We observed that higher consumption of red meat was associated with a higher risk of stroke; both processed and unprocessed red meat contributed to this excess incidence. Poultry intake was associated with lower risk. These associations were independent of other major protein sources, fruits, and vegetables, as well as of other stroke risk factors. Compared to red meat, intake of poultry, nuts, and both whole-fat and low-fat dairy were associated with a lower risk.
Earlier studies have found an increased risk of stroke with red meat intake, with as little as one serving per day 6, 12–13
. In a recent meta-analysis, intake of both unprocessed and processed red meat were associated with a non-significant modestly increased risk of total stroke (14% and 17%) 2
. However, only two studies had separated unprocessed from processed red meat, and the meta-analysis included a total of 1,700 cases. Our findings, with more than twice the number of cases included in the meta-analysis, are largely in agreement with earlier findings on the association between red meat and ischemic stroke. Fish intake has been associated with a lower risk of stroke in some 3–4, 14
but not all 15
studies. We did not observe a statistically significant association between fish intake and stroke risk. Poultry intake was more strongly associated with lower stroke risk than was fish intake, especially among women, for ischemic stroke, and when substituted for red meat; substituting poultry for fish was also associated with a trend toward a lower stroke risk (RR 0.89; 95% CI 0.68,1.16). Both whole-fat dairy and low-fat dairy were associated with lower stroke risk when substituted for red meat. The caloric content for a 1 oz serving of mixed nuts is 168 Kcal and for a cooked or broiled 15% fat hamburger patty is 192 Kcal 16
. Three slices of cooked, broiled, or pan-fried bacon has 129 Kcal and�a cup of plain low-fat yogurt has 154 Kcal 16
. Thus, substituting nuts or yogurt for bacon at breakfast, for example, or yogurt or nuts in a salad for a hamburger at lunch or dinner could maintain energy balance.
Multiple mechanisms might mediate the relationship between protein sources and stroke risk. A diet high in protein, when compared to a diet high in carbohydrate, may lower blood pressure, a major risk factor for stroke, and improve plasma lipids 1
. Prior analyses from our group found that a lower intake of total animal protein was associated with higher risk of hemorrhagic stroke in women 17
but not in men 18
. However, this mechanism alone would not account for the different associations of various protein sources with stroke risk. A lower intake of saturated fat has been associated with a trend toward higher stroke risk in some but not all studies, 19
perhaps mediated by low total serum cholesterol levels, reduced platelet aggregation, increased vascular fragility, and increased risk for hemorrhage 17
. Such mechanisms could partly explain, for example, why dairy foods were associated with lower stroke risk. In a recent meta-analysis of prospective cohort studies, higher milk intake was associated with a nonsignificant trend toward lower stroke risk (per 200 ml/d, RR=0.87, 95% CI: 0.72, 1.05); insufficient data were available to separately evaluate low-fat versus whole-fat milk 20
. In contrast, through its effects on blood pressure, 21
sodium may account for part of the association seen between processed red meat and ischemic stroke risk. Heme iron, found in processed and unprocessed red meat, has been associated with coronary heart disease 22
and diabetes 23
. Higher intakes of potassium, magnesium, and calcium have been associated with reduced stroke risk 24
: one or more of these nutrients may help explain the reduction in risk seen when dairy products are substituted for red meat. The reasons for risk reduction seen with higher poultry intake, especially when compared to fish, are not certain, but we note that processing and cooking methods for poultry likely differ from other protein sources, as does the nutrient content, and higher poultry intake has been associated with lower risk of coronary heart disease 7
. Compared to red meat, poultry has lower amounts of heme iron and higher amounts of polyunsaturated fat.
Our analysis has important strengths and limitations. We separately examined total, hemorrhagic and ischemic stroke outcomes. We accounted for other lifestyle factors and dietary habits over time to minimize effects of time-dependent confounding. The long cohort follow-up with updated dietary data, high follow-up rate, and large number of participants provided power to detect clinically relevant differences in risks. Although we reduced measurement error in these prospective studies by the use of repeated assessment of diet, some error is inevitable. However, this error should be independent of stroke and would likely lead to an underestimate of the true associations. Misclassification of dietary covariates could create bias in unpredictable directions. The ability to control for known cardiovascular disease risk factors, assessed repeatedly during follow-up, in multivariable models reduced bias, but we cannot exclude the possibility of residual and unmeasured confounding.