From a public health perspective, salt shakers have been branded the equivalent of an ash tray, an instrument of ill health. Achtung baby. But a large study published recently in The Lancet, a major British medical journal, is shaking things up.
Researchers followed 133,000 people for more than four years, monitoring their salt intake and whether any of the following bad things happened to them: heart attack, heart failure, stroke, or death. Roughly half the group had already been diagnosed with hypertension, and excess salt intake has long been thought to be a key driver in the physiology that causes hypertension.
Surprise #1: Those on a low sodium diet (less 3,000 mgs/day) did worse than those with moderate sodium intake (4,000 to 5,000 mg/d). That's right: not only did the taste of their food suffer, but their health did too. This finding didn't align well with current U.S. guidelines, which recommend a daily sodium intake of 2,300 mg (that's roughly a teaspoon of salt). U.S. women average around 3,000 mg a day and men around 4,200 mg.
Surprise #2: A high sodium diet (more than 7,000 mgs/day) was harmful only to those with high blood pressure. It didn't get the people with normal blood pressure into any trouble. This salt-sensitive group -- the salty hypertensives -- made up only 10% of the 133,000 study participants, leading the researchers to suggest that "it is unclear whether the remaining more than 90% of the population will benefit from dietary sodium reduction."
A commentary in The Lancet pointed out that the idea of a low sodium diet being potentially harmful is not new, and that better studies are needed to figure out who benefits and who doesn't. The American Heart Association refuted the findings, calling the study "flawed" and suggesting that it shouldn't and couldn't dethrone all of the prior sodium research.
If you wonder why we don't have better evidence on the subject, it's because salt intake studies (and dietary studies in general) are hard to do. Salt is ubiquitous in our foods, and it's hard to account for and measure. For example, processed foods are high in salt, but is it the salt, or the processed foods causing the trouble? This Lancet study was an observational study: they grouped participants by salt intake and then observed how they did. It provides useful information, but it's not as strong as a study that randomly assigns patients to a particular level of salt intake. These researchers have just that kind of study in the pilot stage.
Finally, this salty debate points out the conundrum of some population-wide health initiatives, where you end up treating the many to benefit the few. No doubt, lowering salt intake in a population will save lives, but whose lives and how many, and how many will be either inconvenienced ("I can't taste my food. Am I eating now? My jaw is moving.") or even harmed? Helmets on bicyclists might save lives, but ideally we'd only put them on those riders who were going to fall off. The rest receive no benefit.
To that point, the editor-in-chief of The New England Journal of Medicine's Journal Watch Cardiology, commented: "In a precision medicine era, we will want to know which level of sodium consumption is optimal for each individual. In the meantime, our patients should be informed that low and high sodium intake appears to have risks and that moderating sodium intake is likely best overall for those with hypertension and high sodium diets. And don't expect this study to settle the debate."
That's time-tested advice: everything in moderation. Except cigarettes.