Does excessive salt consumption really lead to hypertension?

As if we didn't know already, salt's prominence in our lives was underlined when the industry research company Freedonia reported that "global salt consumption is forecast to grow 1.9 per cent annually through 2020 to 335 million metric tons, valued at $14.1 billion (€12.1 billion).

Not all of that will be sprinkled over our chips. We have all seen public health warnings about how excessive salt intake promotes high blood pressure (hypertension), risking cardiovascular disease, and so it must be limited.

But is it true?

A recent paper in the American Journal of Medicine asks: "Is salt a culprit or an innocent bystander in hypertension?" Dr James J DiNicolantonio and colleagues claim that the notion of excessive salt consumption leading to hypertension "is based on opinion, not scientific proof". For example, they cite a 2011 Cochrane Review – such reviews are internationally recognised as the highest standard in evidence-based healthcare – of almost 170 studies, which noted that sodium restriction only lowers blood pressure by 1 per cent to 3 per cent in people with normal blood pressure (normotensives) and between 3.5 per cent and 7 per cent in people with high blood pressure (hypertensives).

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Protection

Sugar, not salt, the DiNicolantonio paper claims, is the more likely primary cause of hypertension: “Clinicians should advise their hypertensive patients to cut down on the intake of added sugars (sucrose and high fructose corn syrup) and refined carbohydrates such as white flour, and make dietary salt less of an issue.”

The Food Safety Authority of Ireland (FSAI) told The Irish Times that it "doesn't typically respond to the individual findings of research papers, such as Dr DiNicolantonio's", adding that the FSAI bases its views on salt reduction and the protection of public health on the totality of the evidence as reviewed by its independent scientific committee: "In 2016, our scientific committee reviewed and revised its 2005 salt and health report. This review was (under)taken as new research regarding salt and its impact on public health had come to light in recent years.

"However, our scientific committee concluded that the totality of evidence from around the world supported the approach of continuing to reduce the salt intake in the Irish population to improve public health. As new research and scientific evidence accumulates further reviews of our position on salt reduction may be necessary.” The FSAI recommends the “daily limit for salt intake is 6 grams, and less for children”.

The nature of the salt debate – some of it acrimonious – hardly provides clarity for consumers

As for Dr DiNicolantonio’s assertion that sugar is the culprit behind hypertension, the FSAI responded: “We’ve been aware of this hypothesis over the last few years and noted some research suggesting a link between sugar and hypertension. However, the totality of this research is minimal compared to research related to hypertension and salt. That said, we recognise the dangers of high sugar intakes and its effects on the health of our population.”

A recent article in the Journal of Clinical Hypertension – whose lead author is director of the World Health Organisation Collaborating Centre on Population Salt Reduction – commented that "(c)urrent recommendations to reduce population sodium intake are based on sound scientific evidence. The recent publication of a few paradoxical studies of questionable scientific merit should not delay implementation of salt reduction initiatives worldwide."

Questionable

One of five cited so-called "paradoxical studies of questionable scientific merit" appeared in the Lancet (2016) and was co-authored by Prof Martin O'Donnell, professor of translation medicine at NUI Galway. The study of 133,000 adults found that among hypertensives, a high sodium intake, compared with a moderate sodium intake, was associated with a greater risk of cardiovascular events and death. But no such association occurred among normotensives.

Notably, a low sodium intake was associated with a greater risk of cardiovascular events and death in both hypertensives and normotensives: “These data,” say the authors, “suggest that lowering sodium intake is best targeted at populations with hypertension who consume high sodium diets.”

Dr Neville Wilson, medical director of the Leinster Clinic, acknowledges that excess salt can harm cells, but he told The Irish Times that the salt/sugar/hypertension evidence adduced by Dr DiNicolantonio and colleagues is persuasive: "Despite the conventional view that salt is one of the causes of hypertension and obesity, there is no good science to back up this hypothesis. Since sodium is an essential ion for nerve conduction, muscle contraction and cell signalling, salt restriction may cause harm, and low concentrations of sodium in the urine may indicate inadequate intake."

A low salt intake, Dr Wilson added, may cause altered brain function, especially in the elderly: “Certain drugs for high blood pressure, often prescribed for the elderly, may cause losses of important electrolytes (sodium and potassium) through kidney and urinary excretion, and impair normal brain function in vulnerable people.

"The lack of sodium retention by the kidneys is common in the elderly and in pregnant women and may result in hyponatraemia, or low blood concentrations of salt, swelling in the brain, and may be linked to amyloid plaque formation, a common feature in patients with Alzheimer’s disease. The body cannot make sodium chloride, and depends on diet for a healthy supply of these nutrients.”

The nature of the salt debate – some of it acrimonious – hardly provides clarity for consumers. But as Gary Taubes observed in The (Political) Science of Salt, published in Science (in 1998): "The salt controversy is a philosophical clash between the requirements of public health policy and the requirements of good science; between the need to act and the institutionalised scepticism to develop a body of reliable knowledge."

However, FSAI advice that few would quibble with is “limit high-salt foods such as processed meats, snacks. Use fresh meat, fish, fruit and vegetables wherever possible”.