Salt and Blood Pressure

In ancient times, dietary salt was a scarce and prized commodity. Humans need salt, along with potassium and other minerals, for many essential bodily functions, including the regulation of water flow and storage and the maintenance of a normal heart beat.

Today salt is common and cheap, and people use it to season many foods. However, many scientists believe that too much dietary salt can increase people's blood pressure. High blood pressure increases a person's chances of having a stroke, a heart attack, or kidney failure.

Although excess dietary salt is suspected as a cause of high blood pressure, it has been difficult to prove that it is. Many other factors have been linked to high blood pressure, including alcohol, tobacco, diabetes, stress and obesity.

People who live in primitive tribes, such as the Kalahari bushmen of Africa, have provided some evidence that salt contributes to high blood pressure. The Kalahari normally eat little salt and in general do not suffer from high blood pressure. However, studies showed that when the same people moved into industrialized areas and began eating more salt, their blood pressure often went up.

Skeptics pointed out that the rise in blood pressure was not necessarily caused by increased salt. It could have stemmed from the stress of living in a radically different setting or some other factor.

A new study of chimpanzees, humans' closest relatives, strongly supports the idea that dietary salt can raise blood pressure. Researcher Derek Denton, of the Howard Florey Institute of Experimental Physiology at the University of Melbourne, Australia, and his colleagues studied 26 adult chimpanzees in Gabon, an African country. All the chimps received a mostly vegetarian diet, supplemented with a liquid baby formula.

Half of the chimps received formula spiked with amounts of dietary salt that increased over a 22-week period from five grams to 15 grams a day. Three of the chimps receiving the salted formula would not drink all of it, and data from these chimps were left out of the final analysis.

Blood pressure is measured as a ratio between the pressure in an artery when the heart pumps, called the systolic pressure, and the pressure in an artery when the heart rests between beats, called the diastolic pressure. Normal adult blood pressure in both humans and chimpanzees is about 120/80.

Among the 10 chimpanzees who ate all the salt, three did not get high blood pressure. In seven of the chimps, however, blood pressure rose significantly, going higher than 150/90 in some cases. After three months on a normal diet, the chimps' blood pressure returned to normal. None of the control chimps developed high blood pressure.

The pattern of high blood pressure in the salt-eating chimps resembled that in salt-eating humans. Some humans do not get high blood pressure when they eat high-salt diets. Many others, however, do get high blood pressure. It would be difficult, if not impossible, to duplicate the experiment in humans. The researchers controlled every aspect of the chimps' lives and the only difference between the two groups was their salt intake. Humans would probably not agree to live under such conditions for several months.

Denton published his findings in the October issue of the journal Nature Medicine. Scientists hailed the report as the first solid step in linking excess dietary salt to high blood pressure.

Sources

"Salt Raises Chimps' Blood Pressure." Lisa Seachrist. Science News, October 7, 1995, page 228.

"Study of Chimps Strongly Backs Salt's Link to High Blood Pressure." Harold M. Schmeck Jr. New York Times, October 3, 1995, page C3.