Australian children have a taste for salt that is likely to lead to serious health risks — a pattern already established in their parents, who are estimated to consume 50 per cent more than the recommended daily maximum of six grams of salt.
Susan Torres, a researcher in the centre of physical activity and nutrition research at Deakin University, says salt increases children's blood pressure. A seven-year study of 233 Dutch children aged 5-17 found the rise in blood pressure as they got older was linked to their high salt intakes.
High blood pressure, in turn, can lead to heart disease and other illnesses that unless treated can prove fatal. Yet eating salty foods appears to create an appetite for more.
"One study demonstrated that, in children aged 12 and 13, those who reported eating at fast food restaurants more that once a month had a greater preference for salty foods," says Dr Torres.
In a study with a colleague, Professor Caryl Nowson, Dr Torres set out to find the reasons for this increasing appetite for salt. One suggestion proposed 20 years ago was that stress was a factor.
"Primitive humans developed processes to deal with stress such as higher blood pressure and heart rate that might have been caused by escaping a charging lion, for example," Dr Torres says. "Response to this stress could cause a depletion of salt in the body and consequently an increase in salt appetite to replace the loss."
While conducting research as part of her PhD, Dr Torres explored the relationship between stress and mood, and how it related to the food a person ate. She recruited volunteers from within her university, gave them a stress test and measured their salt preference before and after the test.
"The theory is that stress induces a salt appetite and we wanted to see if this was the case," she says. "We prepared eight solutions of tomato juice with different concentrations of salt and asked those taking part to taste them all and pick which one they liked most out of the eight — both before and after the test."
To generate stress, the volunteers had to perform some relatively simple mental arithmetic in a test that lasted only seven minutes. They had to do sums out loud, starting with the number 2083 and subtracting 13, then continuing the subtraction.
"We told them they had to do it as fast as they could and, to increase the pressure, I told them after every minute they had to go faster. Then, if they made a mistake, I said that was wrong and they had to start again."
Simple as it sounds, the volunteers found the test quite stressful. This was shown by monitors indicating that their blood pressure and heart rate had risen sharply.
Yet there was no change in their choice of tomato juice at the end of the test – suggesting they did not want more salt.
To find out what other researchers had reported, Dr Torres and Professor Nowson conducted a literature search to locate papers in scientific journals where investigations had been conducted on animals and humans to determine if stress induced a salt appetite.
Dr Torres says it took more than a year to locate and read more than 150 research reports in journals from around the world, select 73 for a close study, then write up their own paper for publication.
"Some researchers will only work with an animal model such as rats and mice because they expect the physiological response will be similar to what occurs in a human. But with animals, you can be more in control.
"The researcher can study the effect of chronic stress on animals over several months because of their short lifespan whereas with humans this would be impossible. You can't impose chronic stress on people and that makes it more difficult to do such research."
Dr Torres says she and Professor Nowson began with the hypothesis that stress was unlikely to cause an increase in people's salt intake in Australia because salt consumption was already too high.
Although the experiments with animals such as mice, rats and hamsters confirmed that stress did increase salt intake, there were too few studies involving humans to be able to draw a reasonable conclusion.
There is no doubt, however, that a high salt intake can be harmful. Dr Torres says processed food contributes about 70 per cent of the amount of salt an Australian eats and a high salt intake does have a "negative physiological impact".
"People who consume a lot of salt have high blood pressure and if you put them on a low-salt diet, their blood pressure goes down. Which is just as well because high blood pressure is linked to cardiovascular disease."
The problem facing people who want to cut back on the salt they consume is that it is so widely used in manufactured food. Dr Torres says the manufacturers add more salt because people like the taste.
"Some food has salt to act as a preservative but most is there for taste. Manufacturers are not going to make something that doesn't taste good even if you don't know it contains salt."
She says that because salt is used so widely in processed food, people should choose salt-reduced products, or even those with no salt. Adding salt to food in cooking could be cut by adding herbs and spices to increase the appeal.
The results of limiting salt in a person's diet could be profound. Dr Torres says a 15 per cent reduction over 10 years by people in the 23 developing world countries that account for 80 per cent of chronic disease could prevent 8.5 million deaths.
A paper describing the Deakin study was published recently in the British Journal of Nutrition.