Nearly half of U.S. adults are classified as having high blood pressure under new national guidelines announced Monday, up from one-third of Americans by the old definition.

Call it moving the goalposts, doctor-style. Hypertension now means a reading of at least 130 for systolic blood pressure — the higher of those two numbers you hear at the doctor's office — down from the old cutoff of 140.

Yet most of the millions who suddenly are considered to have high blood pressure will not be counseled to lower it with drugs, according to the standards from the American Heart Association and the American College of Cardiology.

Instead, doctors will advise them to follow common-sense lifestyle changes: get regular exercise, eat a healthy diet, and, if needed, lose weight and stop smoking.

The new guidelines are based largely on a trial that found the number of heart attacks, strokes, and cardiovascular deaths was significantly lower among older people who took drugs to lower their systolic blood pressure to 120, compared with patients who achieved numbers in the low- to mid-130s.

Yet along with that lower blood pressure came higher rates of dizziness and kidney problems, likely explaining why the authors of the new guidelines set the dividing line at 130, not 120, said Raymond Townsend, director of the Hypertension Program at Penn Medicine.

Even setting the cutoff at 130 will mean an increase in dizziness, but Townsend said the tradeoff will be worth it in terms of fewer heart attacks and strokes, so long as physicians manage their patients carefully, keeping an eye on such indicators as sodium and potassium levels.

"You're going to see a little bit more dizziness. You're going to see a bit more kidney problems," Townsend said. "We're going to have to do blood work. It's going to ramp up the intensity of office management of hypertension."

And if unwanted side effects arise, physicians must listen to their patients' wishes, said Daniel Edmundowicz, chief of cardiology at Temple University Hospital.

"If my patient comes in and says, 'I can't tolerate this more aggressive approach,' then you back off," he said.

The reason lower blood pressure can raise the risk of dizziness and kidney problems is simply that the brain and kidneys require a certain amount of pressure to "drive" their function, pushing blood through the organs, said Edmundowicz, who also is medical director of the Temple Heart and Vascular Institute.

The new blood pressure guidelines break down the population into five categories:

Normal: Less than 120 over 80. (i.e., the higher blood pressure number is below 120 and the lower number is below 80.)

Elevated: The top number is between 120 and 129 while the bottom number is below 80.

Stage 1 hypertension: The top number is between 130 and 139 or the bottom number is between 80 and 89.

Stage 2: The top number is at least 140, or the bottom number is at least 90.

Hypertensive crisis: The top number is above 180, or the bottom number is above 120, or both.

The study that prompted these changes found that over a three-year period, there were 243 "events" — strokes, heart attacks, and cardiovascular deaths — in a group of 4,678 people who achieved a systolic blood pressure of 120 through higher doses of medication. That rate was 25 percent less than the 319 events in the group of 4,683 patients that took less medicine, getting their blood pressure down to the low- to mid-130s.

Simply put, a higher blood pressure means the heart is working harder. Over the long term, that has a series of grave consequences. First, the heart muscle becomes thicker, making it harder for its innermost layer to receive blood flow.

"You starve it a little bit," Penn's Townsend said.

That in turn means a higher risk of rhythm problems and heart failure, he said.

Second, a higher blood pressure also increases the rate of plaque formation on the walls of arteries.

The new guidelines included specific instructions for physicians treating African American patients, who are more likely to suffer from high blood pressure and may respond to certain medications differently than white people.

In African American adults with hypertension but without heart failure or chronic kidney disease, the guidelines call for lowering blood pressure initially with one of two types of medicine: a thiazide-type diuretic or a calcium-channel blocker.