Salt raises chimps' blood pressure.

Four thousand five hundred years ago, the Yellow Emperor of China noted in the Nei Ching, an ancient book of medicine, that "if too much salt is used for food, the pulse hardens." That hardened pulse signals high blood pressure.

Researchers today still debate how large a role--if any--salt plays in the epidemic of high blood pressure in Western societies. Some rank salt intake high among determinants of hypertension, while others maintain that factors such as smoking and the stresses of modern society have much greater significance.

Now, research by an international team of investigators goes far in teasing out salt's role from the morass of factors contributing to hypertension. By studying the effects of salt in chimpanzees, the team shows that dietary salt alone can significantly raise the animals' blood pressure.

"The real importance of this experiment in this species, which is closest to humans, is in relation to the basic principles for diet for infants, children, and young adults," says lead investigator Derek Denton of the Howard Florey Institute of Experimental Physiology and Medicine at the University of Melbourne in Australia. "It probably is a good idea to reduce the sodium intake and increase the potassium to make our diet closer to that we evolved with."

Epidemiological evidence to support Denton's advice has existed for years. Preliterate peoples such as the Kalahari Bushmen live on largely vegetarian diets, consuming about half a gram of salt per day compared to the 10 grams a day of the average person in the United States. And the Bushmen's diet is high in potassium. Unlike people in developed countries, Bushmen see no increase in hypertension with age, and the disease itself is virtually unknown.

When the Bushmen move into cities and begin eating a more Western diet, however, their rates of hypertension rise. But such observations fail to prove that salt causes hypertension. As well as changing their diets, urbanized Bushmen may start smoking, drinking alcohol, and eating more fat. Changes in social dynamics may cause stress, which can increase blood pressure.

"It is very difficult in human epidemiological studies to get clear relations between salt and high blood pressure," says Denton. But the chimpanzee study "was a one-variable experiment."

Denton's team studied an established colony of 26 chimpanzees in Gabon, as they report in the October Nature Medicine. The researchers separated the animals into two groups of 13, giving them identical diets of fruits and vegetables. The researchers supplemented the animals' diets with a liquid infant formula that provided calcium--previous studies have indicated that low calcium may lead to hypertension--and, for one group, an amount of sodium that increased over 22 weeks to 15 grams per day.

The researchers measured the animals' blood pressure in millimeters of mercury (mmHg) as a ratio of systolic pressure (when the heart pumps blood) over diastolic pressure (when the heart rests between beats). Normal adult human blood pressure is around 120/80. After 20 months on the high-salt diet, blood pressure in seven chimps rose by an average of 33 mmHg systolic and 10 mmHg diastolic. Three chimps showed no increase, while three others failed to drink all of their sodium-laced formula. The control group experienced no rise in blood pressure. Moreover, 6 months after the researchers weaned the animals off sodium, their blood pressures returned to normal.

Alan R. Dyer of Northwestern University Medical School in Chicago, who wrote a commentary in the same issue, told Science News that "the paper is quite convincing about the effect sodium can have in an animal model that is close to man." He notes that "most of the animals can be described as salt-sensitive."

Denton agrees that the data indicate salt sensitivity in most of his animals, but he observes that some develop no problems with the added salt. He suggests that genetic testing may one day identify which humans are susceptible, but until then he advises everyone to lower their salt intake.