In a study of more than 4,000 adults, scientists revealed that a diet rich in foods that are linked with lower blood pressure — such as fruits and vegetables — did not offset the blood pressure-increasing effects of consuming too much salt.
Co-lead study author Dr. Jeremiah Stamler — who works in the Feinberg School of Medicine at Northwestern University in Chicago, IL — and colleagues recently reported their results in the journal Hypertension.
High blood pressure, or hypertension, occurs when the force of blood that pushes against the artery walls becomes too high. This can cause damage to the heart and blood vessels and raise the risk of stroke and heart disease.
According to the Centers for Disease Control and Prevention (CDC), around 75 million people in the United States — or 32 percent of the country's population — have high blood pressure.
However, revised guidelines from the American Heart Association (AHA) and the American College of Cardiology — which were published in November last year — now mean that almost half of U.S. adults can now be classed as hypertensive.
Under these new guidelines, a person is categorized as having high blood pressure if their systolic blood pressure (or the top number that measures the blood pressure when the heart beats) is 130 millimeters of mercury (mmHg) or higher and their diastolic blood pressure (or the bottom number that measures blood pressure between heart beats) is 80 mmHg or higher.
Salt and blood pressure
Eating too much salt is considered a key risk factor for high blood pressure. So, the 2015–2020 Dietary Guidelines for Americans recommend that we limit our intake of sodium — which is the primary component of table salt — to no more than 2,300 milligrams, or approximately one teaspoon, each day.
But despite this recommendation, most adults in the U.S. consume more than 3,400 milligrams of sodium daily.
Health organizations state that a diet rich in fruits and vegetables can help to lower blood pressure, but can such a diet counteract the effects of a high salt intake? Not according to Dr. Stamler and colleagues.
The researchers came to their findings by analyzing the data of 4,680 men and women who were a part of the International Study on Macro/Micronutrients and Blood Pressure.
Subjects were from the U.S., the United Kingdom, China, and Japan, and they were all between the ages of 40 and 59.
Over a 4-day period, two urine samples were taken from each subject per day. These were tested for levels of sodium, as well as potassium.
Also, on each of the 4 days, participants were asked to recall their food and drink intake over the past 24 hours. The team used this information to calculate the subjects' intake of more than 80 nutrients — including fats, proteins, amino acids, vitamins, and minerals.
The participants' blood pressure was also measured twice per day over the study period.
'Having a low-salt diet is key'
The results of the study revealed that participants who had a high amount of sodium in their diet were more likely to have hypertension than those with low dietary sodium, regardless of their intake of fruits and vegetables.
In detail, the scientists found that each additional 118.7 millimoles of sodium in urine excretion over a 24-hour period was linked with a systolic blood pressure increase of 3.7 mmHg, even after controlling for levels of potassium and other blood pressure-lowering nutrients in the urine.
Based on these results, the researchers conclude that eating high amounts of fruits, vegetables, and whole grains will not offset the negative effects of too much salt.
"We currently have a global epidemic of high salt intake — and high blood pressure. This research shows there are no cheats when it comes to reducing blood pressure. Having a low-salt diet is key — even if your diet is otherwise healthy and balanced."
Co-lead study author Dr. Queenie Chan, Imperial College London, U.K.
The team notes that around 75 percent of salt intake in the U.S. comes from restaurant foods or foods that are prepackaged or processed.
As such, the team calls for food manufacturers to reduce the amount of salt that they add to their products, as a way of helping to protect our heart health.
"We're learning more about the role other nutrients play in influencing the blood pressure-raising effects of sodium, and that the focus on sodium remains important," says Cheryl Anderson, Ph.D., vice-chair of the AHA's Nutrition Committee, who was not involved in the study.
"Restaurant and prepackaged food companies must be part of the solution," she writes, "because Americans desire the ability to choose foods that allow them to meet their sodium reduction goals."