For decades, public health officials have told us that salt is pretty bad for you. Meanwhile, others have repeatedly questioned the scientific basis of that claim. In fact, a new study finds, it's all a bit worse than that: Not only is the science decidedly unsettled, the debate is so polarized that no one's getting any closer to real answers.
"In many respects, the divide between the uncertainty in the scientific literature ... and the certitude expressed by decision makers involved in developing public health policies in this arena is jarring," write Columbia University public health researchers Ludovic Trinquart, David Johns, and Sandro Galea. The World Health Organization, for example, recommends people consume less than two grams of sodium a day, while a recent National Academies of Science report concluded there wasn't evidence to support reducing sodium below even 2.3 grams per day. Within the scientific community, too, there are strong opinions on both sides.
Researchers were 1.5 times as likely to cite studies that reached similar conclusions to their own.
To get a little traction on the debate, Trinquart, Johns, and Galea first gathered 269 studies that investigated a link between sodium and health, including randomized trials and systematic reviews of previous research as well as research comments, letters, and less systematic reviews. All told, 54 percent of those supported the existence of a link between salt and health, while 33 percent contradicted the link, and 13 percent were inconclusive. In other words, there's lukewarm support for a link.
The more interesting question, however, is whether combatants in the salt wars are taking each other's views seriously. Unfortunately, that doesn't seem to be the case. By analyzing citation networks—that is, which papers cite which others—the team found that researchers were 1.5 times as likely to cite studies that reached similar conclusions to their own, compared with studies that reached different conclusions.
Often, even supposedly systematic reviews of the medical literature on salt failed to include all of the past research. In fact, the 10 systematic reviews Trinquart, Johns, and Galea found didn't even focus on the same set of experiments, suggesting that supposedly thorough reviews of the medical literature on salt were, in fact, biased.
"The citation patterns underlying this particular debate suggest that the scientific community is not engaged in a collaborative effort to arrive at a data-informed consensus on the matter and instead appears to be divided into two silos," Trinquart, Johns, and Galea write in the International Journal of Epidemiology.
Why? Like the rest of us, scientists are likely to have "long-held beliefs that resist re-examination when new data become available." Once we develop an opinion—in this case about the health effects of salt—new information isn't likely to change our minds.
In an accompanying commentary, Stanford University professor of medicine John Ionnadis was more blunt: "A wrong opinion is like an epidemic that is difficult to eradicate. But stronger evidence will hopefully help contain these epidemics of opinion."
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