Research suggests a surprising link between blood type and stroke risk, with people who have a specific blood type A experiencing a higher likelihood of stroke before the age of 60.
This discovery, published in a 2022 paper, deepens our understanding of how our unique biological makeup can affect our health.
“This important and surprising finding adds to our current knowledge of nonmodifiable risk factors for stroke — including a person’s blood type,” Mark Gladwin, a physician-scientist at the University of Maryland, said when the study was published.
Related: Expert reveals 8 ‘essential’ ways to reduce stroke risk
Watch the video below for a recap:
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You’ve probably heard of the A, B, AB, and O groups, which refer to different chemical markers, known as antigens, found on the surface of our red blood cells.
Even within these major blood types, there are subtle variations caused by mutations in the genes involved.
The researchers analyzed data from 48 genetic studies, which included approximately 17,000 stroke patients and nearly 600,000 stroke-free controls. All participants were between the ages of 18 and 59.
Their findings revealed a clear relationship between the gene responsible for the A1 blood subgroup and early-onset stroke.
Related: Super-Rare ‘Hybrid’ Blood Type Found in Just 3 People
“The number of people with early stroke is increasing,” said lead author and vascular neurologist Steven Kittner of the University of Maryland.
“These people are more likely to die from the life-threatening event, and survivors potentially face decades of disability. Despite this, there is little research into the causes of early strokes.”
A genome-wide search revealed two loci strongly associated with an earlier risk of stroke. One of these locations coincided with where the blood type genes are located.
A second analysis of specific types of blood type genes then found that people whose genome coded for a type A variation were 16 percent more likely to have a stroke before the age of 60 compared to a population of other blood types.
For those with a gene for the O1 group, the risk was 12% lower.
The researchers noted, however, that the additional risk of stroke in people with type A blood is small, so no additional vigilance or screening is needed in this group.
“We still don’t know why blood type A would confer a higher risk,” Kittner said.
“But it probably has something to do with blood-clotting factors, such as platelets and cells that line blood vessels, as well as other circulating proteins, all of which play a role in the development of blood clots.”
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While the study’s findings may seem alarming — that blood type could change the risk of early stroke — let’s put these results into context.
Each year in the US, just under 800,000 people have a stroke. Most of these events – about three out of four – occur in people aged 65 and over, with risks doubling every decade after age 55.
Also, the people included in the study lived in North America, Europe, Japan, Pakistan and Australia, with people of non-European origin representing only 35% of the participants. Future studies with a more diverse sample could help clarify the significance of the results.
Related: Why women’s risk of stroke is much higher, experts say
“We clearly need more follow-up studies to clarify the mechanisms of increased stroke risk,” Kittner said.
Another key finding of the study came from comparing people who had a stroke before the age of 60 with those who had a stroke after the age of 60.
For this, the researchers used a data set of about 9,300 people over the age of 60 who had had a stroke and about 25,000 controls over the age of 60 who had not had a stroke.
They found that the increased risk of stroke in type A blood became nonsignificant in the late-onset stroke group, suggesting that strokes that occur early in life may have a different mechanism than those that occur later.
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Stroke in younger people is less likely to be caused by a buildup of fatty deposits in the arteries (a process called atherosclerosis) and more likely to be caused by factors related to clot formation, the authors said.
The study also found that people with type B blood were about 11 percent more likely to have a stroke compared to controls without a stroke, regardless of age.
Related: Half of Heart Attacks in Younger Women Don’t Cause Clogged Arteries
Previous studies suggest that the part of the genome that codes for blood type, called the “ABO locus,” is associated with coronary artery calcification, which restricts blood flow, and heart attack.
The genetic sequences for blood types A and B have also been associated with a slightly higher risk of blood clots in the veins, called venous thrombosis.
This work was published in Neurology.
An earlier version of this article was published in September 2022.