Study Strengthens Linkage of Salt to Coronary Ills

A new study of chimpanzees, the species evolutionarily closest to humans, has provided what scientists are calling the first definitive proof that a high salt intake causes high blood pressure and is thus a risk factor in coronary disease.

Although physicians have suspected such a link for centuries, so far the evidence has been indirect, consisting primarily of epidemiological studies in humans and physiological studies in rodents.

Recently, in fact, many critics have begun to question the conventional wisdom, arguing that stressful living conditions, alcohol and smoking are responsible for the increases previously associated with salt.

The new study, published today in the journal Nature Medicine, refutes such arguments, experts say, and gives new impetus to efforts to remove salt from food.

An international team headed by Dr. Derek Denton of the University of Melbourne in Australia studied 26 chimpanzees housed in a colony in Gabon. Over a 2 1/2-year period, all ate a normal chimp diet low in salt, but midway through the study, half were given salt supplements equivalent to the amounts consumed by the average human. Their blood pressure rose dramatically, only to fall back to normal when the added salt was removed from the diet.

Calling the research “as close as feasible to a decisive experiment,” the multinational team called for a sharp decrease in salt use, particularly in processed foods. The craving for salt was appropriate when humans were hunter-gatherers, Denton said, but is “maladaptive” when salt is cheap and plentiful.

Denton’s findings, “unequivocally demonstrating that high salt intake causes [blood pressure] to rise, have crucial implications in regard to the usual rise of blood pressure with age for populations habitually consuming a high salt diet,” Dr. Jeremiah Stamler of the Northwestern University Medical School, wrote in an editorial in the same issue. Stamler heads one of the largest epidemiological studies of the consequences of salt consumption.

The debate over the importance of salt in hypertension has been heightened by other scientists who have emphasized the roles of alcohol and smoking, and by the efforts of trade groups that promote salt use, added Jeffrey Cutler of the prevention branch of the National Heart, Lung and Blood Institute. “Studies like this help to put us back on course.”

Both the American Heart Assn. and the National Institutes of Health already recommend reductions in salt intake to lower blood pressure and improve health.

“Even a modest reduction in blood pressure may translate into a significant reduction of cardiovascular events,” said Dr. Theodore Kotchen of the Medical College of Wisconsin, a member of the heart association’s nutrition committee. “I think it’s an important study.”

The one dissenting voice came from Rhona Applebaum, executive vice president for scientific and regulatory affairs for the National Food Processors Assn. “This is an interesting study in chimpanzees, but being able to extrapolate findings from this study . . . to supporting public health policy for the general population of Americans . . . is a reach.”

The idea that salt intake increases blood pressure is at least 4,500 years old, showing up in one of the first medical texts ever written. A host of studies in rats and mice have since confirmed the concept, but detractors have argued that rodents are not close enough to humans to permit extrapolation of results and that most of the studies have been conducted on genetically defective rats bred for their susceptibility to salt.

Studies in aboriginal societies of humans have drawn the same conclusions. Hunter-gatherer tribes, in general, have an extremely low incidence of hypertension, but when individuals from such tribes move into contemporary societies, their blood pressure typically rises dramatically within weeks. Again, however, critics attribute the results to alcohol, smoking, stress and other factors.

Scientists say that it would be difficult, if not ethically impossible, to perform experiments such as the chimpanzee study in humans who have not been previously exposed to high levels of salt in their diet. Not only are such groups few and far between, but most researchers do not want to run the risk of habituating them to high salt content with the possibility of raising their blood pressure and risk of cardiovascular illnesses.

Chimpanzees are very expensive to purchase and maintain for laboratory experiments, experts said. The current studies were prompted in part by the renewed controversies over the risk of salt intake.

Although most scientists now agree that controlling salt is desirable, the process is extremely difficult because of the growing influence of prepared foods and fast foods in the American diet. More than 75% of the salt people consume comes not from the salt shaker, but from processed and fast foods, where it is added to improve the taste, according to the heart, lung and blood institute.

Generally, the consumer is not even aware of the presence of salt. A chocolate milkshake purchased at a fast-food restaurant, for example, often has more salt than an order of french fries.

The institute now recommends that Americans consume no more than 2.4 grams of sodium per day. That is the amount in six grams of salt, the equivalent of about one teaspoon. The American Heart Assn. recommends a cap of three grams per day. The average American consumes 15 to 20 grams per day.