Studies that have explored the direct relationship between salt and heart disease have not fared much better. Among them, a 2006 American Journal of Medicine study compared the reported daily sodium intakes of 78 million Americans to their risk of dying from heart disease over the course of 14 years. It found that the more sodium people ate, the less likely they were to die from heart disease. And a 2007 study published in the European Journal of Epidemiology followed 1,500 older people for five years and found no association between urinary sodium levels and the risk of coronary vascular disease or death. For every study that suggests that salt is unhealthy, another does not.
Part of the problem is that individuals vary in how they respond to salt. "It's tough to nail these associations," admits Lawrence Appel, an epidemiologist at Johns Hopkins University and the chair of the salt committee for the 2010 Dietary Guidelines for Americans. One oft-cited 1987 study published in the Journal of Chronic Diseases reported that the number of people who experience drops in blood pressure after eating high-salt diets almost equals the number who experience blood pressure spikes; many stay exactly the same. That is because "the human kidney is made, by design, to vary the accretion of salt based on the amount you take in," explains Michael Alderman, an epidemiologist at the Albert Einstein College of Medicine and former president of the International Society of Hypertension.
Some physicians argue that although tiny blood pressure drops will not have a big effect on individuals—they will not really affect your risk of having a heart attack—they may end up saving lives at the population level, in part because a small percentage of the population, including some African-Americans and elderly individuals, seem to be hypersensitive to salt. For instance, a study published in February 2010 in the New England Journal of Medicine estimated that cutting salt intake by about 35 percent would save at least 44,000 American lives per year. But such estimates are not evidence, either; they are conjecture. And low-salt diets could have side effects: when salt intake is cut, the body responds by releasing renin and aldosterone, an enzyme and a hormone, respectively, that increase blood pressure.
Rather than create drastic salt policies based on conflicting data, Alderman and his colleague Hillel Cohen propose that the government sponsor a large, controlled clinical trial to see what happens to people who follow low-salt diets over time. Appel responds that such a trial "cannot and will not be done," in part because it would be so expensive. But unless we have clear data, evangelical antisalt campaigns are not just based on shaky science; they are ultimately unfair. "A great number of promises are being made to the public with regard to this enormous benefit and lives saved," Cohen says. But it is "based on wild extrapolations."