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By Maggie Fox

Lowering blood pressure to recommended levels can prevent dementia and the memory and thinking problems that often show up first, researchers reported Wednesday.

People whose top blood pressure reading was taken down to 120 were 19 percent less likely to develop mild cognitive impairment, the loss of memory and brain processing power that usually precedes Alzheimer’s, the study found. And they were 15 percent less likely to eventually develop cognitive decline and dementia.

“This is really exciting,” said Heather Snyder, senior director of medical operations for the Alzheimer’s Association. The results were presented at the annual meeting of the association in Chicago.

“These results really do cement the adage that what is good for the heart is good for the brain,” said Dr. Mark Supiano, a geriatrician at the University of Utah Health system and the VA Salt Lake City Health Care System who worked on the study.

It may take a few more years before the study conclusively shows whether the risk of Alzheimer's was actually reduced because of the lower blood pressure,the researchers said.

It’s the first intervention that has been clearly demonstrated to lower rates of mental decline. And it was one of two promising studies presented at the Alzheimer's Association annual meeting in Chicago. A second study found an experimental drug might slow progression of Alzheimer's disease in early stage patients.

Alzheimer’s disease is the sixth-leading cause of death in the United States. More than 5 million Americans have been diagnosed with Alzheimer’s, including 200,000 under the age of 65.

There’s no cure, and while there have been indications that a healthy lifestyle can lower the risk, no controlled trial comparing people given a treatment with people not given the treatment has been able to show a reduction in cases of either cognitive decline or dementia.

This one has.

The findings come from a large trial of blood pressure called the Systolic Blood Pressure Intervention Trial, or SPRINT.

It has already found that lowering systolic blood pressure — the top number in a blood pressure reading — to 120 or less can prevent stroke, heart attacks, kidney disease and other problems.

The findings were so compelling that the trial was stopped, and the American Heart Association and American College of Cardiology released new guidelines for blood pressure in 2017 that recommended getting systolic blood pressure to 120 or lower in everyone.

Previously, people were not considered to have high blood pressure until the top reading hit 140.

The trial also was designed to tell if getting blood pressure down would affect dementia risk.

It does, said Dr. Jeff Williamson of the Wake Forest School of Medicine and colleagues. They studied 9,361 people with an average age of 68 at the start. Half had their blood pressure lowered to 120 by whatever means it took, while the other half got standard blood pressure therapy.

“These results support the need to maintain well-controlled blood pressure, especially for persons over the age of 50,” Williamson said in a statement.

There had been worries that it might be too much.

“There was a lot of concern that there could be cognitive harm in lowering blood pressure,” Supiano told NBC News.

The fear was that less blood would get to the brain if blood pressure was taken down so much.

But that did not seem to happen. People whose blood pressure was taken lower were not any more prone to dizziness or falls. Serious adverse events were more common in the people whose blood pressure was intensively lowered, but that was just 4 percent of cases compared with 2 percent of those who got less intense treatment.

There was also concern that it would take a lot of drugs to get blood pressure low enough in some people, but Supiano said this was not the case.

“On average, people randomized to intensive arm took one more medication compared to people in the standard arm,” he said. It worked out to about three drugs, compared with two in the group getting standard care.

Gregg Donovan, a 41-year-old software engineer from Port Washington in New York, takes these recommendations to heart.

Hus father died with Alzheimer's at the age of 77, and three of his grandparents had the disease, so Donovan is worried he is at risk himself.

"I've learned that the biological processes that underline Alzheimer's go for decades before people start to show symptoms before being diagnosed," Donovan told NBC News.

"I would say that the effort is worth it," said Donovan, who keeps an eye on his blood pressure, tries to get enough sleep and tries to exercise and eat a healthy diet in the hope of reducing his own risk. "I'm willing to go to great lengths to prevent it myself."

A low-salt diet heavy on fruits and vegetables can lower blood pressure, as can exercise. There are several different classes of drugs approved to lower blood pressure. Doctors are advised to start with a diuretic, to lower the total volume of blood, and add other classes such as calcium channel blockers as needed.

High blood pressure damages the brain in the same way it damages other organs, by affecting the blood vessels.

“Anywhere from 50 percent to 70 percent of people with Alzheimer’s also have vascular changes,” Snyder of the Alzheimer's Association told NBC News.

Supiano said he thinks there’s more overlap between what’s called vascular dementia and what’s called Alzheimer's than has been appreciated. Alzheimer’s is usually defined by blockages of a protein called amyloid in the brain, and many drugs designed to treat Alzheimer’s target amyloid.

“So much of the focus on Alzheimer’s disease has been centered on amyloid. I think increasingly there is evidence that the vascular damage over time may be potentially more important,” Supiano said. “Perhaps amyloid is an innocent bystander.”

Right now, the drugs approved to treat Alzheimer's merely mask symptoms for a while.

“There isn’t a whole lot we can do to treat Alzheimer’s disease once it develops,” Supiano said.

“If we can effectively prevent it, even by a few years, the public health impact health of this is enormous."

Parminder Deo and John Torres, M.D. contributed.