Monday, July 25, 2011

Do high salt diets impair reading comprehension?

While I'm certainly not one to suggest people shouldn't champion their opinions, I think it's important to champion them based off facts and not headlines.

These past few weeks have seen a flurry of activity in the pro-salt camp that suggests we've been barking up the wrong tree.

A great many folks have linked to a recent article published in Scientific American. The article, It's Time to End the War on Salt, appears to be explicitly written in response to a newly published meta-analysis on salt reduction.

The meta-analysis, Reduced Dietary Salt for the Prevention of Cardiovascular Disease is reported by Scientific American as finding,

"no strong evidence that cutting salt intake reduces the risk for heart attacks, strokes or death in people with normal or high blood pressure"
Scientific American then goes on to report on another study from May that found an association between dietary salt reduction and increased mortality in folks with congestive heart disease, to make it's case that, "it's time to end the war on salt".

So what's my beef? Certainly if the data suggests we've been barking up the wrong tree, we really ought to change our tune, after all, that's what evidence-based medicine's all about.

While that's true, it's also important to look critically at the actual data.

So let's look at those two studies that Scientific American uses to conclude that it's time to end the war on salt.

The Harvard School of Public Health explains that the increased mortality study cited by Scientific American, used only a single day of sodium excretion upon which to base all of their findings; that they didn't control for even basic confounding variables such as height, physical activity and total calories; and that there was a great deal of missing data from participants.

Unfortunately those are enormous problems, and certainly calls into question, if not outright negates, the utility of the paper's findings to draw any firm, actionable, opinions.

And what of the Cochrane Review?

Why don't I just quote directly from the opening line of the Review's Author Conclusion's section,
"Our findings are consistent with the belief that salt reduction is beneficial in normotensive and hypertensive people"
They then go on with their call to action,
"The challenge for clinical and public health practice is to find more effective interventions for reducing salt intake that are both practicable and inexpensive."
That sure doesn't sound like ending a war, it sounds like an RFP for new weapons.

So while I think healthy debate is in fact healthy, I would have thought that magazines like Scientific American, and many of the intelligent commentators on this and other blogs, would in fact do their due diligence to read and critically appraise studies, before getting on any particular bandwagon.

To put this another way, while it's wonderful to question, make sure your questions are based on sound science, not on sound bites, and while you might think salt's harmless, basing your conclusion on these recent papers and media reports isn't a reasoned decision.

Reminds me of a wonderful Yiddish proverb, "What you don't see with your eyes, don't invent with your mouth".

[And for other takes on some of this salt spin, have a read of the Globe and Mail's Carly Weeks' discussion on some of the conflicts of interest inherent to the spin, and Science Based Medicine's Scott Gavura who relates salt's recent reporting to confirmation bias]