Are the benefits from eating oily fish limited to those with existing cardiovascular disease?

With cohort studies suggesting health benefits from eating fish, what remains unclear is if these benefits are related to underlying risk.

The recommendation to eat at least two portions of fish, especially oily fish, each week, is believed to help reduce the risk of cardiovascular disease. The purported benefits from eating oily fish are thought to be related to the presence of the long-chain omega-3 fatty acids, docosahexaenoic acid and eicosapentaenoic acid. While there is evidence from interventional studies to show that eating oily fish improves cardiovascular risk factors such as triglycerides and blood pressure, the evidence from meta-analyses have been inconsistent. For instance, one such analysis from 2012 did not show any benefit from omega-3 supplementation on major cardiovascular outcomes. In contrast, a 2019 analysis concluded that marine omega-3 supplementation did lower the risk of adverse outcomes such as myocardial infarction and cardiovascular heart disease (CHD) death. However, what has remained unclear from the existing evidence is whether the magnitude of any benefit is dependent on the presence of underlying vascular disease. In an attempt to provide some clarity on this issue, an international team of researchers, have pooled individual participant data from 58 countries to examine the relationship between omega-3 intake and cardiovascular events and mortality, among those with and without existing cardiovascular disease (CVD). The team used existing data from a number of large and ongoing prospective studies involving nearly 200,000 individuals from 58 countries. The data on fish consumption was derived from habitual food intake questionnaires and recordings of major cardiovascular disease (CHD) events and death during follow-up were collated from the different studies. The main outcomes and measures examined were mortality and major CHD events.

Findings
Overall, 191,558 participants with a mean age of 54.1 years (47.9% male) were included in the analysis and the median duration of follow-up was 7.5 years (ranging from 4.9 to 9.4). Overall, there were 8949 deaths (6.4%) among individuals without prior CVD but 6762 (13.1%) among those with existing CVD. In addition, there were 6825 and 8565 major CVD events in those without and with prior CVD respectively. Collectively, among patients with prior CVD, a serving of at least two portions of fish per week was associated with a lower risk of CVD events (hazard ratio, HR = 0.84, 95% CI 0.73–0.96) and total mortality (HR = 0.82). In contrast, among those without prior disease, there was no significant association between fish intake and health outcomes. In their conclusion, the authors noted that fish consumption only appeared to be of benefit among high-risk individuals or those with existing vascular disease but not the general population.

Citation
Mohan D et al. Associations of fish consumption with risk of cardiovascular disease and mortality among individuals with or without vascular disease from 58 countries. JAMA Intern Med 2021