Why official food intake recommendations should be taken with pinch of salt
For years we’ve been told to eat less salt to cut our risk of stroke and heart attacks. As they have been responsible for about a third of deaths in the UAE, the Ministry of Health and Prevention has been doing what it can to help us. Even bakeries in Abu Dhabi have been instructed to reduce the amount of salt in their bread.
But now researchers are claiming that cutting back on salt to officially approved levels could increase risks to our health.
Welcome to the flip-flopping science of nutrition, whose “insights” often seem about as solid as a souffle.
It’s hard to sit down at the dinner table without wondering which bit of research applies to what’s on our plate – and whether it’s still valid.
The latest “spat du jour” has been sparked by a study of the link between salt, blood pressure and cardiovascular disease.
Health experts have long warned of the dangers of too much salt for so-called hypertensives – people whose blood pressure is at least 140 over 90.
But more recently, the rest of us have also been told to cut back on salt.
According to the World Health Organisation, adults should consume no more than five grams of salt a day – half the global average.
Yet not everyone has been convinced by the evidence, with some suggesting that such blanket advice may do more harm than good.
Their concern stems from studies suggesting that the risks from salt follow a U-shaped curve. Decreasing salt consumption does indeed cut the risk of cardiovascular disease – but only up to a point. Cut back even further and the risks start to rise again.
If true, this means official calls for us all to slash our salt consumption may drive our intake below the level that is healthy for most people – with potentially tragic consequences.
These concerns have now been backed by a worldwide study of the salt consumption of more than 130,000 people in 49 countries and its link with cardiovascular disease and death.
The findings, published last month in the leading medical journal The Lancet, confirmed that hypertensives face a higher risk if they consume too much salt: about 15g per day is the limit.
Healthy people also face an increased risk, although the study found that it too only kicks in for more than 15g of salt a day – three times the official recommendation.
But the researchers also confirmed that the link between salt consumption and health risk follows a U-shaped curve, implying that if salt intake is too low, the risks start to rise again.
More worrying still, the researchers found a U-shaped curve for hypertensives and healthy people.
The results suggest that all of us face an increased risk of stroke, heart attacks and death if we consume less than about eight grams of salt a day.
The implications are stark: in their determination to combat the dangers of too much salt for hypertensives, officials may have gone too far – and set a limit that’s too low for everyone.
According to the researchers led by Prof Andrew Mente of McMaster University and Hamilton Health Sciences in Canada, these findings are in line with basic biochemistry.
About 40 per cent of salt is sodium, and this is essential for the correct functioning of cells. There’s also evidence that our disease-fighting immune system needs reserves of sodium to deal with infections.
Although everyone accepts there is such a thing as too little salt intake, the new research suggests that the minimum is surprisingly high – much higher than the official guidelines.
According to Prof Mente, these are based on flawed assumptions and small studies, and they need to be updated.
The study and its implications have not exactly been welcomed in some quarters, however.
Within hours of the research’s release, the American Heart Association (AHA) put out a strongly-worded rebuttal, declaring the study “flawed” and “offers no credible evidence” that low salt intake can be harmful.
Instead, it reiterated its own recommendation of no more than about four grams per day – lower even than the WHO’s limit of five grams.
So where did the AHA get its own limit? In its rebuttal, it states that “the association’s guidelines are based on the expert review of an expansive body of the best available scientific research over time”.
Compared to this, a single study seems like a flimsy basis for changing official advice. But anyone hoping to find the truth by reviewing the published scientific literature faces an uphill struggle. That’s because the whole field of salt research shows signs of having turned into a tribal dispute, with each side throwing hand-picked evidence at each other.
In February, the International Journal of Epidemiology carried an analysis of papers on the link between salt and health published between 1978 and 2014. Overall, about half of the papers supported the link, while a third did not – a clear majority in favour.
But the analysis, by a team led by Dr Ludovic Trinquart of Columbia University in New York, also showed that researchers on either side of the debate had a habit of citing studies that backed their own views, ignoring those that did not.
That may hardly seem like a revelation: who has not tried winning an argument by selective use of evidence?
Yet scientists are supposed to be above such tactics, especially in the health sciences, where findings inform public guidance that affects all our lives.
In the same issue of the journal, Prof John Ioannidis, of Stanford University, arguably the world’s leading analyst of medical evidence, called for long-term, randomised trials to settle the arguments between the two camps. He warned that “potentially millions of lives” could be jeopardised if hard evidence does not triumph over factionalism.
History does not give much hope that peace and harmony will break out any time soon. Despite huge studies into the link between heart disease, saturated fat and cholesterol, the research community remains split, with each side refusing to give way.
It is hard to escape the suspicion that both are vying for the credit of deciding what the world should eat.
Somewhere along the line, they seem to have overlooked the fact that all of us are different. Until they do, blanket advice about salt should be taken with a large pinch of it.
Robert Matthews is visiting professor of science at Aston University, Birmingham. His new book, Chancing It: The Laws of Chance and What They Mean for You, is out now.