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Monday, May 14, 2012

Dietary advice with a grain of salt

Salt is one of the most ancient ingredients used as a food preservative, particularly for the preservation of meat. Roman soldiers pay was named “salarium” from which the word salary is derived because it was expected that salt would be one of their main items of expenditure. Cities such as Salzburg were associated with salt and Mahatma Gandhi marched with many thousands of Indians on the "Dandi March" or "Salt Satyagraha", where they made their own salt from sea water in a protest against the tax levied on salt by their British rulers. Today, however, salt is seen as an food ingredient which is associated with high blood pressure and drives to lower the salt levels of processed foods are operational in many countries, Processed foods provide about 90% of salt intake while the salt cellar accounts for a mere 10%.
In 1949, an MD from Durham, North Carolina by the name of Walter Kempner published a paper in Annals of Internal Medicine in which he showed a dramatic effect of a diet based on rice on a number of cardiovascular risk factors, among which was hypertension. This was a very low salt diet and thus the association between salt and hypertension had gained momentum. In 1964, Lewis Dahl, working at the Brookhaven National Laboratory in New York found that about 25% of his rat colonies were resistant to increased blood pressure when given a high salt diet. Thus he genetically bred two strains of Dahl rats, an R-strain that was resistant to salt-induced hypertension and an S-strain that was sensitive in this respect. These rats were widely used to see how salt interacted with other nutrients such as potassium, in moderating blood pressure and thus the salt-blood pressure story grew. It is important to note that by my calculations (others have done like wise and agree), the quantity of sodium ingested by these rats would translate into about 400g per day for a 70kg human. Bearing in mind that a high salt intake in free-living humans might be 15g per day, the relevance of these rat studies (as is generally the case for animal models) to human physiology is laughable.  However, a spate of poorly designed human intervention studies followed putatively confirming the rat work and so salt was served up on the first ever set of dietary guidelines published by the US Senate Select Committee in the mid 1970’s. That a reduction in sodium intake would reduce blood pressure became accepted wisdom and in the world of nutrition guidelines that is as sacred and immutable as the dogmas of the Vatican or the Kremlin.

Professor Roger McCarron of the Department of Medicine at the University of Oregon published studies, which began to criticise these data. Of course he was generally regarded as a heretic by the high priests of healthy eating. But he stuck to his guns and began to provide data that other nutrients were more important such as calcium and that obesity was a significant factor. Slowly, evidence began to emerge which suggested that a broad modification of diet might be better than  a single nutrient-based approach and so the DASH (Dietary Approaches to Stop Hypertension) trial was initiated, funded by the US National Institute of Health. This was a very large randomly controlled intervention study involving 459 adults who were put on a control diet for the initial 3 weeks. This was a typical US diet high in fat and low in fruit and vegetables. Half the subjects were then put on the same diet but with increased fruit and vegetable intake for 8 weeks and the other half also had this diet but, additionally, had high intakes of low fat dairy products together with a low fat, low saturated fat diet, again for 8 weeks. Very importantly, no changes were made to salt intake, either as table salt or salt in normal foods. The results showed that the diet high in fruits, vegetables and dairy products and low in total and saturated fat significantly reduced blood pressure to a clinically significant level in subjects whose blood pressure was normal or elevated.

But that did not detract from the salt reduction zealots who have protected their dogma with great passion. However, in 2011, a series of papers published in prestigious journals have shown that all is not rosy in the salt garden.   In 2011, an international consortium reported on an 8-year study of 3681 subjects based on 24-hour urinary sodium data, collected at the outset of the study[1]. The reason for using a 24-hour urine collection to measure sodium output is that dietary data on salt intake is utterly unreliable. In contrast the urinary data doesn’t lie and give an accurate measure of sodium intake. They divided the subjects according to their level of sodium excretion: The lowest, middle and highest thirds of excretion. The 8-year incidence of cardiovascular disease went as follows: 4.1% in those with the lowest sodium excretion, 1.9% at the middle and a mere 0.8% among those with the highest intake. No, I didn’t type it wrongly. That’s how it stood. Low salt intakes had a higher risk of hear disease that the low salt group. Higher salt intakes increased diastolic blood pressure but not systolic blood pressure and overall, increasing sodium excretion did not increase the levels of clinical hypertension. A second study published in 2011 explored baseline urinary sodium with cardiovascular events over a 56 month period and they found that the relationship was “J-shaped”[2]. In other words, heart disease was higher at low levels of sodium excretion (by about 30%), which fell to a minimum at about the average for sodium excretion and then rose again at the higher level of sodium excretion (by about 70%). Finally, we had a major review of existing literature where 167 studies, covering high and low sodium diets were examined[3]. Salt reduction reduced blood pressure by 1% in those with normal blood pressure and by 3.5% in those with high blood pressure. However, plasma lipids rose by 7%, which may explain the adverse effects of low sodium diets on cardiovascular risk, observed in the other two papers

One might imagine that the salt-hypertension debate might take a major turn in fortune in light of these papers but as I have pointed out before, scientific dogmas are like oil tankers – hard to turn around. The WHO has issued a call for public comments as part of its forthcoming review of its current recommendations on salt and hypertension. It will be interesting to see the outcome.

[1] JAMA. 2011;305 (17):1777-1785
[2] JAMA.2011; 306 (20):2229-2238
[3] Am J Hyper (2012), 1, 1-15

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