People with a lower body-mass index (BMI) — a measure of body weight that takes height into account — tended to experience greater cognitive impairment after a stroke, according to a new study published in the journal Scientific Reports.
While lots of studies have looked at cardiovascular and weight-related risk factors for having a stroke, there hasn’t been much research on how body weight affects cognitive outcomes in stroke survivors. For this study, researchers looked at 335 people who had an ischemic stroke (involving a clot, as opposed to a hemorrhagic stroke, which involves bleeding), with an average age of about 65. Participants underwent cognitive testing three months after their stroke, and also had the function of their frontal lobe (an area of the brain) assessed while completing different types of cognitively demanding tasks. Based on their body weight and height, participants were divided into four different BMI groups, from highest to lowest BMI.
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BMI linked to cognitive assessment scores
After adjusting for stroke severity, the researchers found that cognitive assessment scores were significantly different in the four BMI groups (although stroke severity didn’t differ significantly between the groups, as noted in a Healio article on the study). The group with the lowest BMI — which included people who were underweight — had lower overall cognitive function scores than the groups with the second-lowest and the highest BMI. But specifically in the area of word or speech fluency, members of the group with the highest BMI had worse outcomes than the group with the second-lowest BMI. Other differences between the groups in these areas didn’t reach the point of statistical significance.
The researchers noted that these results appear to fit what is widely known as the “obesity paradox,” meaning that while a greater body weight increases certain cardiovascular risks, it also appears to reduce other risks — including those related to the likelihood of survival after certain serious cardiovascular events in older people. This study indicates that not just survival, but cognitive outcomes after a stroke may be harmed by being underweight or having a lower body weight in general. It’s unclear, though — both in the latest study, and in general — whether the cardiovascular risks linked to a lower body weight apply to all people based on their weight, or just certain people whose lower body weight reflects poorer health or inadequate nutrition. Past studies, the researchers noted, have shown mixed results when it comes to how BMI is linked to stroke severity — something the latest study didn’t look at, and in fact specifically tried to eliminate as a factor in its analysis.
The current study’s limitations, the researchers wrote, include that it looked only at BMI as an indicator of body weight — not other measures related to body fat such as waist circumference or waist-to-hip ratio. It’s possible, then, that the study missed crucial differences between simply being lean and being underweight when it came to cognitive outcomes after a stroke. More studies are needed, looking at a variety of weight-related measures, to gain a broader understanding of how body composition affects cognitive function and other outcomes after a stroke or other major cardiovascular events.