Salt may not be as damaging to health as is usually claimed, according to a controversial new study which suggests campaigns to persuade people to cut down may only be worthwhile in countries with very high sodium consumption, such as China.
The World Health Organization recommends cutting sodium intake to no more than 2g a day – the equivalent of 5g of salt – because of the link to increased blood pressure, which is in turn implicated in stroke.
But no country has ever managed to get population salt or sodium intake that low, the authors of the study published in the Lancet medical journal point out. Their research, the Canadian academics say, shows it may be pointless to try in countries such as those in the European Union.
The study by Prof Andrew Mente from the Population Health Research Institute of Hamilton Health Sciences and McMaster University and colleagues is large, involving more than 90,000 people in more than 300 communities in 18 countries. But it immediately reignited a simmering row with other scientists who are on a crusade to reduce our salt consumption to near zero.
Mente and colleagues found that the harmful effects of sodium – raised blood pressure and stroke – only occurred in countries like China, where the liberal use of soy sauce leads to sodium levels over 5g a day, the equivalent of 12g of salt. And they found that very low levels of salt actually led to more heart attacks and deaths, suggesting moderate salt intake may be protective.
“Our study adds to growing evidence to suggest that, at moderate intake, sodium may have a beneficial role in cardiovascular health, but a potentially more harmful role when intake is very high or very low. This is the relationship we would expect for any essential nutrient and health. Our bodies need essential nutrients like sodium, but the question is how much,” said Mente.
Two years ago, the same team published a study with similar results, also in the Lancet, looking at individuals. It was lambasted by critics, who called it "bad science" and its findings were rejected by the American Heart Association.
The latest observational study – not a randomised controlled trial which compares different groups of people – looks at communities rather than individuals. It immediately came in for heavy criticism. The chief complaint was that it did not accurately measure the amount of sodium in people’s urine, which needs to be done over a 24-hour period.
“The authors have not addressed any of the serious criticisms from the wider scientific community of their 2016 study,” said Graham MacGregor, professor of cardiovascular medicine at Queen Mary University of London, and founder of the salt-reduction campaign Cash (Consensus Action on Salt and Health). “These criticisms include the use of ill participants in the study, leading to reverse causality (ie those suffering with heart disease don’t eat much food, and consequently eat less salt, but it is the illness that leads to death rather than lower salt intake), and the use of spot urine measurements.”
The Irish Heart Foundation says our bodies need just 4g of salt each day and “an acceptable maximum level is six grams, or one teaspoon, of salt per day. According to the Food Safety Authority of Ireland, “the average daily salt intake in Ireland is high – approximately 10g in adults”.
Tom Sanders, professor emeritus of nutrition and dietetics at King's College London, said campaigns to lower salt intake have been beneficial in some countries. "Japan used to have a very high prevalence of high blood pressure and high rates of stroke, and took action to cut salt intake in the 1970s and now has much lower rates," he said.
But it is not easy to persuade people to forgo salt, say Franz Messerli and Louis Hofstetter, experts from Switzerland and New York in a commentary on the Lancet findings. They cite Sir George Pickering, regius professor of medicine at the University of Oxford, who wrote more than half a century ago: "The rigid low-sodium diet is insipid, unappetising, monotonous, unacceptable, and intolerable. To stay on it requires the asceticism of a religious zealot."
The new study measured potassium as well as sodium levels in people’s urine and found that higher potassium, which is found in fruit and vegetables, cut rates of stroke, heart disease and death. “Perhaps salt-reduction evangelists and salt-addition libertarians could temporarily put aside their vitriol and support the hypothesis that diets rich in potassium confer substantially greater health benefits than aggressive sodium reduction,” they write.