Experts have just redefined what counts as high blood pressure, which means the number of people with it is about to rise by tens of millions in the US alone – and all those people are advised to start thinking about lifestyle changes.

In the United States, the definition of high blood pressure comes from recommendations by the American Heart Association and the American College of Cardiology, and this is the first significant change to their official guidelines for 14 years.

The purpose of the reclassification – which will see the number of US adults with high blood pressure or hypertension rise to 103 million from 75 million – is to spot the problem and tackle it earlier than ever.

"We want to be straight with people – if you already have a doubling of risk, you need to know about it," says the lead author of the new guidelines, physician Paul K. Whelton.

"It doesn't mean you need medication, but it's a yellow light that you need to be lowering your blood pressure, mainly with non-drug approaches."

The distinction between those who now have high blood pressure and those who need medication is important: while the number of US adults with hypertension will rise to 46 percent from 32 percent, only a small fraction of those will need drugs to treat it, say the authors of the guidelines.

More exercise, a healthier diet, and less alcohol are some of the ways that blood pressure can be reduced without getting anything new from your doctor.

The new definition of high blood pressure is 130/80 millimetres of mercury or greater: the first number represents the pressure on the blood vessels when the heart contracts, and the second is when the heart relaxes.

The old high blood pressure level was 140/90, which remains in place in other parts of the world like the UK and Australia, at least for the time being.

Experts say they want people to understand more about the complications that can occur at those lower numbers, and have also stressed the need for check-ups by qualified health professionals to get accurate readings.

And the problem is a significant one – every year some 7.5 million people worldwide are estimated to die from heart problems caused by high blood pressure. In the US, it's second only to smoking as a preventable cause of heart attacks.

The authors of the new guidelines are hoping that they'll be particularly helpful in diagnosing younger people at an earlier stage, before serious damage is done.

Meanwhile, scientists have also issued new warnings about the dangers of fluctuating blood pressure.

A team from the Intermountain Medical Center Heart Institute in Utah has found that variations in systolic blood pressure (the higher number) can be as deadly as consistently high blood pressure.

The findings were based on 10,903 patient records covering a period between 2007 and 2013. If the upper blood pressure readings vary by 30-40 between doctor visits, the researchers say, that's a big red flag.

"The call to action for patients as a result of this study is to do everything they can to control their blood pressure on a regular basis," says one of the team, Brian Clements.

"Eat healthy foods, exercise regularly, and if your doctor has prescribed you medications for your blood pressure, be sure and take them consistently."

You can find out more about the new high blood pressure guidelines on the American Heart Association website.