The team of Swedish researchers behind the study were surprised by the results, which have just been published in the British Journal of Nutrition.
“We weren’t expecting this. Our hypothesis was that the fatty acids contained in fish would help protect against the risk of blood clots,” said Maria Wennberg, a dietician from Umeå University.
The researchers also anticipated that subjects with high levels of mercury in their blood would be more prone to strokes.
But the only group to stand out in the study of 1,100 people in the Norrland region were men who ate fish more than twice a week.
This group was found to be most prone to strokes even when taking into account factors such as diabetes, high blood pressure, high cholesterol levels and smoking.
“We have asked ourselves whether this might just be a coincidence or whether there could be some pollutant other than mercury in fish that increases the risk of stroke,” said Wennberg.
The same risk was not apparent among women with similar fish consumption habits. In fact, the opposite was the case, which the researchers tentatively attributed to a difference in lifestyles.
“Women who eat fish maybe do so as part of a healthy lifestyle whereas men maybe don’t work the same way,” said Wennberg.
Since the research group did not look at lifestyle factors such as vegetable consumption, exercise and alcohol habits, Wennberg believes that this may be a possible explanation.
But she is keen to stress that men should not be put off from eating fish as a result of the study.
“We absolutely do not want this to result in men abstaining from eating fish. We believe that the protective effects when it comes to heart disease for example are greater than any risk of stroke,” she said.