Everything You Know is Wrong, Part I

19 05 2010

Salt isn’t bad for you

I just found something to add to my reading list. It’s an article called The (Political) Science of Salt (pdf) that Gary Taubes wrote in 1999 for Science. It’s not available for free on their site, but it is available at the National Association of Science Writers site because it won their 1999 Science in Society Journalism Award for a magazine article. The bottom line is that there is no scientific evidence that reducing your salt intake will reduce your blood pressure if you’re not hypertensive. If you are, it might have a very small effect. The article is long, but good. I’ve known for a while that dietary recommendations to reduce saturated fat and cholesterol intake are based on garbage science, which made me wonder about salt. I never managed to find anything on it until I heard an interview with Taubes where he mentioned that it was the salt issue which first got him into dietary science.

Emphasis mine:

Through the early 1980s, the scientific discord over salt reduction was buried beneath the public attention given to the benefits of avoiding salt. […]

Not until after these campaigns were well under way, however, did researchers set out to do studies that might be powerful enough to resolve the underlying controversy. The first was the Scottish Heart Health Study, launched in 1984 by epidemiologist Hugh Tunstall-Pedoe and colleagues at the Ninewells Hospital and Medical School in Dundee, Scotland. The researchers used questionnaires, physical exams, and 24-hour urine samples to establish the risk factors for cardiovascular disease in 7300 Scottish men. This was an order of magnitude larger than any intrapopulation study ever done with 24-hour urine samples. The BMJ published the results in 1988: Potassium, which is in fruits and vegetables, seemed to have a beneficial effect on blood pressure. Sodium had no effect.

With this result, the Scottish study vanished from the debate. Advocates of salt reduction argued that the negative result was no surprise because the study, despite its size, was still not large enough to overcome the measurement problems that beset all other intrapopulation studies. When the NHBPEP recommended universal salt reduction in its landmark 1993 report, it cited 327 different journal articles in support of its recommendations. The Scottish study was not among them. (In 1998, Tunstall-Pedoe and his collaborators published a 10-year follow-up: Sodium intake now showed no relationship to either coronary heart disease or death.)

[…]

Of all these studies, the one that may finally change the tenor of the salt debate was not actually about salt. Called DASH, for Dietary Approaches to Stop Hypertension, it was published in April 1997 in The New England Journal of Medicine. DASH suggested that although diet can strongly influence blood pressure, salt may not be a player. In DASH, individuals were fed a diet rich in fruits, vegetables, and low-fat dairy products. In 3 weeks, the diet reduced blood pressure by 5.5/3.0 mmHg in subjects with mild hypertension and 11.4/5.5 mmHg in hypertensives — a benefit surpassing what could be achieved by medication. Yet salt content was kept constant in the DASH diets, which meant salt had nothing to do with the blood pressure reductions.

That reduction is far greater than even the most wildly optimistic projections for extreme salt reduction (from 10g/day to 4g/day). I was first introduced to Taubes via this 72 minute video.

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5 responses

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23 05 2010
coldequation

The problem with salt is that it makes you want to eat more. Just try eating one and only one potato chip.

26 05 2010
wolfgang

So, unless I’m misreading, you’re claiming that 327 different studies are negated by 2 studies that claim something different? Any statistician would call shenanigans.

26 05 2010
unfrozencaveman

You are misreading. If you’re interested in this topic you should really read the entire article. I only quoted two tiny pieces.

You’re assumimg that the 327 journal articles mentioned in my quote reference conclusive stuides. They do not. The only studies that have ever shown a difference between high and low salt intake are the so-called ecological stuides which are the least scientific.

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