NEW ORLEANS – For the first time, physicians have evidence independently linking high dietary levels of fat and sodium to an increased risk of ischemic stroke, based on findings from an epidemiologic study in New York.
Experts who heard these findings at the 30th International Stroke Conference hailed them as a key step for changing public health guidelines in the United States.
Until now, “there has not been evidence of a strong association” between dietary fat and sodium and stroke, commented Lawrence Brass, M.D., professor of neurology, epidemiology, and public health at Yale University, New Haven.
“These new data will allow us to make a stronger statement,” added Dr. Brass, a member of the American Stroke Association's committee that writes guidelines for the primary prevention of stroke.
“I can't speak for the whole committee, but this is the type of information that may prompt us to [look again at] the guidelines,” he said.
A link between fat and sodium and stroke “may sound like common sense, but guidelines are based on the available scientific data. Without the data we can't include them,” said Ralph Sacco, M.D., professor of neurology and epidemiology at Columbia University in New York and senior investigator for both new studies.
Both studies used data collected in the Northern Manhattan Study, which enrolled 3,298 people who lived in northern Manhattan in 1993. The subjects were thoroughly assessed at baseline, and 3,183 answered a food frequency questionnaire that was used to calculate their average, daily intake of fat and sodium. At entry, this group had an average age of 70 years; 63% were women, 21% were white, 24% were African American, and 52% were Hispanic. They were followed for an average of 5.5 years, during which 142 had an ischemic stroke.
The analysis that examined the link between stroke incidence and dietary fat assessed fat intake both as a continuous variable and as a dichotomous variable, with 65 g/day as the dividing line between a low- and high-fat diet. Current diet recommendations of the National Cholesterol Education Program say that fat should be about 30% of total calorie intake, which means that if a person eats 2,000 calories per day, their daily fat intake should be no more than 65 g. About 36% of people in the study reported eating more than 65 g of fat daily.
In the dichotomous analysis, the incidence of ischemic stroke was 60% higher in people who consumed more than 65 g of fat daily, compared with those who ate 65 g or less daily, reported Halina White, a researcher at Columbia. This analysis controlled for a variety of demographic, clinical, and dietary variables, including age, gender, race, physical activity, body mass index, sodium intake, and the number of fruits and vegetables in the diet. When also adjusted for total calorie intake, the increased risk of stroke linked with a high-fat diet rose to 90%.
“These are reasonable hazard rates that are close to the increased risk for stroke in patients with hypertension,” said Dr. Sacco, who is also director of stroke and clinical care at Columbia.
He speculated that atherosclerosis may be the mechanistic link between high dietary fat and stroke. High-fat diets usually lead to increased serum levels of total cholesterol and LDL cholesterol, he said.
The sodium analysis divided the study group into three groups: those with an average, daily sodium intake of 4 g or more per day, those with a daily intake of 2.4 g or less per day, and those who ate between 2.4 and 4 g per day. In an analysis that controlled for age, gender, race, hypertension, heart disease, diabetes, smoking, body mass index, physical activity, alcohol intake, and other variables, people who ate 4 g or more per day had an 84% higher risk of ischemic stroke, compared with people who consumed 2.4 g or less per day, reported Armistead D. Williams III, M.D., a neurologist at Columbia.
People who ate between 2.4 and 4 g per day had a slightly increased risk of stroke, compared with those with the lowest intake level, but the difference was not statistically significant.
Because the analysis controlled for differences in blood pressure, it's possible that some other, unknown biologic mechanism explains the link between dietary sodium and stroke risk. But blood pressure may still play a role, Dr. Sacco said at the conference, sponsored by the American Stroke Association.
“We controlled for differences in blood pressure, but there might have still been small differences in blood pressure that we couldn't detect. We can't exclude a blood pressure effect,” he said.