WASHINGTON — The nation’s heart experts tightened guidelines for high blood pressure Monday, a change that will sharply increase the number of US adults considered hypertensive in the hope that they, and their doctors, will address the deadly condition sooner.

Acting for the first time in 14 years, the American Heart Association and the American College of Cardiology redefined high blood pressure as a reading of 130 over 80, down from 140 over 90.

The change means that 46 percent of US adults, many of them under the age of 45, now will be considered hypertensive. Under the previous guideline, 32 percent of US adults had high blood pressure.

‘‘We’re recognizing that blood pressures that we in the past thought were normal or so-called ‘pre-hypertensive’ actually placed the patient at significant risk for heart disease and death and disability,’’ said Robert M. Carey, cochairman of the group that produced the new report. ‘‘The risk hasn’t changed. What’s changed is our recognition of the risk.’’

But the report’s authors predicted that relatively few of those who fall into the new hypertensive category will need medication.

Rather, they hope, that many found with the early stages of the condition will be able to address it through lifestyle changes such as improving their diet, getting more exercise, consuming less alcohol and sodium, and lowering stress.