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]

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6 comments:

  1. Paulette8:01 am

    Thanks for the excerpts from the study and better description of the study itself. No surprise that the media take was so far off base. As someone who is salt sensitive and have been restricting salt intake for many years,(I target 1500 mg a day with variable success), I took the media reports with the proverbial grain of salt. I also understand that the scientific search for answers is not linear and not all studies agree all the time. So the flurry that followed was entertaining while being frustratingly predictable.
    The real conclusion about the (lack of) effectiveness of dietary interventions is a much better story to my mind as it points to the need for more work in the food supply itself, but lets face it that's long term hard work and not a catchy sound byte to sell today's paper.

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  2. You might find Gary Taubes' article "The (Political) Science of Salt interesting.

    I have the whole article. An abstract is here.

    Email me if you want the whole thing.

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  3. Roman Korol10:32 am

    It appears that the bulk of our salt intake (75%) comes from processed food products, especially processed grains and meats http://www.ncbi.nlm.nih.gov/pubmed/19935850

    Plus, it seems that increased dietary fructose intake stimulates salt absorption in the body making it worse. Think: liquid candy. http://www.ncbi.nlm.nih.gov/pubmed/21143427

    It seems it's a two-headed hydra. To deal effectively with the one, you also have to deal with the other. Interestingly, both problems are extravagantly pushed upon the populace from all directions by . . . yea, you guessed it.

    Got these links and related video commentary at: summertomato.com

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  4. whoever wrote the Cochrane “Plain Language Summary” headline is the first culprit:

    Cutting down on the amount of salt has no clear benefits in terms of likelihood of dying or experiencing cardiovascular disease


    Apart from all the other issues discussed it did show that “encouragement to reduce salt intake did lead to a reduction in salt eaten and a small reduction in blood pressure after more than six months”

    OK that is not a cure – but a small reduction in 6 months in a risk factor for diseases that develop over 20-30 years is i think a clear benefit. Just DO NOT expect short trials to show a clear benefit in “likelihood of dying or experiencing cardiovascular disease”

    the plain language summary is easy to understand, pity it is plain wrong

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  5. Anonymous7:04 pm

    @Roman Koral, it seems clear to me that these studies simply further an important point; cooking wholesome meals with real unprocessed ingredients will fix just about any problem, including salt issues.

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  6. Jason Harrison10:15 am

    Hmm, perhaps another reason to read the entire paper rather than just the abstract.

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