http://www.nytimes.com/2009/04/07/science/07tier.html?ref=science

Public Policy That Makes Test Subjects of Us All
By JOHN TIERNEY

Suppose you wanted to test the effects of halving the amount of salt in people’s diets. If you were an academic researcher, you’d have to persuade your institutional review board that you had considered the risks and obtained informed consent from the participants.

You might, for instance, take note of a recent clinical trial in which heart patients put on a restricted-sodium diet fared worse than those on a normal diet. In light of new research suggesting that eating salt improves mood and combats depression, you might be alert for psychological effects of the new diet. You might worry that people would react to less-salty food by eating more of it, a trend you could monitor by comparing them with a control group.

But if you are the mayor of New York, no such constraints apply. You can simply announce, as Michael Bloomberg did, that the city is starting a “nationwide initiative” to pressure the food industry and restaurant chains to cut salt intake by half over the next decade. Why bother with consent forms when you can automatically enroll everyone in the experiment?

And why bother with a control group when you already know the experiment’s outcome? The city’s health commissioner, Thomas R. Frieden, has enumerated the results. If the food industry follows the city’s wishes, the health department’s Web site announces, “that action will lower health care costs and prevent 150,000 premature deaths every year.”

But that prediction is based on an estimate based on extrapolations based on assumptions that have yet to be demonstrated despite a half-century of efforts. No one knows how people would react to less-salty food, much less what would happen to their health.

Dr. Frieden has justified the new policy by pointing to the “compelling evidence” for the link between salt and blood pressure. It’s true that lowering salt has been shown to lower blood pressure on average, but that doesn’t mean it has been demonstrated to improve your health, for a couple of reasons.

First, a reduced-salt diet doesn’t lower everyone’s blood pressure. Some individuals’ blood pressure can actually rise in response to less salt, and most people aren’t affected much either way. The more notable drop in blood pressure tends to occur in some — but by no means all — people with hypertension, a condition that affects more than a quarter of American adults.

Second, even though lower blood pressure correlates with less heart disease, scientists haven’t demonstrated that eating less salt leads to better health and longer life. The results from observational studies have too often been inconclusive and contradictory. After reviewing the literature for the Cochrane Collaboration in 2003, researchers from Copenhagen University concluded that “there is little evidence for long-term benefit from reducing salt intake.”

A similar conclusion was reached in 2006 by Norman K. Hollenberg of Harvard Medical School. While it might make sense for some individuals to change their diets, he wrote, “the available evidence shows that the influence of salt intake is too inconsistent and generally too small to mandate policy decisions at the community level.”

In the past year, researchers led by Salvatore Paterna of the University of Palermo have reported one of the most rigorous experiments so far: a randomized clinical trial of heart patients who were put on different diets. Those on a low-sodium diet were more likely to be rehospitalized and to die, results that prompted the researchers to ask, “Is sodium an old enemy or a new friend?”

Those results, while hardly a reason for you to start eating more salt, are a reminder that salt affects a great deal more than blood pressure. Lowering it can cause problems with blood flow to the kidneys and insulin resistance, which can increase the risk of strokes and heart attacks.

Salt deprivation might also darken your mood, according to recent research by Alan Kim Johnson and colleagues at the University of Iowa. After analyzing the behavior and brain chemistry of salt-deprived rats, the psychologists found that salt, like chocolate and cocaine, affected reward circuitry in the brain, and that salt-deprived rats exhibited anhedonia, a symptom of depression characterized by the inability to enjoy normally pleasurable activities.

Dr. Frieden has predicted that people “won’t notice the difference” if salt is gradually reduced, but how can he be sure? What if they respond by eating more food, or a different mix of foods and stimulants? What if the food industry turns to salt substitutes that cause new health problems? “We have no way of knowing the health effects of eating less salt, yet we’re supposed to forge ahead with this new policy that affects the whole population,” said Michael Alderman, an expert in hypertension at the Albert Einstein College of Medicine. Like other critics, he has compared the antisalt campaign to the campaign against fat that began several decades ago.

That antifat campaign, like the antisalt campaign, was endorsed by prominent groups and federal agencies before the campaigners’ theory was tested in rigorous trials. It too seemed quite logical — in theory.

But in practice the results were dismal, as demonstrated eventually by clinical trials and by the expanding waistlines of Americans. People followed the advice in the “food pyramid” to reduce the percentage of fat in the diet, but they got more obese, perhaps because they ate so many other ingredients in foods with “low fat” labels.

You might think that experience would inspire caution among public health officials, but instead they seem to be gaining confidence. When Dr. Frieden and Mr. Bloomberg decided several years ago that trans fats were dangerous, they didn’t simply issue a warning or a set of voluntary guidelines. They insisted on outlawing trans fats in New York’s restaurants.

At the time, it seemed extraordinary for a city to be forbidding its diners to order a legal food product, particularly given the scientific uncertainties about trans fats and the possible harms resulting from the ban (see TierneyLab at nytimes.com/tierneylab).

But that local restaurant policy now seems fairly modest by comparison with Mr. Bloomberg’s and Dr. Frieden’s plans for salt. Soon, wherever you live, wherever you eat, you could be part of their experiment.