Federal health officials said yesterday that 45 million Americans with blood pressure levels previously considered normal are at increased risk of stroke, heart failure and kidney disease.

Launching an aggressive new approach to one of the nation's leading health problems, they issued new guidelines urging those Americans to make diet and exercise changes designed to lower their blood pressure and significantly reduce their risk of future health problems. They also urged more intensive drug treatment for anyone with higher blood pressure levels.

Labeled "prehypertensive," this new group joins an estimated 50 million Americans -- roughly one in four adults -- who already have high blood pressure, a symptom-less condition that places stress on blood vessels and greatly raises the risk of stroke, heart failure and kidney disease. It is the most commonly diagnosed and treated condition in the United States, resulting in 35 million visits a year to doctors' offices and clinics.

Despite that, "Americans' lifetime risk of developing hypertension is much greater than we'd thought," said Claude Lenfant, director of the National Heart, Lung and Blood Institute, which issued the guidelines. "We also now know that damage to arteries begins at fairly low blood pressure levels, those formerly considered normal and optimal."

Blood pressure readings are expressed in two numbers that reflect the pressure on artery walls when the heart contracts (the "top" number, measured in millimeters of mercury) and when it is between beats (the "bottom" number).

Normal blood pressure is considered anything less than 120 over 80, but studies have shown that the risk of death from heart disease and stroke begins to rise at levels as low as 115 over 75. For people between 40 and 70 years old, each 20-point rise in the top number (systolic pressure), or each 10-point increase in the bottom number (diastolic pressure), doubles the risk of heart disease.

A 55-year-old with normal blood pressure today, furthermore, has a 90 percent chance of developing high blood pressure in the next 25 years, according to decades of data from the Framingham Heart Study. That is why "the new guidelines ask Americans to change how they think about high blood pressure," said Edward J. Roccella, coordinator of the National High Blood Pressure Education Program.

The problem is that awareness of the dangers of high blood pressure has not kept pace with the new data. The public's perceptions have remained unchanged during the past decade, and treatment has increased by less than 10 percent.

As the Mayo Clinic's Thomas E. Kottke and his colleagues note in an editorial that will accompany the guidelines' publication in next week's Journal of the American Medical Association, only one in three people with hypertension succeeds in lowering his or her blood pressure to safe levels. That is 50 percent short of the goals set by the Department of Health and Human Services for the year 2010.

The new guidelines are designed to improve public awareness and medical treatment -- a step that public health officials say could significantly reduce complications, cut medical costs and save lives. Studies suggest that effective antihypertensive treatment decreases strokes by as much as 40 percent, cuts heart attacks by 25 percent and reduces heart failure by half.

Under the new guidelines, the "prehypertensive" category is defined as 120 to 139 for systolic blood pressure and 80 to 89 for diastolic pressure.

People with these blood pressure levels are urged to get down to a healthy weight if they are overweight or obese; exercise more; reduce sodium and alcohol intake; and follow a diet regimen developed by the heart institute that emphasizes fruit and vegetables, low-fat dairy products, low saturated fat and whole grains. Known as Dietary Approaches to Stop Hypertension, or DASH, the plan has been shown to often reduce blood pressure within two to four weeks as effectively as antihypertensive medication.

Drug treatment is not recommended for those in the prehypertensive group unless they have other medical conditions -- diabetes or kidney disease, for example -- that could accelerate their health problems. The report also recommends that for overall health reasons, smokers in the prehypertensive group should quit.

The new guidelines also try to put to rest a debate about drug treatment for those who already have full-blown high blood pressure, defined as levels at or above 140 over 90. Doctors now prescribe a wide range of medications, including diuretics, beta blockers, angiotensin-converting enzyme (ACE) inhibitors and calcium channel blockers.

After reviewing more than three dozen studies, the authors of the guidelines concluded that diuretics -- one of the oldest and least expensive medications -- should be the first-line therapy for most people. These drugs have remained underused despite many studies supporting their effectiveness in preventing cardiovascular complications of high blood pressure, the guidelines note.

If blood pressure does not fall to safe levels with diuretics and lifestyle changes, the guidelines recommend adding other types of drugs -- as many as necessary -- until blood pressure is under control. "The guidelines stress that most patients will need more than one drug to control their hypertension," said Aram V. Chobanian, dean of the Boston University School of Medicine and chairman of the committee that issued the guidelines.

Because treatment and control rates "are still too low," Chobanian said, "the new guidelines zero in on this problem, recommending factors that often lead to inadequate control, such as not prescribing sufficient medication."

John H. Laragh, director of the Cardiovascular Center at New York Hospital-Cornell Medical Center, praised the guidelines for establishing the new prehypertensive category. "I have no quarrel with setting a prehypertensive category, recommending lifestyle modifications or clean living."

But starting treatment with diuretics "is a flawed approach," said Laragh, who still favors using other classes of drugs first.

"He'll have to show me the numbers to support that," responded the institute's Roccella. "They don't exist."