March 28, 2024
BANGOR DAILY NEWS (BANGOR, MAINE

Jury still out on link between salt, hypertension

There are two incontrovertible facts we all know about salt. One is that it is necessary to sustain life and has been prized from antiquity to preserve and enhance the flavor of foods. The other is that too much salt is bad for you and reducing salt intake will lead to lower blood pressure and a longer, healthier life.

But is this second “fact,” one that has been preached as gospel for decades, actually true? In the Aug. 14 issue of Science, Gary Taubes writes that the supposed benefits of salt reduction in the diet have generated heated controversy pitting federal agencies, medical groups, and scores of nutritionists against a growing body of data that indicates it has little benefit for most people.

Epidemiological studies involving large numbers of people are used to bolster both sides of the argument, and these are invariably open to criticism because of variables that cloud the effect being scrutinized. Advocates of salt reduction use a so-called “ecologic study,” in which the salt intake of peoples with little hypertension, such as the Yanomamo Indians of Brazil, is compared to high-intake populations such as a westernized industrial nation.

While it is true that salt intake is less in the groups with lowest hypertension, they also consume fewer calories, eat more fruit and vegetables, and exercise more. Those who claim salt reduction has little or no effect use “intrapopulation studies,” in which comparisons of salt intake vs. blood pressure are made within the same population.

A study of males in Chicago found no evidence linking salt consumption with high blood pressure. A 1980 study of 20,000 Americans by the American Center for Health Statistics also found no link between the two. The flaw in these studies, as suggested by epidemiologist Geoffrey Rose, is that developed nations consume so much salt that any data relating salt and high blood pressure are simply overwhelmed.

Despite the uncertain data, groups such as the National High Blood Pressure Education Program confidently assert that a reduction of salt intake from 10 grams daily, the national average, to 6 grams will save countless lives. Ed Roccella, director of the program, even criticized Science for writing about the controversy, for it “serves only to undermine the public health of the nation!”

The major study used to advocate the reduction of salt intake is Intersalt, led by Geoffrey Rose and Jeremiah Stamler of Northwestern University Medical School. They measured salt intake and blood pressure in groups of people from 52 communities around the world that bracketed the highest to lowest extremes of salt consumption. These 52 intrapopulation studies were then combined into a single huge ecologic study.

In a 1988 report, Intersalt found no measurable correlation between salt intake and incidence of hypertension. It did claim a relationship between salt and a rise in blood pressure with age but even this was suspect because the study was not designed to look at this possible link.

In 1993, NHBPEP cited the Intersalt study as support to its call for universal sodium reduction even as new controversy erupted over Intersalt’s reworking of its old data. The group corrected for a statistical problem called regression dilution bias. By doing this, their ambiguous original result became a “strong, positive” association between salt intake and hypertension.

British epidemiologist Malcom Law, who co-authored a 1991 study finding a salt-blood pressure relation “substantially larger” than expected, found that the reworked Intersalt data confirm “salt as an important determinant of blood pressure.”

On the other hand, Jamie Robins of the Harvard School of Public Health found that the report was “embarrassing to read” and its arguments “arcane, bizarre, and special pleading.”

As expected, people are weighing in hotly on both sides of the debate. Taubes writes that the study which may finally settle this argument was published in a 1997 issue of the New England Journal of Medicine. Called Dietary Approaches to Stop Hypertension, the study strongly suggests that while diets rich in fruits, vegetables, and low-fat dairy products go a long way toward reducing hypertension, salt seemed to have little to do with it.

So does this mean we can wield a salt shaker over our food with impunity? Not at all. What Taubes is saying is that the question is as yet unsettled, has become too politicized, and needs to be decided by research, not emotion.

Clair Wood taught physics and chemistry at Eastern Maine Technical College for 13 years.


Have feedback? Want to know more? Send us ideas for follow-up stories.

comments for this post are closed

You may also like