Fish and Mortality Risk: Benefits Depend on Fish Type and Sex, Japanese Cohort Study Finds

A questionnaire filled out in the second half of the 1990s is now the basis for a new look at what fish does, and does not, appear to do for the people who eat it. Between 1995 and 1998, more than 90,000 adults in Japan reported their eating habits as part of the Japan Public Health Center-Based Prospective Study, a large project that follows health outcomes among adults in the country. Researchers have now mined that data, and their findings, published in The Journal of Nutrition, are less tidy than the usual advice to eat more fish.

The analysis covered 90,944 adults. Each had completed a food frequency questionnaire covering 138 food and beverage items, reporting how often they ate each one, from never to several times a day, and their usual portion size over the previous year. Nineteen of those items were fish and shellfish, among them salted fish, salmon, canned tuna and sardines.

The researchers sorted those 19 items into groups. Four were salted products. The other 15 were unsalted fish and shellfish, and a subset of those was classed as omega-3-rich fish, a category that includes salmon, trout, sea bream, horse mackerel, sardines and eel. The team then tracked participants' health over several years, looking at deaths from all causes and from specific causes, including cerebrovascular disease, the group of conditions that affect blood flow to the brain, such as stroke, aneurysms and vascular malformations.

The results split along lines of fish type, cause of death and sex. Among women, moderate intake of fish and shellfish was associated with a lower risk of death from any cause. More specifically, higher intake of omega-3-rich fish was linked to lower all-cause mortality and lower cerebrovascular mortality. Among men, the pattern was different: higher intake of unsalted fish and shellfish was associated with lower cerebrovascular mortality.

The researchers could not say why men and women diverged. They suggested that differences in smoking and alcohol consumption might play a part, but their analysis did not clearly establish those habits as the explanation.

The study carries limits that its authors acknowledge. Diet was assessed at a single point in time, so any changes in what participants ate during follow-up would not have been captured. And although the researchers adjusted for several potential influences, other unmeasured factors could still have shaped the results. This is an observational study, so it points to associations rather than proving that fish itself changed anyone's risk.

The article's reviewing dietitian, Madeline Peck, RDN, CDN, frames the findings as consistent with fish being a source of protein and omega-3 fatty acids, and with the Dietary Guidelines for Americans, which recommend eating seafood regularly. For people who rarely cook it, the practical suggestions are modest: swap fish or shellfish into tacos or salads, use canned tuna, salmon or sardines as budget-friendly shelf-stable options, keep frozen fillets and shrimp on hand, and use garlic, lemon, herbs or sauces such as pesto, salsa or tzatziki for those put off by a fishy taste.

The study adds to existing evidence on seafood rather than settling the question. Until the sex-specific differences are explained, and ideally tested with repeated dietary measurements, the clearest reading is that variety in seafood appears to be associated with benefit, with the details still open.