After 25 years, the great salt debate rages on
Counselling patients when there are no definites
While it may be the salt of life, for decades doctors have admonished hypertensives they must reduce their sodium intake in order to lengthen their lives. Case closed.
But now the great salt debate is once again swirling through medical circles as the waters are muddied by emerging evidence that sodium chloride may not be as bad as we've all believed. The bitter debate has set researchers against one another, amid accusations one side is intractable and the other side has sold out to the salt lobby.
While dozens of studies have looked at the connection between salt, heart disease, and high blood pressure, their methodology and interpretation are under fire. (See chart on studies, p. 61.)
"In the last 20 to 25 years, things have changed greatly in terms of what we know," says Sanford Miller, PhD, dean of graduate studies for biomedical sciences at the University of Texas Health Science Center in San Antonio, and the former director of food safety and applied nutrition at the U.S. Food and Drug Administration who helped set federal salt consumption recommendations 20 years ago.
"Even back then, the data were not overwhelming," he says. "There were some studies that showed substantial effect, some that showed very little effect, and other experiments that showed no effect. But 25 years ago, we didn't have any other data so [the group of researchers] agreed that of all the things we knew about coronary artery disease and hypertension, salt was a pretty good thing to take a shot at because reducing salt intake wasn't going to hurt anybody."
The National Heart, Lung and Blood Institute urges a maximum salt intake of 2,400 mg a day. That's 50% to 75% less than the average American consumes. While the human body only needs about 500 mg daily as a nutrient, researchers are puzzled about the human desire for amounts as great as 12 times what the body needs. Many experts have pointed to the amounts of salt added to baby food which may contribute to a lifelong acquired taste for salt.
Although Miller is far from becoming a traitor to the legions of salt cautionaries he fostered 20 years ago, he now believes that society is composed of people who are "salt-sensitive" and "salt-insensitive."
"If you do a study that has a number of people who are salt-sensitive, you get an effect of reducing sodium. If you do a population that has a larger group of salt-insensitive people, you don't get the effect," he says. But at the same time, Miller says a reduction in salt intake is a good idea. "People ought to be conscious about how much sodium they use, but I don't think they ought to be drastic about it."
Rather than a blanket admonition to avoid salt, he recommends identifying salt-sensitives through urinalysis, sodium levels in blood, and a variety of means, then making recommendations on salt consumption based on test results.
Edward J. Roccella, PhD, MPH, coordinator of the National High Blood Pressure Education Program (NHBPEP) in Bethesda, MD, says he considers himself a traditionalist in the salt debate. "When we look at the preponderance of the evidence, we know that there's a relationship between salt intake and a rise in blood pressure," he says.
Roccella, who concedes the strength of the effect of salt on blood pressure is in question, says his viewpoint is based on broad public health considerations rather than specific effects on individual patients. "A reduction of 2 or 3 mm of mercury isn't much to an individual patient," he says. "But spread out over a large population, 2 mm of mercury or so could mean a reduction in strokes of 12% on an average in the population."
A large part of the debate seems to center around the conflict between laboratory sciences and public health sciences.
Miller says, "Public health science is concerned with assuring human health. It will decide to accept a set of data as being sufficient to taking public health action if there is some indication it might help and there is a lot of indication it won't hurt anybody. Laboratory science has a much more rigorous set of standards to assure scientific certainty."
In its August issue, an article in Science magazine surveyed 80 researchers and found a wide divergence of opinion on both sides of the controversy: "This has led to a literature so enormous and conflicting it is easy to amass a body of evidence that appears to support a particular conviction definitively, unless one is aware of the totality of data that doesn't."
The evidence of reducing salt intake in hypertensives has been somewhat muddied by accompanying dietary changes likely to affect blood pressure. "Choosing low-salt foods, for instance, inevitably leads to changing other nutrients, as well, such as potassium, fiber, and calories," states the Science article.
And some experts say that harping on the benefits of salt reduction among patients who are notoriously lax in compliance with physician guidelines may actually cause them to be more resistant to other known beneficial behaviors such as weight loss and healthy diets in general.
Roccella emphasizes that NHBPEP has never touted salt reduction as a panacea. "We've also advocated increased physical activity, losing weight if the patient is overweight, decreasing alcohol consumption, and increasing potassium intake. There's no silver bullet. If there were, we'd just say, 'Do it.'"
Despite their fundamental differences on the value of salt reduction on hypertension, the opposing sides come together on the basic issue of what constitutes a healthy level of salt consumption.
Roccella and Miller agree that 2,400 mg of sodium per day, a little more than a teaspoon, is reasonable.
Roccella says, "We're all talking about a goal, an upper limit of 2,400 mg, which is a lot more than the minimum daily requirements of 500 mg."
Miller echoes, "There's no reason for people to consume as much salt as people tend to consume. But if you want to add a little salt to the boiling of spaghetti or put some salt on fresh tomatoes, a little salt isn't going to hurt you."
Compliance is a perennial problem, particularly in hypertension where guidelines call for lifelong dietary changes. The difficulty of compliance is compounded by the large amount of salt in processed foods. According to Roccella, about 80% of our daily salt intake comes from processed foods. That means simply removing the salt shaker from the table is not the answer to bringing salt consumption within the guidelines.
The NHBPEP is lobbying the food industry to reduce the addition of salt to virtually every type of processed foods.
"We are pleased with the progress food manufacturers are taking," says Roccella. "And they are making small but progressive reductions of salt in the food supply. When we went up to Pillsbury, we saw they are reducing salt in their processed foods."
Food manufacturers have told the NHBPEP the reduction of salt in processed foods, including baby food, will be driven by consumer demand, and they will do so gradually so it is not noticeable and foods don't taste flat.
And finally, Roccella says, "People have got to make some choices. We're asking them to consider herbs and spices to increase the flavor of food. They can choose fresh or frozen, and they can choose food lower in salt by reading the label."