New research looking at the risk factors contributing to stroke burden — the number of healthy life-years lost due to stroke — has shown that behavioral factors play a larger role than metabolic factors.
"Our study provides robust evidence of the very large contribution that behavioral risk factors make to stroke burden. These risk factors are avoidable," lead author Valery L. Feigin, MD, from Auckland University of Technology, New Zealand, commented to Medscape Medical News.
"Poor diet, smoking, alcohol, lack of exercise, et cetera, are responsible for more days of stroke disability than factors such as hypertension, hyperglycemia, high cholesterol, et cetera. Our data show that stroke is definitely a disease of lifestyle, and as such a great proportion of stroke disability can be avoided. This is a new paradigm and has clear practical implications."
The other major and unexpected finding of the study is the large contribution that air pollution makes to stroke burden, Professor Feigin said.
"Our data show that air pollution is responsible for one third of stroke burden. We knew there was an association between air pollution and stroke, but we didn't expect to see such a large effect," he said. "This has not been studied before. A number of studies have looked at air pollution and risk of cardiovascular disease and seen quite strong evidence of a causal link, but in terms of magnitude of its contribution to stroke burden this is the first report."
Their findings were published online in Lancet Neurology on June 9.
The new data come from Global Burden of Disease (GBD) study, which Professor Feigin explained is the largest repository of epidemiologic studies and official statistics on medical conditions. "It is regularly updated and analyzed with sophisticated techniques so that it allows models of the burden of different diseases in each nation using all available information."
For the current analysis, the researchers estimated the disease burden of stroke associated with 17 risk factors in 188 countries. They calculated the population-attributable fraction of stroke-related disability-adjusted life-years (DALYs) for each risk factor: that is, the estimated proportion of disease burden in a population that would be avoided if exposure to a risk factor were eliminated. The study is the first to analyze the global risk factors for stroke in such detail, especially in relation to stroke burden on global, regional, and national levels.
"Our main results show that over 90% of stroke burden can theoretically be avoided by controlling risk factors," he said. "This is a highly preventable condition."
The GBD Study: A "Monumental Task"
In an accompanying comment, Vladimir Hachinski, MD, University of Western Ontario, London, Canada, and Mahmoud Reza Azarpazhooh, MD, Mashhad University of Medical Sciences, Iran, note that stroke imposes possibly the greatest burden of DALYs of all neurologic disorders.
They point out that a systematic analysis of 119 studies has shown the loss of more than 100 million DALYs related to stroke in 2010, and a significant increase since 1990, especially in low- and middle-income countries.
They describe the GBD study as a "a monumental task," and although there are inevitably going to be some limitations given its ambitious scope, they say the systematic analysis is a "timely, authoritative, and a data-based guide to stroke prevention and disease burden reduction," which provides "a firm basis for policy makers to implement preventative measures."
"Our results suggest that 74% of the stroke burden is caused by behavioral risk factors compared with 72% for metabolic risk factors," Professor Feigin said. "This is an important and novel finding of our study."
He explained that the percentage contribution of individual risk factors adds up to well over 100% because they all interact with each other: One risk factor is often mediated through other risk factors.
The finding that behavioral risk factors play such an important role should change the focus on addressing stroke, Professor Feigin says. "Previously we have been focused predominantly on the metabolic/medical risk factors, but we need a paradigm shift and to pay more attention to diet, exercise, smoking et cetera."
"No one denies prevention is important, but it has never been the first priority. Acute care always comes first, then targeting preventative treatments at high-risk individuals. Primary prevention of stroke doesn't really get a look in, but our data suggest primary prevention would have the greatest effect."
Although this can be done on an individual level by providing more education and counseling to motivate people to change their lifestyle, he stresses that interventions on a population level are more important.
"Governments need to make more changes to reduce salt in food, and persuade people to reduce alcohol intake and stop smoking. Population-wide interventions will have a much bigger reduction in stroke burden than similar individual recommendations, which are normally just targeted at high-risk individuals."
He gives the example of reducing salt intake by 5 g per day, "which could easily be done without people noticing any difference in taste." This would reduce systolic blood pressure by 5 mm Hg, which if applied to the whole population would lead to a 25% reduction in the incidence of stroke.
Professor Feigin believes the most effective way of making this happen is through taxation. "Unhealthy food should be taxed at a higher level. Similar actions can be taken with sugar, alcohol, and smoking, and the revenues generated could be used to further improve health."
The Perils of Air Pollution
One of the most surprising findings of the study was that 29.2% of global disability associated with stroke is linked to air pollution (including environmental air pollution and household air pollution). This is especially high in developing countries (33.7% vs 10.2% in developed countries).
Professor Feigin says there are some actions individuals can take to lessen the impact of air pollution, such as reducing the time spent outside in traffic-crammed streets. "Don't go jogging through the city at busy times — go in the park or at times when there is less traffic around," he suggests. But he emphasizes that air pollution really needs to be tackled on a population level. "We need to put more pressure on governments, and evidence from studies such as this one will help do this."
In their comment piece, Dr Hachinski and Dr Azarpazhooh describe the air pollution findings as "alarming."
"Although air pollution is known to damage the lungs, heart, and brain, the extent of this threat seems to have been underestimated," they write. "Air pollution is not just a problem in big cities, but is also a global problem. With the ceaseless air streams across oceans and continents, what happens in Beijing matters in Berlin."
There were no surprises on what was found to be the biggest risk factor for stroke — hypertension, defined as a systolic pressure over 120 mm Hg, accounting for 64% of stroke burden "If we lower blood pressure to under 120 mm Hg in everyone in the world, then 64% of the stroke burden would be avoided," Professor Feigin said.
After hypertension, the other leading risk factors for stroke were a diet low in fruit, high body mass index, a diet high in sodium, smoking, a diet low in vegetables, environmental air pollution, household pollution from solid fuels, a diet low in whole grains, and high blood sugar.
The authors say that because of a lack of data, they could not include some important risk factors for stroke, such as atrial fibrillation, substance abuse, or other health conditions.
This study was funded by the Bill & Melinda Gates Foundation; American Heart Association; National Heart, Lung, and Blood Institute; Columbia University; Health Research Council of New Zealand; Brain Research New Zealand Centre of Research Excellence; and National Science Challenge, Ministry of Business, Innovation and Employment of New Zealand. Professor Feigin reports that the Stroke Riskometer app, mentioned in the Lancet article, is copyrighted by the Auckland University of Technology, and funds resulting from the sale of the professional version of the app go into further research and education for stroke prevention.
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