BY ANDY HO, SENIOR WRITER

Salt: Too little is worse than too much

Most authorities recommend a daily intake of ¾ teaspoon to one teaspoon of salt for preventing artery disease, heart attacks and strokes, but studies just published in the New England Journal of Medicine suggest that one may actually take so little
Most authorities recommend a daily intake of ¾ teaspoon to one teaspoon of salt for preventing artery disease, heart attacks and strokes, but studies just published in the New England Journal of Medicine suggest that one may actually take so little salt that it jeopardises one's health.ST FILE PHOTO

Life's so unfair. Seems like anything that's tasty is unhealthy. For example, we like salt but we must have much less of it to prevent hypertension and/or heart disease. Or so the experts say.

But reducing salt intake for any extended period is very hard work for most of us. The reason is that "salt improves the sensory properties of virtually every food that humans consume", according to Sodium Intake in the United States. This was a 2010 report from the Institute of Medicine, a non-profit organisation that provides the United States government with independent advice on health issues.

The report explained that salt can enhance flavours in ways such that even unpalatable food tastes better. The experience of taste is made up of five primary qualities - sweet, sour, salty, bitter and savoury, the last being that which monosodium glutamate (MSG) imparts.

Sweetness is simple, which is why sugar substitutes are easy to make. In contrast, salt adds more than just a salty taste to food. In taste tests, salt has been shown to make one perceive product "thickness, fullness, sweetness and overall balance" more favourably.

It also "masks metallic or chemical off-notes", the report added. Salt also suppresses the bitterness of certain compounds that are to be found in various types of food.

But how salt achieves all this remains largely unknown. So making a salt substitute with all these attributes apart from saltiness remains a huge challenge.

Without a salt substitute, maintaining a low- or no-salt diet is difficult for most of us.

But here comes some good news finally: We may not - or should not - go on very low salt diets at all. So suggest three huge, well-designed, multi-country studies just published in a top journal, the New England Journal of Medicine (NEJM).

Hypertensives, especially those who also have heart disease, usually watch their salt intake religiously as they have been told salt raises blood pressure and thus increases heart mortality risks.

The adverse effects of a high-salt diet were thought to accumulate over decades: blood pressure would be raised and blood vessel walls stiffened, raising the patient's risks for artery disease, heart failure, heart attacks, strokes and kidney failure.

What most patients don't know, however, is that while studies do show a modest association between lowering salt intake and better control of blood pressure in hypertensives, there are actually no good studies to show that reducing blood pressure by cutting salt intake actually reduces death risks or serious heart problems.

That is, while salt intake and blood pressure are related - where hypertension is a known risk factor for heart attacks and strokes - the relationship between salt intake, on the one hand, and heart attacks, strokes or death rates, on the other, is not as clear.

Last year, another Institute of Medicine study reported it could find no evidence that reducing daily salt intake below 1.5g to 2.3g of sodium - or ¾ teaspoon to one teaspoon of salt - ever reduced the risks of heart disease.

Today, most authorities recommend a daily intake of ¾ teaspoon to one teaspoon of salt for preventing artery disease, heart attacks and strokes. But this guideline - the upper daily limit of no more than a teaspoon of salt - was always a bit of guesswork and not all experts agree on its merits.

The three recent NEJM studies - where urine levels of sodium were used to derive the daily salt consumption accurately - just rekindled the debate over how much to reduce one's salt intake.

The studies suggested that very high salt intake is indeed associated with increased death rates from cardiovascular causes such as heart attacks and strokes (by 54 per cent). On the other hand, very low salt intake is associated with even higher death rates from those causes (by 77 per cent).

What this means is that one may actually take so little salt that it jeopardises one's health.

Up till these three studies, the optimal levels of salt consumption were not known with any certainty. The upper daily limit of no more than a teaspoon of salt was largely extrapolated from short-term studies.

The NEJM studies affirm, first, that too much salt - defined as more than 2½ teaspoons a day - was associated with raised risks of heart attacks, strokes and death.

Second, the safe range with the lowest risk levels was associated with a daily consumption of 1¼ teaspoons to 2½ teaspoons of salt. This safe level is, in fact, the usual pattern of consumption in nearly three quarters of all countries around the world. So most people don't heed the message to cut salt consumption too much.

Third, a daily intake of under 1¼ teaspoons of salt a day was associated with the highest risks of heart attacks, strokes and death.

But why would an extremely low salt intake be hazardous at all? Note that sodium is necessary for many of the body's physiological processes. When one takes too little salt, this leads to low sodium levels in the blood.

Low sodium levels, in turn, cause an increase in the production of an enzyme in the kidney called renin as well as a hormone called aldosterone, which together can raise blood pressure.

Putting all this together, the general recommendation of no more than a teaspoon of salt per day may actually fall outside the safe range. In summary, the evidence does not support existing guidelines for salt restriction for the general population. These guidelines may well be too extreme.

Granted there are indeed some hypertensives whose blood pressure is very hard to control, who do experience an increase in blood pressure whenever they ingest some salt. In such individuals whose blood pressures are unusually sensitive to sodium, salt intake does need to be restricted. Such salt-sensitive patients need to be advised by their physicians accordingly.

But most of us are not salt-sensitive, so people who heed too zealously the public health message urging them to restrict salt could end up taking far less than may be needed for health.

The obverse may also be true: more salt may not be as sinful as we have been led to believe. For once, maybe, life's not too unfair.

andyho@sph.com.sg