So why the mixed messages on salt?

Despite the received wisdom, recent studies seem to say that reducing salt offers no benefits

WE HAVE ALL heard that high blood pressure increases our risk of heart disease, stroke and kidney disease. In order to maintain normal blood pressure we are advised to adopt a “healthy lifestyle”, ie eat a “healthy” diet and take plenty of aerobic exercise (walking, swimming, cycling). We are exhorted to keep salt intake low, because salt raises blood pressure.

However, the results of research on the ill-health effects of eating salt are ambiguous. The most recent study ( American Journal of HypertensionVol 24, 2011) found no strong evidence that lower intake reduces the risk for heart attacks, strokes or death in people with normal or high blood pressure. Other studies had similar conclusions, but many others again have reached opposite conclusions and these inform conventional medical advice on salt intake. Confusing, isn't it? Nevertheless, I strongly believe we should take care to minimise salt intake.

Chemically, salt is sodium chloride, an essential element for life. If you cut sodium from your diet you would die. We need just enough to maintain good health, but too much can be bad for us.

If you take a tube made of semi-permeable membrane (permeable to water but not to sodium), eg a length of intestine tied-off at one end, partially fill it with a salty solution and suspend it in a beaker of pure water, you will observe that water is drawn into the tube and the volume inside increases. This is called osmosis and is the reason why too much salt in the blood raises blood pressure. The blood circulation is a closed system of tubes and the tube walls are semi-permeable. Blood pressure is the force of blood against the blood vessel walls, generated as the heart pumps blood around the system.

The body must regulate blood sodium concentration within narrow limits. This is mainly done by the kidneys, which filter the blood. If your blood- regulation mechanisms are faulty, for example in essential hypertension, your blood-sodium levels will tend to be raised. If you take in too much salt, blood-sodium levels rise further, drawing more water into the blood vessels, increasing blood volume and therefore the pressure of the blood on the vessel walls. But frequent or chronic ingestion of salt can cause a problem, even in a person with normal regulatory mechanisms.

Blood pressure is expressed as two numbers: systolic and diastolic. The higher systolic number is the pressure as the heart beats, and the lower diastolic number is the pressure when the heart relaxes between beats. Pressure is measured in terms of millimetres (mm) of mercury (Hg). Normal pressure would be less than 120/80 (systolic over diastolic) mm Hg. A pressure greater than 140/90 is considered high. Pressure tends to rise in older age and about two-thirds of people over-65 have high blood pressure.

High blood pressure increases the risk of heart disease, stroke and kidney disease. For example, it can cause leaks in weakened blood vessels in the brain leading to a brain bleed and a stroke. Also, arteries tend to stiffen with age, causing the heart to work harder to pump the blood, and, like any muscle, the heart grows bigger with constant strenuous exercise, leading to complications. High blood pressure is a major risk for heart attack.

Why do many studies find no correlation between reduced salt intake and a reduced risk of ill-health? I have seen two reasons suggested: (a) it is very hard to find a large group of people nowadays on a low salt intake to compare with people on a high salt intake, because much of the salt we ingest is “hidden” in food, particularly processed foods; (b) many people seem to be able to tolerate higher salt intake without adverse ill-health effects, compared to a significant minority who are very susceptible to high salt intake.

So, when research compares health outcomes between “low” and “high” salt intake groups, what is really being studied is outcomes between high and higher salt intake groups, and the relative health effects here are small. So, what to do? My advice is to use the salt shaker sparingly. Initially you will find some foods taste bland, but your taste buds soon re-adjust and food will taste salty again.

Finally, the best advice I ever heard on healthy diet was given by the American food journalist Michael Pollan: “Eat food, not a lot, mostly plants.” By food, Pollan means food your grandmother would recognise.

William Reville is a UCC professor in the Biochemistry Dept. and is Public Awareness of Science Officer. understandingscience.ucc.ie