It’s been traded, taxed, fought over. It’s been ploughed into the fields of defeated city states. It is used in some religious ceremonies. It is essential to life and one of the fundamental tastes mediated by the human tongue. It’s the difference between a herring and a kipper, pork and bacon because it is an effective preservative. It has been used as currency and it gives us the word salary (although the Roman Army was never paid in salt).
Solntsata in Bulgaria was possibly the first city in Europe. The name translates as salt works and they have been in use since 5400 BC. There is evidence of even older salt extraction processes in China.
It is essential to our physiology, without it we couldn’t even generate a nerve impulse. But In excess it increases blood pressure which in turn increases the risk of heart disease and stroke. So how much is the right much?
The Australian guidelines are formulated by the National Health and Medical Research Council and can be accessed at this government website which also sets out the logic behind the recommendations. They deal in milligrams of sodium. Salt is sodium chloride. 1 unit of sodium = approximately 2.5 units of salt once you add the chloride.
The suggested daily target is 2 grams of sodium a day for adults (roughly 5 grams of salt). The NH&MRC wrestled with an upper limit but concluded that
… because the relationship between sodium intake and blood pressure is progressive and continuous, it is difficult to set a UL precisely.
So no upper limit was set.
The average Australian is ingesting about 9 grams of salt a day currently. Some is an inescapable part of the raw ingredients of our diet but much of that figure is added to restaurant and take away food, processed and packaged food, and drinks such as Coca Cola and sports drinks. It makes food more palatable by dialing down the sensations of bitterness and over sweetness. In the case of drinks it increases thirst – how convenient for the manufacturers.
So far as it goes the NH&MRC guideline purports to have an evidence base, is easy to understand and is aimed at getting the community’s blood pressure down.
The low carb intelligentsia seem to give the issue very little concern. How come?
A low carb diet has a real food base. McDonalds is out. Most packaged foods are out. If you start with the raw ingredients of a keto diet the challenge is to get enough salt especially if you are also into sweaty exercise.
The phrase lower limit doesn’t occur in the government website. Whilst the essential nature of salt is acknowledged the assumption seems to be that less is best. There is evidence to the contrary.
An article by Mente and an insane number of other authors (28 other authors – I imagine they wrote every 29th word and then had a committee meeting on where to put the punctuation marks) )from the Lancet May 2016 entitled Associations of urinary sodium excretion with cardiovascular events in individuals with and without hypertension: A pooled analysis of data from four studies, reports on a study of 133,118 individuals roughly half of whom were hypertensive. The daily intake of salt was compared to the likelihood of death and major cardiovascular events and blood pressure over a median period of 4.2 years.
Compared with moderate sodium intake, high sodium intake is associated with an increased risk of cardiovascular events and death in hypertensive populations (no association in normotensive population), while the association of low sodium intake with increased risk of cardiovascular events and death is observed in those with or without hypertension.
There is a subtlety there that I will return to but first let’s take a look at the results as a graph …
On the vertical axis we have the hazard ratio, on the horizontal axis the daily excretion of sodium in mg. Daily excretion of sodium is pretty much the same thing as daily intake of sodium. The amount of sodium in blood is regulated by the kidneys and there is no storage mechanism. The U-shaped curve comes as no surprise, sodium is essential therefore as you reduce the intake you’d expect to reach a point where risk starts to increase. Excretion of less than 3g of sodium a day increases the risk of cardiovascular catastrophes and death. The graph shows that the risk increases above 7g a day. The lowest point of the curve falls between 4000 mg and 6000mg of sodium or ~11 grams of salt.
If instead of looking at the whole cohort we look at the hypertensive and normotensive subgroups steepness to the left of the low point stays the same for both but to the right it rises more steeply for those with high blood pressure. Why should this be?
The kidneys play a pivotal role in the maintenance of blood pressure and not all kidneys are equal. Researchers have bred salt sensitive rats that develop high blood pressure. If their kidneys are transplanted into normal rats the recipients develop high blood pressure on a high sodium diet. Normal rats receiving kidneys from normal rats do far better. The problem follows the kidney. This is borne out in human transplantation as well. If the donor has a family history of hypertension the recipient has a ten fold increased risk of developing hypertension.
Mente et al looks like a pretty robust study. If we accept their findings it follows that –
- The Australian public are not far from an ideal salt intake.
- The NH&MRC guidelines are set at a level which will increase risk.
- and unless you have hypertension there are more important things to think about.
If only things were that easy. Not all studies bear this out. Cook et al …
found an increased risk at high sodium intake and a direct relation with total mortality even at the lowest levels of sodium intake.
They suggest that other studies are in error …
While several studies suggest beneficial effects of lower sodium on cardiovascular disease, the relationship with total mortality remains controversial. Some have reported a J-shaped curve, but this may be due to poor quality measurement of sodium or confounding bias.
Clearly the definitive answer still eludes us although I am suspicious of any study of an essential nutrient that doesn’t find detriment at a low level of intake.
Which study do we choose to ignore?
I think the best strategy is to ignore neither despite their contradiction. Rather we should take our blood pressure. It’s easily done at home. The hypothetical guy I studied in Physiology weighed 74kg had a pulse rate of 72 and a BP of 120/80. Blood pressure tends to rise through life and the doctor tends to reach for his prescription pad at about 140/90. If you’re border line and want to stave off the pills there are some non-drug remedies to consider …
- Lose weight
- Reduce your salt intake
If you’ve got it to lose then 10 kg off should drop your BP by about 10 mm of mercury. For each gram reduction of salt a hypertensive person can hope to drop about 2 mm of mercury.