Science on Salt Is Polarized.

An analysis of scientific reports and comments on the health effects of a salty diet reveals a sharp divide between those who support the hypothesis that population-wide reduction of salt intake is associated with better health and those who do not.

In all, 54 percent of reports reviewed were supportive of the hypothesis, while 33 percent were not. The other 13 percent were inconclusive.

In an article in the International Journal of Epidemiology, a research team that includes Dean Sandro Galea systematically reviewed 269 academic reports published between 1979 and 2014, including primary studies, meta-analyses, clinical guidelines, consensus statements, comments, letters, and narrative reviews. Each was classified according to whether it supported or refuted the link between reduced sodium intake and lower rates of heart disease, stroke, and death.

More than half of the reports were published since 2011—suggesting an increasing level of interest in the question, even if there was no emerging consensus.

The analysis was led by researchers from Columbia University’s Mailman School of Public Health.

A citation review found that papers on either side of the hypothesis were more likely to cite reports that drew a similar conclusion than to cite reports drawing a different conclusion. Dominating the literature was a small number of influential papers that presented strong evidence for or against, leading researchers to identify what they called “two almost distinct bodies of scholarship,” with little collaboration or overlap.

“We pay quite a bit of attention to financial bias in our work,” says Galea. “We seldom pay attention, however, to how long-held beliefs bias the questions we ask and the results we publish, even as new data become available.”

The researchers’ analysis of systemic reviews on salt intake revealed very little consistency in the selection of primary studies. If a primary study was selected by a review, the chance that another review would select the same study was less than a third. That finding points to uncertainty and disagreement about what should count as evidence, the authors argue. Moreover, the choices about which studies to cite as primary evidence directly influenced the conclusions of systematic reviews.

Even while the scientific debate over salt continues, public health officials, from the local to the global level, have enacted policies to lower consumption. World Health Organization guidelines recommend limiting salt intake. In December 2015, New York City became the first US city to require chain restaurants to label foods high in sodium.

The authors said the citation analysis method used in their study represents a new way of understanding the relationships between academic research papers and authors. In the future, they said, the same method could be applied to other topics, including controversial issues such as e-cigarettes or topics on which there is greater agreement.

–Lisa Chedekel