Last week, discussing the Federal Trade Commission's call for slashing the sodium content of food marketed to children, I noted that the scientific case for population-wide reductions in salt consumption is pretty shaky. A new study in The Journal of the American Medical Association reinforces that point. The researchers, led by Jan Staessen, a professor of medicine at the University of Leuven in Belgium, tracked 3,681 middle-aged Europeans for an average of eight years, measuring their daily sodium intake through urine collected at the beginning and the end of the study. None of the subjects had high blood pressure or cardiovascular disease at the outset. But over the course of the study, those who consumed less salt were more likely to die of heart disease: There were 50 such deaths in the lowest third, compared to 24 in the middle third and 10 in the highest third. The subjects who consumed more salt did have slightly higher blood pressure, but they were no more likely to develop hypertension. "If the goal is to prevent hypertension" by reducing sodium consumption, Staessen told The New York Times, "this study shows it does not work."
Is it plausible that eating less salt could be harmful? "If one lowers sodium intake to lower blood pressure," says Staessen, "this change in sodium activates several systems (including the renin-angiotensin aldosterone system) that conserve sodium, and those systems are implicated in disease processes such as damaging the arterial wall and kidneys." Michael Alderman, who edits the American Journal of Hypertension, also worries that sharply reducing sodium intake, while good advice for people with high blood pressure, could raise the risk of cardiovascular disease in the rest of the population, perhaps by increasing insulin resistance. His own research has generated results similar to Staessen's. "Diet is a complicated business," he told the Times. "There are going to be unintended consequences."
Officials at the U.S. Centers for Disease Control and Prevention were quick to defend the low-salt orthodoxy, noting that Staessen's study did not randomly assign subjects to different diets. While such studies have found that eating less salt reduces blood pressure, they have not lasted long enough to measure the impact on mortality. What's needed, Alderman says, is a large, long-term experiment in which subjects are randomly assigned to diets with different levels of sodium. Low-salt loyalists say such research is too expensive and difficult to conduct. They argue that it would be impractical to make sure that subjects in the low-salt group stick to their restrictive diets for years. Alderman points out the contradiction:
The low-salt advocates suggest that all 300 million Americans be subjected to a low-salt diet. But if they can't get people on a low-salt diet for a clinical trial, what are they talking about?…It will cost money, but that's why we do science. It will also cost money to change the composition of food.
The CDC says no one should consume more than 2,300 milligrams a day, while anyone over 50 should adhere to a 1,500-milligram limit. Since the average daily intake is 3,436 milligrams, these guidelines would require dramatic reductions. Whether or not eating less salt is dangerous, there is no reason to expect the entire population to adopt a diet that even its advocates concede they will not like if it benefits only a minority.