Eating more fiber may modestly reduce the risk of stroke, although details remain uncertain and it might just be a surrogate for other healthy behaviors, a meta-analysis determined.
Each additional 7 g of daily dietary fiber intake was associated with a significant 7% lower risk of hemorrhagic and ischemic stroke combined, Diane Threapleton, MSc, of the University of Leeds, England, and colleagues reported online in Stroke.
“Our study supports current guidelines to increase fiber consumption,” the researchers concluded, although they noted that too little data were available to narrow down what sources or types of fiber were most protective.
They called a 7-g per day boost in roughage doable, it being the equivalent of an extra serving of beans or two servings of fruit like apples and oranges.
The average American falls short of the daily recommended fiber intake by more than that, though, getting an average of just 13 g for women and 17 g for men compared with the 21 to 25 g and 30 to 38 g, respectively, called for by guidelines.
Although the observational data couldn’t ascribe causality, a role for dietary fiber is plausible, Threapleton’s group noted.
“Soluble types of fiber form gels in the stomach and small intestine, slowing the rate of nutrient absorption and slowing gastric emptying, which increases satiety and influences the overall amount of food eaten, resulting in lower levels of overweight,” they wrote. “Bacterial fermentation of resistant starch and soluble fibers in the large intestine produces short-chain fatty acids which inhibit cholesterol synthesis by the liver, consequently lowering serum levels.”
Prior studies have shown links to stroke risk factors, including hypertension and high cholesterol, as well as insulin resistance.
The literature search turned up eight prospective cohort studies from the U.S., northern Europe, Australia, and Japan reporting on fiber intake in healthy individuals (defined as not recruited based on history of disease or poor health) and incidence of first ever stroke.
Pooled results showed a steadily declining stroke risk with higher total fiber intake, with a relative risk of 0.93 per 7 g per day (95% CI 0.88 to 0.98).
Few individuals had fiber levels above 25 g per day, “so extrapolation of risk at higher intakes should be undertaken with caution,” the researchers warned.
There was some evidence of heterogeneity among the studies, with a difference by study size.
Ischemic stroke appeared less common with higher total dietary fiber intake in two of the four studies that reported on this outcome, while a third showed a similar trend but with wide confidence intervals.
Hemorrhagic stroke occurred less often at higher fiber levels in one of the three studies looking at that outcome.
Soluble fiber showed a nonsignificant trend, with 6% lower relative risk of stroke per 4 g/day increase in daily intake across the studies.
The insoluble fiber results couldn’t be pooled. One of the three studies reporting on this measure suggested a 38% lower stroke risk, while the others indicated no association.
Fiber from grain sources appeared protective in several studies, as did vegetable fiber, but again results couldn’t be pooled.
The reviewers cautioned about the “inherent problem of unadjusted confounding” since fiber may be acting as a surrogate for other healthy behaviors like less smoking and more exercise that also would reduce stroke risk.
“All of the pooled studies did, however, include adjustment for potentially important confounding variables such as age, body mass index, blood pressure or history of hypertension, smoking status, alcohol intake, physical activity, and sex (where applicable), and also a variety of other health and lifestyle variables,” Threapleton and colleagues noted.
The main systematic review was funded by the U.K. Department of Health for England.
Threapleton reported that her PhD studies are sponsored by Kellogg Marketing and Sales Company.
From the American Heart Association: