The researchers suggest heart function could prove to be a major risk factor for dementia and Alzheimer's disease.
Alzheimer's disease - the most common form of dementia - is a progressive disease that affects the parts of the brain that control memory, thinking and language. It starts with mild memory loss and may lead to inability to have conversations, live a normal life and take care of oneself.
According to the Centers for Disease Control and Prevention (CDC), there are around 5 million Americans living with Alzheimer's, which commonly strikes after the age of 60. Although we are learning more and more about the disease - for instance, we know there are certain biological hallmarks of it in the brain - we still have no cure and no clear idea about what triggers it.
Scientists believe there is probably more than one cause of Alzheimer's disease. Age is the best-known risk factor, and genetics may also play a role. Evidence is also gathering that suggests some of the risk factors for heart disease and stroke - such as high blood pressure and high cholesterol - may also be involved in raising the risk of Alzheimer's.
And now, this new study adds more weight to the idea that Alzheimer's and heart health are linked, as lead investigator Dr. Angela Jefferson, director of the Vanderbilt Memory and Alzheimer's Center, notes:
"Heart function could prove to be a major risk factor for dementia and Alzheimer's disease."
Study is first to use cardiac index as a risk factor for dementia
For their study, Dr. Jefferson and colleagues analyzed data from the Framingham Heart Study - an ongoing study that began in 1948 with the main aim of identifying risk factors for heart disease.
The analysis compared a measure of heart function - called the cardiac index - with the development of dementia in 1,000 participants from Framingham's Offspring Cohort who were followed for up to 11 years.
Cardiac index is an indicator of heart health that measures the amount of blood that leaves the heart and is pumped through the body - adjusted for body size. The lower the index, the less blood leaving the heart.
Over the 11 years follow-up, 32 participants developed dementia, including 26 who developed Alzheimer's.
The analysis revealed that participants with a clinically low cardiac index had twice the relative risk of developing dementia, compared with those whose cardiac index was normal.
At first, the team thought heart disease might be driving the increased risk of dementia and Alzheimer's disease. But, says Dr. Jefferson:
"When we excluded participants with heart disease and other heart conditions, we were surprised that the risk of dementia and Alzheimer's disease got even worse."
Their analysis found the relative risk of developing dementia and Alzheimer's was nearly three times greater in individuals with no heart conditions but whose cardiac index was low, compared with individuals with normal cardiac index.
Dr. Jefferson notes that while researchers have long linked heart health with brain health, cardiac index has not been seen as a risk factor for significant memory loss or dementia before.
Findings could reflect a subtle, progressive reduction in supply to the brain
The brain is a hungry organ. In the average adult, it accounts for only 2% of body weight but receives as much as 15% of the blood pumped out of the heart.
Dr. Jefferson explains that if there is a drop in the amount of blood the heart pumps out, the brain is resilient and can regulate blood flow to compensate so brain tissue receives a consistent supply.
"But as we age, our vessels tend to be less healthy," she notes. "They become less adaptable to blood flow changes, and those changes may affect brain health and function."
The finding may reflect a process that over a lifetime subtly reduces the supply of oxygen and nutrients to the brain, says Dr. Jefferson, adding that this is a worrying possibility given that a third of the study participants had a low cardiac index.
She emphasizes that their finding only points to a risk factor and, at present, we have no proven method for preventing dementia or Alzheimer's disease. However, an encouraging side of their finding is that you can do something about heart health. Dr. Jefferson notes:
"You may not be able to change your genetics or family history, but you can engage in a heart-healthy lifestyle through diet and exercise at any point in your lifetime."
Funds for the study came from the National Institute on Aging, the National Heart, Lung and Blood Institute, and the Alzheimer's Association.
Meanwhile, Medical News Today recently learned about a study that found clumps of Alzheimer's amyloid protein in the brains of people as young as 20. The finding is surprising because it was thought amyloid protein only began to accumulate in older brains.