Letter by Jong et al regarding article, "dietary fish and omega-3 fatty acid consumption and heart rate variability in US adults".

To the Editor: We read with interest the recent report from Mozaffarian et al1 that consumption of ω-3 fatty acid containing fish is associated with increased heart rate variability (HRV) markers of vagal heart rate modulation. Importantly, in their analysis, higher HRV was accompanied by a lower risk of subsequent cardiac death. Their findings, which demonstrate the potential use of HRV indices as surrogate or secondary measures of treatment efficacy when evaluating preventive strategies for cardiovascular disease, raise 2 important questions for research on the HRV response to interventions such as diet. First, how would one characterize the magnitude of treatment effects such …