NEW YORK - According to a new analysis of past studies, people with the most protein-heavy diets are about 20 per cent less likely to have a stroke than those who eat the least protein.
Since the analysis includes studies with different designs, there is a chance that something other than protein intake might explain the results, said co-author Dr. Xinfeng Liu of Nanjing University School of Medicine in China.
"Dietary protein intake tends to be associated with other nutrients that may prevent stroke, such as potassium, magnesium and dietary fiber," so the findings should be interpreted with caution, Liu told Reuters Health in an email.
Still, the relationship between protein and stroke persisted when the researchers only looked at studies that took those factors into account, he noted.
Close to 800,000 people in the United States have a stroke each year, according to the Centers for Disease Control and Prevention.
The new review includes seven studies, each of which followed a group of adults for at least 10 years. Participants either periodically filled out diet questionnaires or were asked to recall everything they had eaten over the past 24 hours in order to gauge their protein intake, and researchers recorded which of them had a stroke during the follow-up period.
The studies included a combined total of about 255,000 people.
Four of the studies took place in the US, two in Japan and one in Sweden. Most considered any type of stroke, but two focused on fatal strokes.
Six of the studies found that as protein intake increased, stroke risk decreased. But in three of those studies the relationship was weak enough that it could have been due to chance, according to findings published in Neurology.
All together, the review authors found that people who ate the most protein were about 20 per cent less likely to have a stroke than those who ate the least. An extra 20 grams of protein per day was linked to a 26 per cent lower stroke risk.
Most of the studies accounted for age, sex and diabetes history as factors in stroke risk.
Eating protein may help lower blood pressure, which in turn lowers stroke risk, the authors write. Still, there could be other explanations for the findings.
"The results of this meta analysis should be interpreted carefully," said Dr. Arturo Tamayo, who studies stroke prevention at the University of Manitoba in Winnipeg. He co-wrote an editorial accompanying the analysis.
Diet can influence stroke risk, but in many ways, not just through protein intake, he told Reuters Health by email. Genetic predisposition for increased cholesterol, age and other medical conditions like high blood pressure or heart disease can also influence risk, he said.
"The results of this study show us a 'trend' of a positive factor in diet that can contribute to decrease the risk of stroke," he said.
Animal versus plant protein
Animal protein was linked to a greater reduction in stroke risk than other sources of protein in the new report. But another recent analysis published in JAMA Internal Medicine found that vegetarian diets are associated with lower blood pressure than diets that include meat.
In the current analysis, there wasn't a big range in plant protein intake - those who ate the most ate only 16 more grams per day than those who ate the least - compared to animal protein, where the daily range was 35 grams, Liu said.
Since there wasn't a big range in plant protein consumption, it was harder to find an association with stroke, he said.
Relatively few people in the included studies got their protein from grains, which could have factored in as well, Tamayo said.
"Among different protein sources, fish consumption has been associated with decreased risk of stroke, whereas red meat consumption has been associated with increased stroke risk," Liu said.
The saturated fat and cholesterol in red meat may increase stroke risk, so red meat isn't the best source of protein for people worried about stroke, he said.
"Thus, stroke risk may be reduced by replacing red meat with other protein sources such as fish," Liu said. "Fish contains omega-3 fatty acids and some other nutritional elements including protein that may protect against stroke."
"As stroke neurologists we aim to change as much as possible all those factors that can potentially be modified, like cholesterol and hypertension, among others and lifestyle factors such as lack of exercise, cigarette smoking, alcohol consumption and diet," Tamayo said.
It is important that people understand how much diet can influence the advent of multiple diseases, he said.
"Global strategies to prevent stroke should start early in life with an adequate diet education for all people regardless of their risk of stroke," he said.
The human diet and stroke prevention are both incredibly complex, he said, but current evidence tilts toward a largely vegetarian Cretan Mediterranean diet high in whole grains, olive oil, fish and fruit, and low in red meat as beneficial for stroke reduction.