Sunday, April 25, 2010

When is a Low Salt Diet Beneficial (if ever)

I was upset, both from a medical and a mathematical point of view, after I read the paper published in "Annals of Internal Medicine" by Crystal Smith-Spangler and others: "Population Strategies to Decrease Sodium Intake",Ann.Int.Med. 2010;152:481-487, the lead article in the Annals volume published on April 20 of this year. I have been a reviewer for Annals for over 10 years. I can assure you that I would have rejected this article immediately. I never had an article I rejected overridden by the editor-in-chief, but sometimes the article in question would be published in a journal with less strict criteria.

I object to this paper on epidemiological grounds, on mathematical grounds, and on moral grounds as well.I think we all agree that blood pressure is a monotonically increasing function of the amount of daily NaCl consumption. We also note that increased blood pressure puts one at risk for strokes (CVA's) as well as heart attacks (MI's). The question of course is twofold: does lowering blood pressure reduce the risk or rate of CVA's and MI's? And furthermore, by how much can we reduce blood pressure "the silent killer, by decreasing the salt in the diet.

I will pass over the fact that some heart failure patients on NY Class III or IV worsen their faliure when their salt intake is lessened. I will also ignore the fact that there is no city-wide Institutional Review Board to evaluate the safety of this intervention in various groups of patients, and, of course, the "patients" in an entire city or country are not all consulting adults. So we are treating/testing a group of involuntary "volunteers".

Since correlation is NOT causation, we first have to see if lowering dietary salt lowers cardiovascular events (presumably by lowering blood pressure). Giving a patient a diuretic makes his kidneys excrete extra salt, so a daily water pill mimics a low salt diet somewhat. The result of diuretic treatment to lower blood pressure was that the incidence of CVA's was reduced. but not that of MI's.

To add further spice to the stories, the authors claimed that a 9.7% reduction in salt intake, as achieved by the British Government, would attain a 1.25mm (i.e. 1%) reduction in systolic blood pressure, and this would prevent 513,885 strokes and 480,358 heart attacks.

This not only looks too good to be true, it is too good to be true. The intra-observer error/statistical deviation in measuring blood pressure is+/- 2 mm.The interobserver error is worse, and is +/- 3 to 4 mm. Hg. (Not unlike the radiologist ' mammogram report that says they miss 10% of cancers). This would be similar to my claiming that since the measured speed of light is 186,272 miles/second, and I measured it to be 186,273 +/- 2 miles/sec., it is time to change the accepted value of the speed of light.

This is also like the situation where the government, seeing that pumping auto brakes helps to reduce the skidding of a car insisted on installing an automatic pumping system on the brakes, the ABS. You got a car insurance reduction for doing this,but it did not decrease the number of accidents after a skid, and now it is no longer mandatory. I also am skipping over the fact that the NTSB knew that if they required air bags to explode/expand rapidly enough to protect you even when you were not belted in, then children in the front seat might be hurt. They deliberately did not mention this fact to Congress, in fear that the air bag law might not be pass. They just admitted the week of May 16 that the air bags cause more damage to front-seat passengers and drivers who are belted-in rather than those who are not. Why believe ANY government reports or recommendations unless you can study the basic data from which the recommendations were derived?

Apropos of poor math training, tonight we attempted to give our waitress the check with a coupon good for an $25 discount, and asked her to divide the bill into 2/3 and 1/3 with tax and tip. It took her only 20 minutes (!). Hooray for the new math.

What I am saying again is that correlation is not causation, and we can only hope that the low salt diet will be of benefit to some patients, and not too harmful to others.

BTW, if you have a blood pressure problem, please make sure your doctor measures the blood pressure in both arms.


  1. Not to say "I told you so", but a recent article in JAMA (2011:305(17):pp1777-1785) in the issue of May 4, 2011 on Fatal and Non-Fatal Outcomes Associated With a Low Salt Diet, showed, using urine sodium excretion as a surrogate measurement of salt intake, showed that higher sodium intake DID NOT correlate with a higher risk of high blood pressure or complications from ASCVD, and a LOWER urine salt excretion as associated with a HIGHER mortality from ASCVD. There are critics of this article who say that association is not causation, which is generally true, but they cannot explain away the results.

  2. In the latest volume of the American Journal of Hypertension, a Cochrane review found NO evidence that lowering salt in the diet was of any health benefit.