Until now, most studies have looked at the association between food and all types of stroke, or focused on ischaemic stroke only. However, the current study, published in the European Heart Journal, investigated ischaemic stroke and haemorrhagic stroke separately.
The study found that while higher intakes of fruit, vegetables, fibre, milk, cheese or yoghurt were each linked to a lower risk of ischaemic stroke, there was no significant association with a lower risk of haemorrhagic stroke. However, greater consumption of eggs was associated with a higher risk of haemorrhagic stroke, but not with ischaemic stroke.
Ischaemic stroke occurs when a blood clot blocks an artery supplying blood to the brain or forms somewhere else in the body and travels to the brain where it blocks blood flow. Haemorrhagic stroke occurs when there is bleeding in the brain that damages nearby cells. About 85% of strokes are ischaemic and 15% are haemorrhagic. Stroke is the second leading cause of deaths worldwide.
Dr Tammy Tong, the first author of the paper and a nutritional epidemiologist at the Nuffield Department of Population Health, University of Oxford (UK), said: "The most important finding is that higher consumption of both dietary fibre and fruit and vegetables was strongly associated with lower risks of ischaemic stroke, which supports current European guidelines. The general public should be recommended to increase their fibre and fruit and vegetable consumption, if they are not already meeting these guidelines.
"Our study also highlights the importance of examining stroke subtypes separately, as the dietary associations differ for ischaemic and haemorrhagic stroke, and is consistent with other evidence, which shows that other risk factors, such as cholesterol levels or obesity, also influence the two stroke subtypes differently."
The researchers say the associations they found between different foods and ischaemic and haemorrhagic stroke might be explained partly by the effects on blood pressure and cholesterol but they realise that, as with all association studies, the causation is not known.
Dr Tong and her colleagues analysed data from 418,329 men and women in nine countries (Denmark, Germany, Greece, Italy, The Netherlands, Norway, Spain, Sweden and the United Kingdom) who were recruited to the European Prospective Investigation into Cancer and Nutrition (EPIC) study between 1992 and 2000.
The participants completed questionnaires asking about diet, lifestyle, medical history and socio-demographic factors, and were followed up for an average of 12.7 years. During this time, there were 4281 cases of ischaemic stroke and 1430 cases of haemorrhagic stroke.
Food groups studied included meat and meat products (red meat, processed meat and poultry), fish and fish products (white fish and fatty fish), dairy products (including milk, yogurt, cheese), eggs, cereals and cereal products, fruit and vegetables (combined and separately), legumes, nuts and seeds, and dietary fibre (total fibre and cereal, fruit and vegetable fibre).
Diet was assessed using validated country-specific questionnaires which asked about habitual intake over the past year, using 24 hour recalls.
Lower risk of ischaemic stroke was observed in participants with higher consumption of fruit and vegetables, dietary fibre and dairy products, while higher risk was observed with higher consumption of red meat. For haemorrhagic stroke, higher risk was associated with higher egg consumption.
The total amount of fibre (including fibre from fruit, vegetables, cereal, legumes, nuts and seeds) that people ate was associated with the greatest potential reduction in the risk of ischaemic stroke. Every 10g more intake of fibre a day was associated with a 23% lower risk, which is equivalent to around two fewer cases per 1000 of the population over ten years.
Fruit and vegetables alone were associated with a 13% lower risk for every 200g eaten a day, which is equivalent to one less case per 1000 of the population over ten years. No foods were linked to a statistically significant higher risk of ischaemic stroke.
Based on UK estimates, two thick slices of wholemeal toast provide 6.6g of fibre, a portion of broccoli (around eight florets) provides about 3g, and a medium raw, unpeeled apple provides about 1.2g of fibre. The European Society of Cardiology (ESC) and the World Health Organization Regional Office for Europe recommend consuming at least 400g of fruit and vegetables a day; the ESC also suggests people should consume 30-45g of fibre a day.
The researchers found that for every extra 20g of eggs consumed a day there was a 25% higher risk of haemorrhagic stroke, equivalent to 0.66 extra cases per 1000 (or around two cases per 3000) of the population over ten years. An average large-sized egg weighs approximately 60g. Egg consumption in the EPIC study was low overall, with an average of less than 20g eaten a day.
Although dairy products are a substantial source of saturated fats in Europe, higher dairy consumption in this study was associated with slightly lower non-HDL-C concentrations.
The authors note that it has been hypothesised that the high calcium and potassium content of dairy products might have a role in stroke prevention.
In the current study, high yogurt consumption was associated with lower SBP, although SBP was similar across categories of milk and cheese consumption, and the risks for all dairy products were similar after additional adjustment for SBP.
It was not possible to differentiate low-fat and high-fat dairy products in the current study so they could not assess whether the associations might differ by their fat content.
The strength of the association for red meat in this cohort was relatively modest, and the association attenuated when adjusted for the other significant foods or for fibre alone, which suggests that the positive association observed might be partly due to an inverse association with the consumption of these other foods.
Most types of food were included in the study, although information on diet was collected at only one point in time, when the participants joined the study. As the study is observational it cannot show that the foods studied caused an increase or decrease in risk of ischaemic or haemorrhagic stroke, only that they are associated with different risks. Information on medication use (including statins) was not available.
The importance of overall diet
Dr Richard Francis, head of research at the Stroke Association, points out that whilst this study is helpful to back up prior knowledge, it's important that the general public understands that it is their overall diet that is most important.
“The study asked people to recall what they had eaten in the past and didn’t look at what people actually ate over time, so we can’t say for certain that specific foods had caused their stroke. Your risk of stroke depends on a number of different things too, such as blood pressure and levels of physical activity.
“Although this research found specific foods may be linked to risk of stroke, it’s better to look at your eating and drinking habits overall. It is not the case that removing one type of food, such as eggs, from your diet will stop you from having a stroke. To reduce your risk of stroke, focus on eating a healthy diet, keeping active and monitoring your blood pressure. If you’re concerned about your stroke risk, talk to your GP.”
Source: European Heart Journal
Tong. T. Y. N., et al
"The associations of major foods and fibre with risks of ischaemic and haemorrhagic stroke: a prospective study of 418 329 participants in the EPIC cohort across nine European countries"