(Reuters) - About half of the 65 million people in the United States who have high blood pressure now have it under control, up from 27 percent two decades ago, U.S. researchers said on Tuesday.

But the overall rate of Americans who have high blood pressure has not changed in recent years, reflecting the need for better prevention efforts, they wrote in the Journal of the American Medical Association.

The Institute of Medicine earlier this year declared high blood pressure, or hypertension, a "neglected disease" that costs the U.S. health system $73 billion a year.

High blood pressure, or too much force exerted by blood as it moves against vessel walls, is easily preventable through diet, exercise and drugs, yet it is the second-leading cause of death in the United States.

The IOM, one of the National Academies of Sciences, urged the U.S. Food and Drug Administration to regulate the amount of salt added to foods to help Americans cut their high sodium intake, which can lead to high blood pressure, kidney failure and strokes.

Dr. Brent Egan of the Medical University of South Carolina and colleagues studied changes in hypertension rates, awareness, treatment and control among nearly 43,000 U.S. adults over two periods -- 1988-1994 and 1999-2008.

People in the group reported whether they were taking blood pressure control medications.

The team defined high blood pressure as being at least 140 for the systolic or top reading, and 90 for diastolic or lower the lower reading. Normal blood pressure is considered to be 120 over 80 or lower.

Between the two periods, the team saw a steady improvement among people who were able to get their hypertension under control, rising from 27.3 percent in the first period to 51.1 percent in the period ending in 2008.

These improvements reflected increased awareness of the need to control blood pressure and a greater proportion of people who were taking medications to keep their blood pressure in check.