Shaking Up Conventional Thinking About Sodium and Hypertension
Controversy continues to churn as to whether reducing your salt intake will help to keep your blood pressure in check. In fact, some experts even say that low-salt diets may be detrimental to your health. Who do you believe?
In May 1996, two leading medical journals reported vastly different conclusions about whether or not people with normal blood pressure should be concerned about their salt (sodium) intake.
On one hand, the Journal of the American Medical Association (JAMA) reported that while low-sodium diets slightly reduced blood pressure in people that already had hypertension , these same diets had no effect on healthy people. The study's co-author, Dr. Alexander Logan of the University of Toronto, concluded that "the present level of [sodium] consumption is just fine for people with normal blood pressure".
On the other hand, a report from the British Medical Journal (BMJ) determined that low-sodium diets are of benefit to everyone—regardless of blood pressure. That finding was based on a re-analysis of data from a 1988 study, INTERSALT, that was initially unable to confirm the sodium-hypertension link.
These two studies came on the heels of an earlier study published in Hypertension , a journal of the American Heart Association. In that four-year study of 3000 people, men with high blood pressure who ate salt had four times as many heart attacks as those who did not.
So, who's right?
Some Grains of History
In the late 1940s and early 1950s, physicians routinely prescribed low-sodium diets for patients with high blood pressure (hypertension.) However, in 1954, the US Food and Nutrition Board warned that "harmful results may follow the restriction of sodium intake...these changes are potentially dangerous, perhaps leading to reduced kidney function." At least two doctors came forward publicly to state that self-imposed sodium restriction was of no benefit to healthy people, and that such a restriction would neither prevent nor produce high blood pressure.
The advice that people with high blood pressure reduce their sodium intake has never really been in dispute. But somewhere along the way, the distinctions between prevention and treatment grew blurry. In what is now considered the origin of the "sodium hypothesis," Dr. Lewis K. Dahl theorized that there was a direct correlation between the sodium intake of a population and the subsequent development of hypertension. As the result of well-placed advocacy efforts, physicians suddenly began prescribing low-salt diets for all their patients, not just those with documented high blood pressure.
Over time, the sodium hypothesis grew to mammoth proportions and was accepted as fact. In the 1970s and 1980s, however, skepticism began to inch forward with the realization that the assumptions about sodium and hypertension were not supported by scientific evidence.
In the early 1980s, researchers began to question whether recommendations for universal sodium restriction were necessary or even safe. Studies began to show unanticipated risks of low-sodium diets, including elevated blood pressures, elevated LDL (bad) cholesterol, and higher fasting insulin levels.
To answer the challenge against the sodium hypothesis, a massive international study, INTERSALT, was undertaken to finally confirm the relationship between sodium and hypertension. By 1986, the researchers had examined numerous studies purported to link sodium and hypertension. When the final data were analyzed, only three studies demonstrated any sodium-hypertension association. The results were summarized in BMJ : "Salt has only small importance in hypertension".
Despite the lack of results from INTERSALT, the US government has continued to recommend that everyone should reduce their consumption of sodium, even though the 1988 US Surgeon General's Report admitted that the relationship of universal sodium restriction to lowering blood pressure had never really been tested.
The Controversy Goes On
Dr. Michael Alderman, past president of the American Society of Hypertension, is an outspoken adversary of a universal sodium restriction. The coauthor of the Hypertension study, he says that, "We have no scientific basis for recommending a low-salt diet to the public. Such diets do lower blood pressure in some people, but they are not necessarily healthier for it. We could also lower blood pressure by putting leeches on people, but no one would advise doing that."
Dr. Alderman believes that low-sodium diets may actually be harmful. When salt intake is reduced, plasma renin increases. Renin, an enzyme produced by the kidneys, can swell the lining of the small arteries, forcing the heart to pump harder.
Jeffrey Cutler, at the National Heart, Lung, and Blood Institute (NHLBI), on the other hand, is an outspoken advocate of salt restriction. He says there is an abundance of evidence showing that blood pressure rises along with sodium consumption.
"I don't think that we can say with a great degree of certainty what the optimally low-sodium intake should be, but we can say that the vast majority of people eat far too much salt."
When studies and experts are in conflict, one of the best approaches is to conduct one or more systematic reviews of the evidence. The systematic reviews performed to date are probably closer to confirming Dr. Alderman’s viewpoint: reducing sodium has a small effect on lowering blood pressure that has not yet been shown to reduce the risk of complications. Reducing sodium also does raise plasma rennin–though any harm from this remains to be proven.
So What Should We Do?
Sodium is only one risk factor for hypertension. Controversy aside, the experts all agree that there are precautions you can take to keep your blood pressure under control.
- Drink your milk.
- Several studies show that low-calcium levels may play a role in the development of hypertension. Although the effect of calcium on blood pressure is probably modest, people should take this opportunity to examine the amount of calcium in their diets. This may have a positive effect on hypertension, and will definitely have a positive effect on osteoporosis , a win-win proposition.
- Lose some weight.
- Overweight people have a hypertension risk six times greater than their slim counterparts. Dropping even a few pounds can reduce your blood pressure by several points.
- Move it.
- Thirty minutes of brisk walking can lower the blood pressure of hypertensive people by about 10 points. Sedentary people have up to a 50% higher chance of developing hypertension than do their active peers.
- Eat your fruits and vegetables.
- A study in the New England Journal of Medicine showed that a diet low in saturated fat, high in low-fat dairy products, and containing 8-10 servings of fruits and vegetables can dramatically reduce hypertension.
- Watch the wine and beer.
- More than two drinks a day can substantially raise your blood pressure. Health experts advise no more than one drink a day for women and two for men.
As with all nutrients, moderation is the key. Regardless of your hypertension status, it makes sense to eat salt in moderation. Add to this 4-5 servings of calcium-rich foods, 8-10 servings of vegetables and fruits daily, and exercise.
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Last reviewed June 2006 by Lawrence Frisch, MD, MPH
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