Could drinking higher-salinity water help lower blood pressure?
A recent study has linked drinking water of higher salinity to lower blood pressure in people living in a coastal region of Bangladesh. Sources of drinking water in the region can vary in salinity due to the influx of seawater.
While water of higher salinity contains more sodium, which can raise blood pressure, it also has more calcium and magnesium. The researchers explain this in a Journal of the American Heart Association paper about the study.
"Calcium and magnesium are protective; they decrease blood pressure," says lead study author Abu Mohammed Naser, who is a postdoctoral fellow in the Rollins School of Public Health at Emory University in Atlanta, GA.
He and his co-authors attribute the study's findings to the benefits of magnesium and calcium outweighing the harms of sodium.
Data on water salinity, blood pressure limited
High blood pressure, or hypertension, is the "leading preventable cause" of early deaths worldwide, according to a 2016 Circulation study that estimated that 1.39 billion people were living with the condition in 2010.
Having blood pressure that is too high increases the force that circulating blood exerts on artery walls. If the condition persists, it can damage the heart and raise the risk of stroke and other health problems.
According to the Centers for Disease Control and Prevention (CDC), there are around 75 million adults with high blood pressure in the United States, where the condition contributed to or caused more than 410,000 deaths in 2014.
Studying people who live in coastal regions offers a useful way to compare the effects of varying water salinity on health.
Naser and his colleagues note that groundwater is the main source of drinking water for more than 1 billion people who live in coastal regions.
Of this population, around a fifth live in areas in which seawater flows into groundwater, giving rise to varying levels of mineralization.
However, they note that data on "drinking water salinity, mineral intake, and cardiovascular health of the population," are limited.
Calcium and magnesium 'counteract' sodium
Their analysis took in data from two studies that had kept track of people in various parts of coastal Bangladesh. The measurements covered periods in which the salinity of drinking water varied as a result of monsoons and dry weather.
The team found that people who drank water of mild or moderate salinity had more sodium in their urine than people who drank fresh water of low salinity. Also, those with higher levels of urinary sodium also had higher systolic blood pressure.
In addition, the analysis revealed that those who drank water of mild and moderate salinity had higher levels of calcium and magnesium in their urine. Having higher levels of these minerals has associations with lower systolic and diastolic blood pressure.
For example, people who drank "mildly-salinated" water had an average systolic blood pressure that was 1.55 of mercury (mm Hg) lower and an average diastolic blood pressure that was 1.26 mm Hg lower than those who drank fresh water.
Systolic blood pressure is the pressure of blood in arteries during a heartbeat while diastolic is the pressure between heartbeats. Systolic is typically the higher of the two numbers.
The authors hypothesize "that the [blood pressure]-lowering effects of [calcium] and [magnesium] counteracted the harmful effects of [sodium] [...]."
They cite studies that have found similar effects in other parts of the world. Some of these studies have linked drinking calcium- and magnesium-rich water to a reduction in deaths due to cardiovascular causes.
Proving the case for fortifying drinking water
Dr. Robert M. Carey, who is a professor of medicine at the University of Virginia in Charlottesville, helped to produce the American Heart Association (AHA) and the American College of Cardiology's latest guidelines on blood pressure. He was not involved in the study and made some comments about it.
He notes that while the reductions in blood pressure are not great, they are large enough to make a difference, and therefore, these results warrant further investigation.
He continues, "I think it's pretty clear from many different studies that a small reduction in blood pressure, done consistently, can have a major impact in reducing cardiovascular disease and stroke."
He points out that the study does not show that adding calcium and magnesium to drinking water actually lowers blood pressure. It is for further studies, conducted in clinical settings, to investigate this, he explains.
If further research indeed establishes that fortifying drinking water with calcium and magnesium can lower blood pressure, then that could be a completely new approach to dealing with hypertension as a public health issue.
The convention until now, Dr. Carey explains, has been to "wait until someone becomes hypertensive" and then proceed with lifestyle changes and drug treatments to help them manage their blood pressure. "I think we need to do both," he suggests.
The AHA recommend that people should get the vitamins and minerals that they need by following a healthful diet. The Academy of Nutrition and Dietetics also support this and do not recommend the use of supplements as a way to protect against chronic disease.
However, the researchers point out that most people in the U.S. do not meet the daily recommended intake for minerals: they don't eat sufficient amounts of the foods that are rich in them.
Naser states that depleting mineral levels in the soil due to "over-farming" and alteration in rainfall from climate change may also be a factor. Fortifying drinking water with beneficial minerals could make up the shortfall, he suggests.
He also points out that the body absorbs minerals better from drinking water than from food, where their "bioavailability" could be lower.
The authors conclude:
"Ensuring optimum concentrations of [calcium] and [magnesium] in drinking water may be an important public health and nutritional intervention to ensure fulfillment of daily requirements of these essential macro-minerals since evidence suggests that globally, concentrations of these minerals are decreasing in the diet."