Research suggests a surprising link between blood type and stroke risk, with people carrying one specific group A blood type facing a higher likelihood of stroke before age 60.
This discovery, published in a 2022 paper, deepens our understanding of how our unique biological makeup can impact our health.
"This important and surprising research finding adds to our current knowledge about non-modifiable risk factors for stroke – including a person's blood type," said University of Maryland physician-scientist Mark Gladwin when the study was published.
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Watch the video below for a summary:
You've probably heard of the A, B, AB, and O groups, which refer to the various 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.
Researchers analyzed data from 48 genetic studies, which included approximately 17,000 stroke patients and nearly 600,000 non-stroke 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.
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"The number of people with early strokes is rising," senior author and vascular neurologist Steven Kittner from the University of Maryland said.
"These people are more likely to die from the life-threatening event, and survivors potentially face decades with disability. Despite this, there is little research on the causes of early strokes."
A genome-wide search revealed two locations strongly associated with an earlier risk of stroke. One of these locations coincided with the spot where genes for blood type sit.
A second analysis of specific types of blood-type genes then found people whose genome coded for a variation of the A group had a 16 percent higher chance of a stroke before the age of 60, compared with a population of other blood types.
For those with a gene for group O1, the risk was lower by 12 percent.
The researchers noted, however, that the additional risk of stroke in people with type A blood is small, so there is no need for extra vigilance or screening in this group.
"We still don't know why blood type A would confer a higher risk," said Kittner.
"But it likely has something to do with blood-clotting factors like platelets and cells that line the blood vessels as well as other circulating proteins, all of which play a role in the development of blood clots."
While the study findings may seem alarming – that blood type could change early stroke risk – let's put these results into context.
Each year in the US just under 800,000 individuals experience a stroke. Most of these events – around three out of every four – occur in people 65 years and older, with risks doubling every decade after the age of 55.
Also, the people included in the study lived in North America, Europe, Japan, Pakistan, and Australia, with people of non-European ancestry only making up 35 percent of participants. Future studies with a more diverse sample could help clarify the significance of the results.
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"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 to those that had a stroke after the age of 60.
For this, the researchers used a dataset of about 9,300 people over the age of 60 who had a stroke, and some 25,000 controls over the age of 60 who didn't have a stroke.
They found that the increased risk of stroke in the type A blood group became insignificant in the late-onset stroke group, suggesting that strokes that happen early in life may have a different mechanism compared to those that occur later on.
Strokes in younger people are 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 to do with clot formation, the authors said.
The study also found that people with type B blood were around 11 percent more likely to have a stroke compared to non-stroke controls regardless of their age.
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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 A and B blood types have also been associated with a slightly higher risk of blood clots in veins, called venous thrombosis.
This paper was published in Neurology.
An earlier version of this article was published in September 2022.