Salt is essential not only to life, but to good health. Doctors often recommend replacing water and salt lost in exercise (see advice for ultraendurance athletes). Increased salt intakes have been used successfully to combat Chronic Fatigue Syndrome, with some very satisfying results. Dramatic deficiencies or "excessive" sodium intakes have been associated with other conditions and diseases, such as stomach cancer. The most talked-about is the association of dietary sodium and elevated blood pressures (hypertension).
It is recommended that we consume a minimum of 500 mg/day of sodium. While individual requirements vary, most Australians have no trouble reaching this minimum and in fact consume "excess" sodium above and beyond that required for proper bodily function. The kidneys efficiently process this "excess" sodium in healthy people. Experimental studies show that most humans tolerate a wide range of sodium intakes, from about 250 mg/day to over 30,000 mg/day. Blood pressure for most people is only marginally affected up to the 7,000 - 11,500 mg/day range. The actual range is much narrower. For healthy people, a moderate sodium diet may range from 2,300 to 4,600 mg/day of sodium. But, if you have elevated blood pressures, your doctor may well recommend a much lower intake of salt and sodium (The Merck Manual, for example suggests that restriction to 2,000 mg/day of sodium "may be necessary in severe cases").
Salt and Blood Pressure
One of salt's major functions is to regulate blood volume and pressure including the flexibility of the blood vessels. As long ago as 2,000 B.C. when the famous Chinese "Yellow Emperor" recorded salt's association with a "hardened pulse," we have known of a relationship between salt and blood pressure. But the exact nature of that relationship has proved far more complex than the ancient Chinese might have imagined. The role of salt and sodium in blood pressure remains to this day both imperfectly understood and intensely controversial. Unravelling the complexity of the relationship of sodium and blood pressure is an important priority. High blood pressure, or hypertension, is a significant risk factor for cardiovascular disease. Hypertension afflicts millions each year, despite the fact that improved treatment techniques have reduced the mortality rate of coronary heart disease by 50%, and that of stroke by 57%.
The human heart is a big pump. When it contracts, it forces blood through the arteries of the circulatory system; that pressure is "systolic," the "top" number. Between heartbeats, the heart relaxes. Pressure measured between heartbeats is "diastolic," the "bottom" number. When blood volume increases or the blood vessel walls don't expand enough, blood pressure increases. Normal blood pressure is less than 130/85 according to the National Heart, Lung and Blood Institute of the National Institutes of Medicine. Years ago, doctors considered pressures only above 160/100 to be "hypertension," but more recent evidence suggests that risk increases anywhere above the "normal" level. Even "high normal" pressures above 130/85 are of concern. And treatment of "mild," Stage 1, hypertension may begin at 140/90. Fifty million Americans, one in every five persons, has hypertension, albeit most of the "mild" Stage 1 variety.
A number of "risk factors" are associated with having elevated pressures. The primary risk factor, however, is beyond our control since we can't choose our parents. Genetic factors explain a quarter to half of blood pressure variability - five times more than environmental factors such as stress, physical activity/exercise, smoking and, of course, diet. Among dietary risk factors, obesity is generally recognised as the most important, followed by excess alcohol consumption and then salt intake.
A 1996 meta-analysis in the Journal of the American Medical Association (JAMA) which examined the clinical trials of salt intake and blood pressure found no significant association for people with normal blood pressure. Read a summary or the full article. In May, 1998, JAMA published another, larger meta-analyis confirming the 1996 study and documenting, as well, a series of adverse changes to blood chemistry among those placed on low-sodium diets in the clinical trials. Recent evidence, however, suggests that it would be more accurate to characterise salt as a risk factor only in conjunction with intakes of the other electrolytes-potassium, calcium and magnesium. It is the balance of these interacting electrolytes in the body that is important more than the amount of any one of them, including sodium.
Benefits of Lowering Blood Pressure
While we have known for four thousand years that salt had a blood pressure relationship, it was not until the last half of this century that researchers focused on the sodium ion. Thirty years ago, Dr. Lewis Dahl stimulated the interest of public health authorities with a dramatic and graphic depiction of the "sodium hypothesis," a straight line correlation of population salt intakes with the prevalence of hypertension in those societies. Dr. Dahl's research methodology and conclusions provoked a storm of controversy. Nevertheless, based on the "sodium hypothesis" and the assumption that lowered sodium intakes would reduce population blood pressures and, hence, reduce the documented risks for stroke and heart attack, the U.S. and a number of other countries began actively discouraging sodium consumption in the general population. Only belatedly did the continuing controversy prompt proponents of the "sodium hypothesis" to conduct a large-scale, uniform methodology study to confirm whether the hypothesis was valid. It was not until mid-1988 that medical journals began to publish the results of this massive effort, the Intersalt Study. These findings showed a scant relationship between sodium and blood pressure. "Salt has little importance in hypertension" headlined the accompanying editorial in the prestigious British Medical Journal. The Intersalt researchers measured urinary electrolytes and blood pressures in 10,079 individuals in 52 centers in 32 countries using standard methods and analyzing the samples in a single laboratory. The head of the American Heart Association's Nutrition Committee and member of the U.S. Dietary Guidelines Advisory Committee summarized: "We're trying to back away from our salt recommendation without looking like fools."
Salt Restriction: Treatment or Prevention
While salt restriction can reduce the blood pressures of some hypertensive persons, but not others, salt has never been shown to cause hypertension. As a corollary, salt restriction has not been shown to prevent hypertension. Often in magazines and newspaper coverage of the salt-blood pressure story, the reporter confuses and blurs the two concerns - prevention and treatment. Salt restriction can be effective dietary therapy, a useful treatment for hypertension. Salt restriction as a hypertension prevention strategy is unproven. As the U.S. Surgeon General declared in the late 1980s: "To date, however, no reported studies have tested this hypothesis (that salt restriction can prevent hypertension) directly."
Effective Low-salt Diets
Working with a professional can help you avoid dietary imbalance. None of us eat nutrients, of course; we eat foods. And foods have lots of different nutrients. If your low-sodium diet has you cut back on eating bread and drinking less milk, for example, the biggest sources of dietary sodium in the average diet, you also would lose the other nutrients they contain - calcium, potassium and iron. Your doctor or dietician can help fashion a diet that supplements the loss of these nutrients. If you have a water conditioner, make sure that you mention this to your doctor if you've been placed on a low-sodium diet; depending on influent water quality, these units can add significant amounts of sodium to water -- a concern to those on strict low-sodium diets.
Nothing is risk free. There are adverse metabolic effects of low-salt diets and a June 1995 study in the American Heart Association's journal Hypertension and a 1998 study in the British journal The Lancet found low-salt dieters suffered more heart attacks as those with normal sodium intakes. But, perhaps the greatest risk of a low-salt diet is fooling yourself that you've licked the problem. First of all, unsupervised low-salt, low-sodium dieters regularly overestimate the amount of sodium reduction they have been able to achieve and maintain. Doctors can measure the actual restriction. Second, if achieved, the low salt diet will lower blood pressure only in a minority of the population; don't assume just because you were able to cut back successfully on salt or sodium that your blood pressure is under control. Such a comfortable assumption, for the majority whose blood pressures either remain the same or go up, could lead to abandonment of other worthwhile steps such as reducing obesity, moderating alcohol consumption, stopping smoking or balancing intakes of the other electrolytes, potassium, calcium and magnesium. Therefore, be sure to consult regularly with your medical professional as you continue on a low-salt diet.
The Salt Institute - used with permission 2001.