On the question of whether the population would benefit if people cut back on salt, researchers fall into two camps, according to a new report.
While most studies have concluded that cutting salt would have benefits, about a third do not agree. And researchers on both sides of the issue tend not to take the other side's findings into consideration.
"We have not analyzed why there is a divide, we can only suggest or make assumptions after the fact," said Ludovic Trinquart, the lead author of the new report and a Columbia University Epidemiology Merit Fellow at the Mailman School of Public Health in New York City.
Trinquart and colleagues reviewed 269 primary studies, analyses, clinical guidelines, consensus statements, comments and letters on salt intake and health published between 1979 and 2014. They sorted the papers based on whether or not they supported the link between reduced sodium intake and lower rates of heart disease, stroke, and death.
They found that 54 percent of papers supported the hypothesis, 33 percent refuted it, and 13 percent were inconclusive.
Those that supported the hypothesis tended to cite other papers that supported it, too, while those that refuted tended to cite others that refuted also, with little crossover between the camps.
As reported in the International Journal of Epidemiology, Trinquart's team found 10 systematic reviews that pooled the data from a total of 48 primary studies - but each review did not necessarily include all available studies. Choosing which primary studies to cite influenced the conclusion of systematic reviews, the authors found.
"One side preferentially cites previous papers that found a similar conclusion," Trinquart told Reuters Health by phone.
His own study cannot offer any individual or population-level advice on actual salt intake, he said.
"On the one side, there is strong bias by the salt industry," said John P.A. Ioannidis of the Stanford Prevention Research Center, who wrote a commentary on the study. "On the other side, there is also bias from academics who want to defend their theories."
There is consensus that salt intake needs to be reduced, but by how much is still up for debate, Ioannidis told Reuters Health by email.
"I am a proponent of the idea that salt is bad for you," said Bruce Neal of the George Institute for Global Health in Sydney, Australia, who wrote another of the three commentaries accompanying the study. "Almost everyone is eating more than they actually require."
We can learn more from considering study quality instead of just sorting all studies into two camps, Neal told Reuters Health by phone.
"A lot of the debate is around weak research," he said.
National and international guidelines from institutions like the World Health Organization unanimously recommend salt reduction, he said.
"We lack a large definitive randomized trial that shows that reduced salt protects against heart attack, but there is very little evidence that reduced salt would do harm," Neal said.
Today adults consume an average of 10 grams of salt per day, but any more than two grams a day and the kidneys "are just trying to pee out as much salt as they can," he said.
"There's very good evidence that if you eat too much salt it pushes up your blood pressure," he said.
Studies that refute this hypothesis divide people by how much salt they are already eating, but people who eat very little salt may have already had a heart attack or stroke or may have terminal stage cancer, so their worse health outcomes have other underlying causes, he said.
"The vast majority of those who work in clinical medicine say it is much more likely that salt is bad for you than good for you," Neal said.