The bottom number in a blood pressure reading (the diastolic pressure) has sometimes played second fiddle to the top number (systolic) in clinical settings, but new research confirms that both numbers are important in determining a person's heart disease risk.

The study, from researchers at Kaiser Permanente in California, was published Wednesday in the New England Journal of Medicine.

"Although systolic does count for a little bit more in terms of the risk of heart attack and stroke, diastolic high blood pressure is a close second, and it's an independent predictor of those risks," said lead author Dr. Alexander Flint, a stroke specialist with Kaiser Permanente.

A high diastolic number "really should not be ignored," he added. "We should not declare victory just because one number is under control. We need to pay attention to both."

Systolic refers to the amount of pressure in a person's arteries, when the heart squeezes and sends blood throughout the body. Diastolic is the pressure in the arteries between heart beats.

The study analyzed more than 36 million blood pressure readings from 1.3 million adults. All were members of Kaiser Permanente in Northern California. Most were white; just 7.5 percent were black.

"Recognizing that the diastolic blood pressure also has to be controlled because it can increase risk for stroke is a good step forward in our management for blood pressure," said Dr. Nieca Goldberg, a cardiologist at NYU Langone Medical Center and medical director of the Joan H. Tisch Center for Women's Health, who was not involved with the research.

Over the past decade, mounting evidence has shown that when patients can get their blood pressure down to levels below what was previously considered "normal," they greatly reduced their risk for heart attack and stroke.

In 2017, the American Heart Association and the American College of Cardiology updated their guidelines for optimal blood pressure. Anyone with a measurement over 130/80 mmHg is considered to have hypertension.

Neither Goldberg nor other outside heart experts were surprised by the findings.

"This is something I've believed for years, that we should treat both systolic and diastolic blood pressure," Dr. Angela Brown, an associate professor of medicine and clinical hypertension specialist at Washington University School of Medicine in St. Louis, wrote in an email to NBC News.

"I emphasize this point in training my students, residents and fellows because they often tend to ignore the diastolic pressure," Brown said.

Brown also emphasized that younger people often have higher diastolic readings than systolic readings, such as 130/100mm Hg, compared with older people whose systolic readings may be higher, such as 170/80 mmHg.

Both systolic and diastolic hypertension can be treated with the same kinds of medications, but some doctors may need to switch the medicines or doses to get patients below that 130/80 mmHg mark.

"Everybody's reaction to medication is different, so it really has to be individually tailored," Flint said.

Lifestyle factors are perhaps the biggest driver of hypertension. The American Heart Associationrecommends several ways to lower your blood pressure and keep it in a healthy range. Those include:

eating a well-balanced diet that's low in salt.

limiting alcohol to two drinks a day for men, one for women.

exercising about 30 minutes a day, five days a week.

quitting smoking.

losing weight if your body mass index is over 25.

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