Pacific herring

Regular oily fish intake is one of the better-evidenced dietary associations in cardiovascular research, with multiple meta-analyses of cohort studies reporting lower cardiovascular mortality among people who eat fish once or twice a week compared with rarely. This body of work is about oily fish as a category — herring, mackerel, sardine, salmon — rather than about herring specifically, and the trial evidence for purified omega-3 supplements is considerably more mixed than the food-based observational data.

What may be going on: EPA and DHA are incorporated into cell membranes and into eicosanoid and specialised pro-resolving mediator pathways, and lower circulating triglycerides at higher intakes.

Strong evidence. Consistent findings across multiple good-quality human studies.

May support a lower fasting triglyceride level. Trials feeding oily fish or its oil consistently lower triglycerides in a dose-related way; effects on LDL cholesterol are small and inconsistent, and the trials use oily fish generally rather than herring on its own.

Moderate evidence. Several human studies point the same way, with real caveats.

Has been studied as a source of DHA and iodine during pregnancy. Advisory bodies generally encourage two portions of fish a week including one oily portion, on the basis that the nutritional contribution outweighs the contaminant load of small, short-lived species. The trial evidence behind DHA in pregnancy comes from supplements rather than from eating herring.

Limited evidence. Early human work, or mostly lab and animal research.