Objective Long-chain omega-3 polyunsaturated fatty acids (PUFA) from fish have been inversely associated with coronary heart disease (CHD) risk. Fish may also contain methylmercury, which has been associated with higher CHD risk and may diminish the cardioprotective effect of long-chain omega-3 PUFA. We investigated the associations of serum long-chain omega-3 PUFA and hair mercury with the odds for myocardial ischaemia during exercise.

Methods A total of 2199 men from the Kuopio Ischaemic Heart Disease Risk Factor Study, aged 42–60 years were studied in 1984–89. Of the 2199 men, 342 had history of CHD. The men performed a maximal symptom-limited exercise stress test using an electrically braked bicycle ergometer. ORs for exercise-induced myocardial ischaemia were estimated with logistic regression.

Results In the multivariable analysis, those in the highest versus lowest serum long-chain omega-3 PUFA quartile had 33% lower odds of myocardial ischaemia (OR 0.67, 95% CI 0.51 to 0.87, p-trend=0.006). The association was stronger among those with CHD history (OR 0.10, 95% CI 0.03 to 0.39, p-trend <0.001), than among those without (OR 0.80, 95% CI 0.57 to 1.12, p-trend=0.17) (p-interaction=0.01). Higher hair mercury concentration was associated with increased odds for myocardial ischaemia in the entire population (OR 1.62, 95% CI 1.22 to 2.14, p-trend=0.002).

Conclusion Higher circulating concentrations of the long-chain omega-3 PUFAs, a marker for fish consumption, were associated with lower occurrence of exercise-induced myocardial ischaemia, but only among men with CHD history. Hair mercury concentration was directly associated with the occurrence of exercise-induced myocardial ischaemia in the entire study population.