London - The need to reduce the amount of salt in our diets has been one of the most powerful public health messages of the past decade.
Cutting the average daily consumption of salt from 8g per person to 6g - around one teaspoon - would save an estimated 17,000 lives a year in Britain. This is thanks to a predicted drop of 22 percent in the number of strokes and a 16 percent reduction in heart attacks.
In the longer term, the goal is to drive salt intake down even further. The British government health watchdog, the National Institute for Health and Clinical Excellence (NICE), has called for average consumption to be cut to just 3g a day by 2025.
But some experts are now saying that too little salt might be just as dangerous as too much, and that continuing to drive levels down could actually lead to an increase in deaths from heart disease. A paper published in the American Journal of Hypertension warns that once average daily consumption dips to below 6.25g, the risk of heart attacks and strokes starts to increase once more.
The reasons for this are unclear. However, there is a small amount of evidence that restricting salt consumption increases levels of cholesterol and triglycerides - both of them harmful fats which cause heart disease - and also leads to insulin resistance (the early stages of type-2 diabetes).
Researchers from the Albert Einstein College of Medicine in New York issued the warning after they pooled the results of 23 different studies highlighting the effects of salt on the body.
The lead author of the paper, Dr Michael Alderman, is a former president of the American Society of Hypertension who first raised concerns about universal salt reduction about five years ago.
He warns that consumers should be advised not to try to reduce their intake to the maximum 6g daily, but to keep it to a minimum of 6.25g and a maximum of 15g.
“Salt intakes above and below this range are associated with increased cardiovascular risk,” says Dr Alderman.
“For those whose average salt intake broadly surrounds 8.75g a day, there is no evidence that altering intake will improve health outcomes.
“Accumulating scientific evidence has raised uncertainty about the health effects of universal salt restriction.”
But other blood pressure experts say Dr Alderman’s connections cast doubt on the reliability of his claims. He himself admits he was once paid $750 (about R6 000) in the mid-1990s to attend a meeting on behalf of the Salt Institute, which represents salt manufacturers in the US, and served as an unpaid member of its diet advisory board for 12 years.
“He has been campaigning against salt reduction for many years and he has worked closely with the Salt Institute,” says Professor Graham MacGregor, chairman of Consensus Action on Salt and Health (CASH), a UK body set up in the mid-1990s to campaign for salt reduction.
“These claims are nothing new and are based on poor evidence.”
But Dr Alderman insists: “I have had no other money from the Salt Institute, or any other commercial interest involved in salt. In short, I have no conflict of interest.”
Salt plays a crucial role in the way our bodies function. Its chemical name is sodium chloride and it is made up of 40 percent sodium and 60 percent chloride.
The sodium component is vital for controlling the amount of water circulating in the body, helping to transmit nerve signals, control muscle contractions and maintain the body’s acid/alkaline balance.
The chloride helps with the digestion process, and the transport of carbon dioxide to the lungs for disposal through our breath.
Doubts about the safety of excessive salt intake first emerged more than 20 years ago when a UK government advisory body first called for a crackdown in salt content of everyday foods due to concerns about the link with high blood pressure.
Excess salt is thought to drive up blood pressure by retaining water in the bloodstream that would otherwise get flushed out by the kidneys. This puts added strain on blood vessels around the heart, increasing the risk of stroke and heart attack.
Salt occurs naturally, even if in tiny quantities, in most foods, including fruits and vegetables.
However, around 75 percent of the salt we consume comes from restaurant meals, takeaways and processed foods, with manufacturers using it as a cheap source of flavouring.
For example, a McDonald’s Big Mac contains 2.1g, a full English breakfast around 4.5g, and a pie and mash meal in a pub around 7.5g. Even a bowl of cornflakes for breakfast - which might be perceived as healthy start to the day - contains 1g of salt.
The government warnings on salt caused average adult intake in the UK to fall from 9.5g to 8.1g a day in the past ten years - which is estimated to have saved around 8,500 lives a year.
While Britain is now said to have the lowest salt intake of any developed country in the world, many experts say that more lives could be saved if we consumed even less salt.
But, from time to time, studies have cast doubt on the benefits of salt restriction. In 1972, a paper published in the New England Journal of Medicine showed the less salt that people ate, the higher their level of rennin, a substance secreted by the kidneys. This increase appeared to trigger a series of events that led to an increased risk of heart disease.
Last year, scientists at Exeter University pooled data from seven different studies and found there was too little evidence to say with certainty that cutting salt intake translates into fewer deaths. Worse still, the study uncovered an increase in the risk of death among people who already suffer with heart failure and who were put on a low-salt diet.
The seven studies involved 6,489 patients with normal or high blood pressure who had been put on low-salt diets. But they showed no fall in deaths or heart disease rates.
Research teams warned that other studies showing benefits of salt restriction may simply have ignored the fact that low-salt consumers probably also eat lots of fruit and vegetables, don’t smoke and take regular exercise.
Professor Rod Taylor, who led the Exeter University study, said it was imperative that more studies are done to assess the dangers of cutting back on salt, as well as the advantages.
“With governments setting ever lower targets for salt intake, it’s really important that we do some large research trials to get a full understanding of the benefits and risks of reducing salt intake,” he said.
Meanwhile, in 2010, a Dutch study found a link between low sodium levels and an increased risk of falls and fractures among the elderly.
Researchers said it is possible that too little salt somehow affects bone quality, even though the volunteers showed no signs of osteoporosis.
Consensus Action on Salt and Health claims there are more than 40 studies alone which highlight differences in blood pressure between undeveloped societies, where little or no salt is added to foods, and industrialised nations with salty diets.
It admits there is a shortage of evidence that low-salt diets translate into fewer deaths, primarily because the logistics of running a trial to find out are so prohibitive and there are ethical problems with feeding one group of people a high-salt intake.
But Professor MacGregor, one of Britain’s leading experts on salt and blood pressure, insists the evidence on the dangers of excess salt is overwhelming.
“Some people are trying to paint a picture that salt reduction is dangerous and there is really no good evidence to support it.
“This is similar to when the dangers of smoking were first realised, yet some still insisted tobacco did not cause lung cancer.
“If someone comes up with real evidence [on the risks of reducing salt], then CASH would look at it very closely.
“But at the moment, the evidence that reducing salt intake to below 6g a day is harmful is overshadowed by a lot of epidemiological evidence showing cutting it below 6g reduces blood pressure and lowers the numbers of people suffering heart attacks or strokes.” - Daily Mail