High blood pressure

Scant evidence that salt raises BP, review finds

The evidence for health benefits associated with salt reduction is controversial and the “concealment of scientific uncertainty” is a mistake, American researchers have suggested.

Controversy about what effect too much sodium intake has on the body goes back to the early part of the 20th century, according to Ronald Bayer, PhD, and colleagues from Columbia University Mailman School of Public Health in New York City.

But in the last few years, the discourse has reached a fever pitch, they wrote online in Health Affairs.

In 2011, for example, the Journal of the American Medical Association published a study by Stolarz-Skrzypek et al. that found only a weak correlation between salt and blood pressure. An editorial in the Lancet lambasted the JAMA study as “disappointingly weak” and “likely to confuse public perceptions of the importance of salt as a risk factor for high blood pressure, heart disease, and stroke.”

Also in 2011, the Cochrane Review published two studies finding little or no relationship with all-cause mortality and salt reduction. The Lancet criticized both the Cochrane Library and the authors, saying, “They have seriously misled the press and thereby the public.”

One of those reviews had concluded that “after more than 150 randomized controlled trials and 13 population studies without an obvious signal in favour of sodium reduction, another position could be to accept that such a signal may not exist.”

Bayer and colleagues cited several studies that could not find a link between salt intake and elevated blood pressure, including a 1967 study of the Framingham cohort, and Japanese and Scottish reports in the 1980s totalling 15,000 people that concluded the association between sodium and blood pressure is “extremely weak”.

The researchers noted that most of the evidence pointed to the weakest of correlations between salt and blood pressure. Yet, the cause to reduce salt was taken up by government agencies with special speed.

They cited a 2010 Institute of Medicine report called “Strategies to Reduce Sodium Intake in the United States.” In the report, the IOM claimed that the “harmful relationship of salt with hypertension has been known for 40 years,” which Bayer and colleagues argue is debatable – based on the evidence.

“The [IOM] report was welcomed by the incoming president of the the American Society of Hypertension,” the investigators wrote, “who warned that the ‘outcomes mafia’ might challenge the justification for a regulatory approach.”

In 2011, the FDA also called for data and recommendations “that would help it shape regulatory policy on salt in food”.

“All the while, skeptics still were asking for the evidence,” Bayer and colleagues wrote.

More than 20 years prior to the IOM report, C. Everett Koop, MD, the US Surgeon General, issued a report noting that government agencies were “very quick to embrace the importance of salt reduction in the 1970s and 1980s, which stood in stark contrast for the snail’s pace of recommendations related to reducing blood cholesterol levels”.

The authors cited many more studies finding little association between salt and blood pressure that did not eliminate the stigma attached to the mineral.

Advocates for salt reduction questioned the science behind studies that didn’t conform to their opinion, and proponents partially blamed the food industry because it was in their best interest to muddy the waters and keep the debate going.

One of the interesting things about this debate, Bayer and colleagues pointed out, was that you could find respected academics on both sides.

“At the most fundamental level, we believe that it is essential to recognize the role that judgment and values must play in evidence-informed policy making,” the authors concluded.

“Science must remain open, skeptical, and concerned about unmeasured confounding and selection bias in studies that accompany even the best efforts to articulate the evidence for new interventions,” they added.

The investigators said that one of the reviewers of this paper had asked, “In the end, does the harm of exaggerating certainty do more harm than good? After all, it would be very hard to make any policy from a position of informed, complicated, contextualized ambivalence.”

They concluded that the “concealment of scientific uncertainty is a mistake that serves neither the ends of science nor good policy. Simplistic pictures of translation from evidence to action distort our ability to understand how policy is, in fact, made and how it should be made.”

Source reference:
Bayer R, et al “Salt and public health: contested science and the challenge of evidence-based decision making” Health Affairs 2012; DOI: 10.1377/hlthaff.2012.0554.