The largest study of its kind reveals that the number of people worldwide living with high blood pressure has nearly doubled in the last 4 decades. The huge international effort also reveals a stark contrast between rich and poor countries.

The number of people living with high blood pressure, or hypertension, worldwide has grown from 594 million in 1975 to over 1.1 billion in 2015 – mainly because of population growth and aging – says the study, published in The Lancet.

However, while average blood pressure is high and rising in less affluent countries, especially in south Asia and sub-Saharan Africa, it has dropped to an all-time low in high-income nations like Canada, the United Kingdom, and the United States.

The authors say the reason for this contrast is not clear, but they suggest a major factor could be that people in wealthier nations enjoy better health overall and eat more fruits and vegetables.

Earlier diagnosis and control of hypertension is also more likely to occur in wealthier countries. Taken together, these factors also help reduce obesity, another risk factor for high blood pressure.

Childhood nutrition could be another reason, suggests Majid Ezzati, a senior author of the study and a professor at the School of Public Health at Imperial College London in the U.K., who notes:

“Increasing evidence suggests poor nutrition in early life years increases risk of the high blood pressure in later life, which may explain the growing problem in poor countries.”

High blood pressure major global killer

Blood pressure is the pressure of the blood in the blood vessels. It is assessed from two numbers measured in millimeters of mercury (mmHg): systolic pressure and diastolic pressure.

Systolic pressure is a measure of the heart pumping blood. Diastolic pressure – taken when the heart rests between beats – measures the resistance to blood flow in blood vessels.

High blood pressure is defined as 140 mmHg systolic and 90 mmHg diastolic pressure or higher. This is normally shown as 140/90 mmHg.

Recent research suggests that the risk of death from cardiovascular diseases like ischemic heart disease and stroke doubles with every 20 mmHg systolic or 10 mmHg diastolic increase in people of middle age and older.

“High blood pressure is the leading risk factor for stroke and heart disease, and kills around 7.5 million people worldwide every year,” says Prof. Ezzati.