Study Questions Benefits of Curbing Salt

A new study by a Canadian researcher has questioned the conventional wisdom that reducing salt in the diet will help prevent hypertension, but his research was quickly attacked by experts who challenged his methodology and motivation.

Dr. Alexander Logan of the University of Toronto reports today in the Journal of the American Medical Assn. that a reanalysis of 56 earlier trials showed that reducing salt in the diet did not lower blood pressure in people who were already healthy, and had only minor effects in those with high blood pressure.

Logan’s study also argued that avoiding salt can in some cases be harmful, particularly among people who exercise a great deal. “We can no longer accept on blind faith that restricting salt intake is harmless,” he said.

Experts pointed out, however, that Logan’s results were paid for, in part, by the Campbell Soup Co., which has long been criticized for the high salt content of its products. They also noted that his study included several poorly designed trials and neglected others, including a major new study published last Saturday in the British Medical Journal.

That study, of 10,000 people in 52 countries, found that a salty diet was directly linked with high blood pressure, especially the kind that worsens with age. “Excess dietary salt is a serious public health hazard,” Dr. Malcolm Law of St. Bartholomew’s Hospital in London wrote in an editorial accompanying the report.

“The preponderance of evidence,” said Dr. Claude Lenfant, director of the National Heart, Lung and Blood Institute, “continues to indicate that modest reduction of sodium . . . would improve public health.”

Other research published in the same journal indicates, however, that the greatest reductions in blood pressure can be achieved by exercise, a low-fat diet and low doses of anti-hypertension drugs.

Hypertension, defined as a blood pressure greater than 140/90 millimeters of mercury, is a major public health problem in this country. At least 50 million Americans, one in every five, suffer from high blood pressure and a third of those do not even know they have it. About 2 million people are found to have hypertension every year.

Only 21% of those under treatment have their blood pressure under control.

Dr. Daniel Levy of the National Heart, Lung and Blood Institute reports in the same journal on an analysis of 5,143 people in the Framingham Heart Study, the largest heart study ever undertaken. He found that hypertension was the single most important risk factor for congestive heart failure, contributing to 91% of the cases of the disease that developed over two decades.

Hypertension, he found, doubles the risk of congestive heart failure in men and triples it in women. Smoking, diabetes, obesity and high cholesterol levels--risk factors in their own right--multiplied the effect of hypertension.

Despite the widespread awareness of the risk of hypertension, average blood pressures have been increasing rather than declining, said Dr. William B. Kannel of the Boston University School of Medicine. Analyzing data from a different subset of the Framingham study, he found that the average blood pressure among adults age 30 to 65 has increased by about 20/10 millimeters of mercury over the last four decades.

The increase “indicates an urgent need for primary prevention by weight control, exercise, and reduced salt and alcohol intake,” Kannel said.

Few would disagree with any of those methods--other than salt reduction. Its link to hypertension has been a subject of hot debate since the federal government started warning Americans to go easy on salt in the 1980s.

The National Institutes of Health recommends that Americans consume no more than six grams of salt per day, the equivalent of about one teaspoon. The American Heart Assn. recommends a limit of three grams per day. The average American, however, consumes 15 to 20 grams of salt per day, much of it in processed foods.

Over the last two decades, many studies have examined the relationship between salt and hypertension, often with mixed results. Part of the problem is that many of the studies have not been large enough to provide definitive answers.

A new technique for addressing such problems is called meta-analysis. This mathematical technique allows researchers to combine the results from many studies to obtain a more significant result.

Logan and his colleagues at the University of Toronto and the Samuel Lunenfeld Research Institute in Toronto combined results from 56 such studies. The key findings, he said, are that “dietary salt intake has little effect on blood pressure in the population at large.”

Lenfant and others challenged the inclusion of some questionable studies in the meta-analysis. Some followed patients on salt-restricted diets for only two weeks and did not observe any lowering of blood pressure. That, Lenfant said, “is not enough time” for a salt-hypertension relationship to show up.

An American Heart Assn. statement also noted that Logan’s results are not all that different from previous findings--only his interpretation.