A higher body mass index (BMI) is associated with an increased the risk for ischemic stroke but lower odds of hemorrhagic stroke, a new study suggests.
"This was such a large study that we were able to look at the effect of BMI separately on different stroke types — not just on ischemic stoke, which everyone knows is higher with increasing BMI, but also on different types of hemorrhagic stroke, which although less common are more fatal," said study author Gillian K. Reeves, PhD, professor, epidemiology, Cancer Epidemiology Unit, University of Oxford, United Kingdom.
The findings, published online September 7 in Neurology, should not affect public health recommendations related to target body weight, Dr Reeves told Medscape Medical News.
The analysis included 1,277,129 women from the Million Women Study (mean age at outset, 56.7 years), who were recruited at breast cancer screening centers in England and Scotland between 1996 and 2001. About 1% of the women were lost during follow-up that continued for a mean of 11.7 years.
Researchers used routinely collected National Health Service data and electronic hospital records to gather information on fatal and nonfatal strokes.
During the follow-up, there were 20,549 first strokes, of which 9993 were ischemic, 5852 were hemorrhagic (including intracerebral and subarachnoid), and 4704 were of unspecified type.
Researchers categorized patients into five groups according to BMI (kg/m2): less than 22.5, 22.5 to less than 25, 25 to less than 27.5, 27.5 to less than 30, and 30 or greater.
Higher BMI was associated with an increased risk for ischemic stroke. The relative risk (RR) for ischemic stroke per 5-kg/m2 increase in BMI was 1.23 (95% confidence interval [CI], 1.20 - 1.26) after adjustment for age and region.
The RR was 1.21 (95% CI, 1.18 - 1.23) after additional adjustment for "deprivation" level, physical exercise, alcohol intake, smoking, and height.
In contrast, the corresponding RR for hemorrhagic stroke decreased with increasing BMI. It was 0.88 (95% CI, 0.85 - 0.91) after adjustment for age and region and 0.89 (95% CI, 0.86 - 0.92) when also adjusting for the additional confounders.
Results of a sensitivity analysis that included only hospital admissions with stroke listed as the first diagnostic code did not differ substantially from the main findings. The same was the case for analyses that excluded the first 5 years of follow-up or patients with any prior ill health other than stroke and that adjusted for history of cancer.
The higher risk for ischemic stroke with increased BMI might be related to such risk factors as hypertension and diabetes. It may also have something to do with changes in lipid levels, said Dr Reeves, which can drive atherosclerosis.
Although the link between BMI and hemorrhagic stroke risk appears to be less clear, lipids could be involved here, too. Some evidence suggests that increasing cholesterol levels are associated with decreased risk for hemorrhagic stroke, said Dr Reeves.
The degree to which total or low-density lipoprotein cholesterol might moderate the BMI relationship with hemorrhagic stroke could not be examined in the current study but "would be of interest," said Kathryn Rexrode, MD, Division of Preventive Medicine, Department of Medicine, Brigham and Women's Hospital, Boston, Massachusetts, and Tatjana Rundek, MD, PhD, Department of Neurology, University of Miami Miller School of Medicine, Florida, in an accompanying editorial.
"Additional research on the mechanisms by which low BMI affects hemorrhagic stroke risk is needed," they write.
The study findings are important in that they help researchers better understand what causes some of the rarer types of stroke, said Dr Reeves.
"Now we know that actually being overweight and obese seems to be protective," she said. "Anything that helps our understanding of how the disease is actually occurring is going to ultimately help with prevention."
The researchers also carried out a systematic review and meta analysis that included 12 studies: 5 from Europe, North America, and Australia and 5 from Asia.
"We found that almost all the studies we looked at consistently showed a bigger adiposity-associated relative risk for ischemic stroke than for hemorrhagic stroke," said Dr Reeves.
However, there were some differences in the Asian studies. While there was a 22% increased risk for ischemic stroke for a 5-kg/m2 increase in BMI among European and North Americans, in Asians the risk was increased 35%.
Although it was clear that higher BMI increased the risk for ischemic stroke in Asians, for hemorrhagic stroke, the relative risk was still slightly greater than 1, albeit "a lot lower" than for ischemic stroke, said Dr Reeves.
The difference between studies conducted in Asia and those carried out in western countries could be attributed to the "quite different" etiology of these diseases in Asia, said Dr Reeves. "They tend to have a different distribution of hemorrhagic and ischemic strokes."
In their editorial, Dr Rexrode and Dr Rundek called the association of BMI with both hemorrhagic and ischemic stroke in Asian populations "striking" in light of the rising prevalence of obesity worldwide.
"Given the direct relationship with both ischemic and hemorrhagic stroke in Asian populations, they will endure even greater effects of the scourge of rising obesity."
The editorial writers noted that the Million Women Study was designed as a breast cancer screening study and so is limited in gathering detailed stroke risk factors. "BMI is a crude measure of obesity and does not take into account fitness, nutritional status, or body fat distribution."
Results of this new study should not affect recommendations for ideal body weights, Dr Reeves said. If patients want to improve their overall health and reduce their risk for death, they should stay within the normal BMI range, she said.
The study was funded by the UK Medical Research Council and Cancer Research UK. The study authors and editorial writers have disclosed no relevant financial relationships.
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Cite this: Higher BMI Linked to Ischemic, Not Hemorrhagic, Stroke - Medscape - Sep 09, 2016.