September 21, 2018
In a somewhat curious twist, however, the results were much more pronounced in women than in men.
"Our results suggest that the Mediterranean diet may be especially protective in women over 40 regardless of menopausal status or hormone replacement therapy," lead author Ailsa A. Welch, PhD, professor of nutritional epidemiology at the University of East Anglia, United Kingdom, told Medscape Medical News.
"It is unclear why we found differences between women and men, but it could be that components of the diet may influence men differently than women," she added.
Welch noted that the current study confirmed previous findings suggesting that the Mediterranean diet reduces stroke risk, but the current study gave more reliable results than were available previously.
"Our study has several strengths — it had a prospective design in both men and women with more precise methods of measuring diet than usually used and a long follow-up period of 17 years," she said. The study also included individuals at high risk and individuals at low risk for cardiovascular disease, whereas most previous studies focused mainly on those at high risk. "We included low-risk individuals in our study because we know strokes can occur in people not necessarily thought to be at high risk."
Welch said that although these findings of a protective effect of the Mediterranean diet on risk for stroke were driven by women, they have implications for the general public.
"Our conclusion is that everyone — both men and women and those at high and low risk of cardiovascular disease — should follow a Mediterranean diet to reduce their risk of stroke," she said. "There have also been many studies showing the Mediterranean diet to be associated with a reduced risk of cardiovascular disease, so the reduced risk of stroke is an additional benefit."
The study was published online September 20 in Stroke.
The researchers note that the key components of a traditional Mediterranean diet include olive oil as the main source of fat, high intakes of fish, fruit, vegetables, nuts, and legumes, low meat and dairy consumption, and moderate alcohol consumption.
They add that although the traditional Mediterranean diet is well established, the foods that contribute to it vary between Mediterranean and non-Mediterranean countries, resulting in different nutrient profiles that may affect comparability between studies.
The current study analyzed data from the EPIC-Norfolk study on 23,232 white adults aged 40 to 77 years in the United Kingdom.
Diet was assessed using 7-day food diaries, which the researchers note has not been done before in such a large population. They add that 7-day diaries (in which participants write down everything they eat and drink over the period of a week) are more precise than food-frequency questionnaires.
The cohort was followed for an average of 17 years. Risk for first stroke was calculated using multivariable Cox regression in the whole population and was stratified by sex and cardiovascular disease risk profile, using the Framingham risk score.
Results showed that 2009 first strokes occurred in the follow-up period. Risk for stroke was significantly reduced with greater adherence to the Mediterranean diet. Those in the top quartile of Mediterranean diet adherence had a 17% reduction in stroke risk compared to those in the lowest quartile (hazard ratio [HR], 0.83; 95% confidence interval [CI], 0.74 - 0.94; P-trend < .01)
A stronger effect was seen in women, for whom there was a 22% reduction in risk for stroke in the top vs bottom quartile of Mediterranean diet adherence (HR, 0.78; 95% CI, 0.65 - 0.93; P-trend < .01).
Men in the top quartile showed a 6% nonsignificant reduction in stroke risk vs those in the bottom quartile (HR, 0.94; 95% CI, 0.79 - 1.12; P-trend = .55).
A significant stroke risk reduction was found with higher adherence to the Mediterranean diet in those participants at high cardiovascular risk, with a 13% reduced risk for stroke for those in the highest quartile adherence vs those with in the lowest quartile (HR, 0.87; 95% CI, 0.76 - 0.99; P-trend = .04).
Those at lower cardiovascular risk showed a trend that did not reach significance (HR, 0.77; 95% CI, 0.58 - 1.02; P-trend = .09). After stratification by sex, it was apparent that the associations found in the high-risk group were driven by the findings in women.
Analysis of the individual components of the Mediterranean diet suggested that the reduction in stroke risk was related to the vegetable and alcohol components of the score. But the researchers write: "We observed few significant associations with individual Mediterranean diet components, indicating that the effects of the diet may be attributable to the synergistic or additive effects of the individual foods or nutrients, and that the overall Mediterranean diet pattern was most important in relation to stroke risk."
Commenting on the study for Medscape Medical News, David Spence, MD, professor of neurology and clinical pharmacology at the University of Western Ontario, London, Canada, said: "In my view, there is no doubt that the Cretan Mediterranean diet reduces the risk of stroke."
He noted that stroke was reduced in secondary prevention by 70% in 4 years in the Lyon Diet Heart study, and in the recently reanalyzed and republished Spanish PREDIMED study, stroke was reduced by 47% in 5 years in primary prevention by the Mediterranean diet supplemented with mixed nuts. And in the Israeli Diet Study, the Mediterranean diet was clearly better for lowering insulin resistance and blood sugar levels than either a low-fat or low-carbohydrate diet.
But he pointed out that in his recent study of the intestinal microbiome, women were more likely to adhere to a Mediterranean diet, and this difference may also have been a factor in the current study. "What I could not see in the paper was a key table reporting the distribution of Mediterranean diet scores for men vs women," he said.
Spence said another problem was in regard to defining Mediterranean diet scores at median values. "Furthermore, dairy, meat, and egg were all lumped together, and a median split was used to determine the score. Fish was separated out as a beneficial dietary component, but it looks as if chicken, pork, and other kinds of animal flesh were all lumped together as meat," he noted. "It would be very important to look at intake of eggs and red meat separately," because these two foods contain compounds believed to be particularly atherogenic, he said.
Welch responded that detailed information on the methods and derivation of the Mediterranean Diet Score and on the demographics and other measured risk factors for stroke are provided in the supplemental tables.
"In our analysis, the Mediterranean diet, as a whole, appears to be more strongly protective against the risk of stroke than the individual foods within it. When we analyzed the individual components of the score, there were few significant associations with stroke risk," she said. "The benefits appear to come from the additive effects of combining a diet high in fish, fruits, vegetables, nuts and beans, cereals, and potatoes, with lower intakes of meat and dairy accompanied by a lower ratio of unsaturated to saturated fat."
On the issue of separating meat from eggs, Welch said. "This is an interesting comment, but we do not know in the context of the overall content of the Mediterranean dietary pattern whether the absolute quantity of eggs consumed may or may not be important," she said.
The EPIC-Norfolk study was supported by grants from the Medical Research Council and Cancer Research UK. Dr Welch has disclosed no relevant financial relationships.