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Four seasons in one day: getting the right dose of salt

Some people benefit from salt restriction while others don’t.

Ah, salt. We love its taste and yearn to use it liberally in our cooking. But we’re told to limit our salt consumption for the sake of our health. Much like a low cholesterol product, the low salt option has become synonymous with the healthy alternative.

Most nutritional guidelines advocate restricting our intake of salt to less that 100 mmol (2.3 grams) every day. This is about half of what most adults get each day from their diet.

For those with high blood pressure, diabetes, kidney or heart disease lower targets are recommended (less than 65 mmol or 1.5 grams each day).

The rationale for such recommendations is straightforward. Our daily intake of salt is correlated with our blood pressure levels.

The more salt we eat, the higher our blood pressure. And high blood pressure damages blood vessels, which leads to cardiovascular disease and strokes.

In fact, along with smoking and diabetes, hypertension (high blood pressure) is one of the biggest killers of hearts and minds.

Consequently, if we ate less salt in our diet, our blood pressure would be lower, and the risk of heart attacks and strokes would, in theory, be reduced.

While such logic is appealing, especially as a low-cost public health message, the actions of salt on human physiology are more complicated, and certainly extend beyond blood pressure regulation.

Our need for salt

About a billion years ago, the first known organisms emerged from the salty oceans. To achieve this exodus, the organisms needed to take part of the ocean with them.

These days our inner sea is a fundamental part of our physiology. We can eat our salt and vinegar chips or drink gallons of fresh water, but our salt content hardly changes. We survive(d) by keeping things constant.

Our kidneys are the key regulators of this balance, filtering and reabsorbing over five kilograms of salt every day, losing only 0.5% of the filtered load into the urine.

In healthy individuals, this loss equates almost exactly with the amount of salt you eat (~3 to 5 grams).

Consequently, if we eat a packet of salty chips (and its extra gram of salt) it’s not a major effort to tune reabsorption down slightly and expel sodium through our urine.

Equally, if we didn’t get much salt today, it’s not hard to increase reabsorption very slightly to maintain the salt/water balance.

This balance is under tight neuro-hormonal control, involving a range of chemical signals between kidneys, the heart, the adrenal glands and, of course, the brain.

If salt intake is reduced, these pathways signal the kidney to hang on to more salt.

If salt intake is increased, these pathways are suppressed to increase salt excretion and offset increases in blood pressure that would otherwise occur.

And this is the rub.

These same neuro-hormonal pathways are implicated in the development and progression of human disease, from cardiovascular disease and diabetes, to cancer and mental illness.

The salt wars

Many observational studies have looked at the association between salt intake with heart disease and the risk of premature death.

Some have shown that high salt intake is associated with poor health. Some have found no effect at all. While others have found that individuals with a low salt intake have worse clinical outcomes. This is despite trials that clearly show salt restriction lowers blood pressure.

This inconsistency forms the basis of the so-called “salt wars”, an unscientific and often vitriolic war of words between (blood-pressure centric) salt reformers and salt sceptics.

Each has some scientific data to support their stance, and sound scientific reasons for doubting the opposing viewpoint.

For example, in response to the article published this week in the Journal of the American Medical Association that showed a modestly reduced survival in individuals on a low salt diet, one well-known advocate of salt restriction said “it doesn’t make (any) sense… it’s like saying we don’t think cigarettes are harmful! … I don’t think this is worth paying attention to”.

His rationale is clearly stated. “Everything else that has been shown to lower blood pressure is beneficial in terms of heart attacks and strokes.” So why should salt intake be any different?

The salt sceptics argue that this is not always true. For example, a recent clinical trial in individuals with diabetes showed that while blood pressure was effectively lowered, it did not reduce their risk of heart disease and death.

Moreover, many antihypertensive medications (to lower blood pressure) are prescribed on the belief that they have actions beyond blood pressure lowering that contribute to their efficacy. Why not salt?

A parable once told by Michael Alderman (a notable sceptic) goes something like this. Weight loss is safe and effective for lowering blood pressure in obese and overweight individuals. So once upon a time, pregnant women were advised to limit their weight gain in order to reduce their risks from hypertension (high blood pressure) during pregnancy (known as eclampsia).

From a blood pressure centric point of view, this worked very well. But while eclampsia was prevented and blood pressure lowered, fetal mortality was dramatically increased.

So no one now recommends weight restriction during pregnancy. It might be lowering the blood pressure, but what else is salt restriction doing?

The right amount of salt

The likely truth is that some individuals will benefit from salt restriction, while others may not.

For some, salt-sensitive hypertension is more important than neurohormonal activation.

One example is in the elderly. Here, salt restriction is a logical and important health message.

In other contexts, neuro-hormonal activation may trump the effects of blood pressure lowering, and salt restriction may lead to less favourable or even adverse outcomes.

Of course, the middle ground doesn’t work as a health message. And so the battle lines of the salt wars are drawn. For some, salt is a weapon of mass destruction. For others, it is the spice of life.

But like many recent and ongoing wars, the public are left confused and increasingly skeptical.

Taking each new piece of intelligence, with or without a grain of salt.

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