Anatomy of Hypertension and the Suspected Role of Sodium

The body maintains blood pressure through a complex system. The system works by a means of nerve signals, hormones, and other influences to expand or narrow the arterioles—small, muscular blood vessels that transport oxygen and nutrients from the arteries to the tissues. If a part of the body desires a lot of nourishment at a given time (the stomach during digestion, for example), nearby arterioles expand to allow increased blood flow. Arterioles in other parts of the body constrict to sustain normal blood pressure. In various people, the regulatory system goes awry. Arterioles in the body contract at the same time and stay constricted. Pressure in the larger arteries goes up and stays up. The result is an abnormal, constant rise in blood pressure. In about 10 percent of hypertensive victims, the defective regulatory effect is traceable to kidney disease, to localized narrowing of the arteries going to the kidneys, or to a tumor or over activity of the adrenal glands. Several problems may be corrected by surgery or by specific prescriptions, after which blood pressure generally returns to normal. In many cases of hypertension, on the other hand, the cause of the regulatory breakdown is difficult to understand. When the origin is unknown, the condition is called essential hypertension. Some expects believe that the unknown cause is large salt consumption. As verification, they point to studies of salt ingestion in humans and animals. In the early 1960s, Lewis K. Dahl of Brookhaven National Laboratory revealed that a high salt ingestion caused high blood pressure in about three-quarters of the laboratory rats he fed salted feed. The rest weren't affected. In the course of breeding, Dahl then produced two distinct strains of rats: a sensitive strain that developed severe high blood pressure from salted feed, and a resistant strain that ate the same amount of salt but maintained normal pressures. He finished by saying that the unpleasant reaction to salt was inherited. Dahl also made another observation: The blood pressures of the genetically susceptible rats remained normal on a diet very low in salt. Many hypertension establishments believe that humans react to salt in much the same way as Dahl's rats. Few are resistant to hypertension and never develop the disease even despite the fact that they eat large quantity of salt all their lives. Others appear to inherit a susceptibility to high blood pressure, and a high-salt diet seems to advance the disease. If this hypothesis is correct, susceptible people may be able to prevent hypertension by using salt sparingly throughout life. For obvious reasons, clinical studies likewise to those performed on rats have never been done with people. So, direct facts as to whether high-salt ingestion can cause hypertension in humans (or whether a low-salt diet can prevent it) don’t exist. Some indirect facts are available, moreover. It comes mostly from studies of different populations that eat varying amounts of salt. Various isolated tribes, such as the Kung Bushmen of the Kalahari Desert and the Yanomamo Indians of Brazil, consume diets particularly low in salt, usually less than 500 milligrams a day. Studies of such tribes have found that the people experience virtually no hypertension and that their blood pressures do not rise with age, as do those of people in industrialized societies. In contrast, farmers in northern Japan, who preserve food with salt, consume as much as 30 grams of salt daily (about six teaspoons). Approximately 40 percent of them have high blood pressure, and the most common cause of death is stroke. Related studies have been carried out on over 20 cultures, ranging from Greenland Eskimos to natives of the Solomon Islands in the South Pacific. Taken together, the studies show that hypertension is rare in populations that use very little salt. On the contrary, in societies where a lot of salt is consumed, a significant minority of the persons develop the disease. Such studies suggest—but don't prove—that salt ingestion is associated to essential hypertension in humans. Skeptics point out that the cultures that do not experience hypertension are all ancient or nonindustrial people whose salt ingestion is hardly the only thing that sets them apart from Americans. They are better leaner and more bodily active than Americans, their diets differ in other ways, and they are free from various stresses of modern industrial life. Perhaps these factors, and not just a low-salt intake, would help to explain the lack of high blood pressure in such societies. Meanwhile, the exact way that sodium influences blood pressure is still not firmly established. One view is that it affects blood-pressure levels through its ability to promote water retention and expand the body's fluid volume. This in turn produces increased pressure within the arteries. Another theory, which may or may not involve sodium, is that a generalized decrease in the diameter of the smaller blood vessels causes a resistance to blood flow that affects blood pressure. Whether or not those theories are correct, virtually all authorities agree that low-sodium diets can reduce blood pressure in many patients with mild hypertension. What has not yet been established, however, is that a moderate cutback in sodium intake can prevent hypertension in susceptible people. For the majority of Americans, moreover, there's currently no proof that the level of their salt ingestion affects their blood pressure at all. In a study published in 1983, for example, researchers at Yale University School of Medicine examined the effect of salt ingestion on blood pressure among 3566 Connecticut inhabitants who had never been diagnosed as hypertensive. All of the subjects were interviewed about their dietary salt intake and had their blood pressure recorded. After developing an approximate "index" of salt intake, the scientists compared the mean blood pressure of those in the top 10 percent of the salt-consumption index with the mean of those in the bottom 10 percent. The difference in average blood pressure between the heaviest and the lightest salt users was irrelevant. "These result shows the researchers reported in the Journal of the American Medical Association. They also noted, however, that their findings would not rule out the possibility of a clinically important effect for a minority of "salt-sensitive individuals."