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.
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