD = 0.03, P = 0.79), and the ratio of LF to HF (LF/HF) showed a trend of reduction (SMD = -0.22, P = 0.08). The trend for a treatment effect on LF/HF becomes significant at P = 0.01 when the 2 studies with a dose <1000 mg/d were omitted. Subgroup analyses according to predefined study characteristics showed no significant results.

Short-term fish-oil supplementation may favorably influence the frequency domain of heart rate variability, as indicated by an enhanced vagal tone, which may be an important mechanism underlying the antiarrhythmic and other clinical effects of fish oil.