The nation’s leading heart experts have issued new guidelines for high blood pressure that mean tens of millions more Americans will meet the criteria for the condition and will need to change their lifestyles or take medicines to treat it.

Under the guidelines, issued Monday and formulated by the American Heart Association and the American College of Cardiology, the number of men younger than 45 with a diagnosis of high blood pressure will triple, and the prevalence among women younger than 45 will double.

“Those numbers are scary,” said Dr. Robert M. Carey, professor of medicine at the University of Virginia and co-chairman of the committee that wrote the new guidelines.

The number of adults with high blood pressure, or hypertension, will rise to 103 million from 72 million under the previous standard. But the number of people who are new candidates for drug treatment will rise only by an estimated 4.2 million people, he said. To reach the goals, others might have to take more drugs or increase the dosages.

WHAT THE CHANGES MEAN New guidelines define high blood pressure as 130/80 for anyone with a significant risk of heart attack or stroke. The guidelines set new categories and get rid of “prehypertension”: >> Normal: Less than 120 over 80 >> Elevated: Top number 120-129 and bottom less than 80 >> Stage 1: Top of 130-139 or bottom of 80-89 >> Stage 2: Top at least 140 or bottom at least 90 >> That means 46 percent of U.S. adults have high pressure (stages 1 or 2) versus 32 percent under the old levels. >> The number of adults with high blood pressure will rise to 103 million from 72 million under the previous standard.

Few risk factors are as important to health. High blood pressure is second only to smoking as a preventable cause of heart attacks and strokes, and heart disease remains the leading killer of Americans.

If Americans act on the guidelines and lower their blood pressure by exercising more and eating a healthier diet, or with drug therapy, they could drive an already falling death rate from heart attacks and stroke even lower, experts said.

Now high blood pressure will be defined as 130/80 millimeters of mercury or greater for anyone with a significant risk of heart attack or stroke. The previous guidelines defined high blood pressure as 140/90. The first number describes the pressure on blood vessels when the heart contracts, and the second refers to the pressure as the heart relaxes between beats.

(In Hawaii there were 353,900 — or roughly 1 out of 3 adults — with high blood pressure under the previous guidelines, according to the state Department of Health. Under the new guidelines, that number will increase significantly, said Lola Irvin, administrator for the department’s Chronic Disease Prevention and Health Promotion Division. State health officials did not have estimates for how many more people would have the condition.

(“As people gain weight and they become overweight and obese, it increases the risk of high blood pressure and also diabetes,” Irvin said. “It really is important for people to eat healthy, to be physically active and to quit smoking if they smoke.”

(The state Health Department said it is working to increase data collection of undiagnosed hypertension and to improve early detection and diagnosis to address high blood pressure at an earlier stage.)

Cardiovascular disease remains the leading cause of death among Americans. The new criteria, the first official diagnostic revision since 2003, result from growing evidence that blood pressure far lower than had been considered normal greatly reduces the chances of heart attack and stroke, as well as the overall risk of death.

Recent research indicates this is true even among older people for whom intensive treatment had been thought too risky. That finding, from a large federal study in 2015, caught many experts by surprise and set the stage for the revision.

That calculation must be individualized, and experts are recommending that patients use a calculator developed by the guidelines committee at ccccalculator.ccctracker.com.

Nearly half of all American adults, and nearly 80 percent of those ages 65 and older, will find that they qualify and will need to take steps to reduce their blood pressure.

Even under the relatively more lenient standard that had prevailed for years, close to half of patients did not manage to get their blood pressure down to normal.

“Is it going to affect a lot of people, and is it going to be hard to meet those blood pressure goals?” asked Dr. Raymond Townsend, director of the hypertension program at Penn Medicine. “The answer is a pretty significant yes.”

According to the new guidelines, anyone with at least a 10 percent risk of a heart attack or stroke in the next decade should aim for blood pressure below 130/80.

But simply being age 65 or older brings a 10 percent risk of cardiovascular trouble, and so effectively everyone over that age will have to shoot for the new target.

Younger patients with this level of risk include those with conditions like heart disease, kidney disease or diabetes. The new standard will apply to them, as well.

People whose risk of heart attack or stroke is less than 10 percent will be told to aim for blood pressure below 140/90, a more lenient standard, and to take medications if necessary to do so.

If there is any good news for patients here, it is that nearly all the drugs used to treat high blood pressure are generic now. Many cost pennies a day, and most people can take them without side effects.

In formulating the guidelines, the expert committee reviewed more than 1,000 research reports. But the change is due largely to convincing data from a federal study published in 2015.

That study, called Sprint, explored whether markedly lower blood pressure in older people — lower than researchers had ever tried to establish — might be both achievable and beneficial.

While agreeing that lower blood pressure is better, Dr. J. Michael Gaziano, a preventive cardiologist at Brigham and Women’s Hospital and the VA Boston, worries about having doctors and patients fixating on a particular goal.

It’s true, he said, that doctors ought to be more aggressive in treating people at high risk. But, he added, “If a patient comes in with a blood pressure of 180, I will not get him to 130.”

Lifestyle changes like diet and exercise can help many patients lower blood pressure. But many of the newly diagnosed are likely to wind up on drugs, said Dr. Harlan Krumholz, a cardiologist at Yale University.

“This is a big change that will end up labeling many more people with hypertension and recommending drug treatment for many more people,” he said.

The current treatment strategy has not been so successful for many patients, he noted.

“How they tolerate drugs, whether they want to pursue lower levels, are all choices and should not be dictated to them,” Krumholz said. “Or we will have the same situation as today: many prescriptions that go unfilled and pills untaken.”

More intensive drug treatment for those controlling their pressure with drugs could increase rates of kidney disease, some experts fear. In the Sprint trial, the incidence of acute kidney injury was twice as high in the group receiving drugs to reduce their systolic pressure to 120.

“Although the lower goal was better for the heart, it wasn’t better for the kidney,” said Townsend, who is a kidney specialist. “So yeah, I’m worried.”

Honolulu Star-Advertiser reporter Kristen Consillio contributed to this report.