Too little and too much are both harmful, researchers find

Results from the largest study of its kind ever conducted -- involving 18 countries and more than 100,000 people -- indicate that the current recommended maximum sodium intake for the population is actually too low and may even be unsafe. However, high sodium is also harmful, so an ‘optimal’ range is the best target.

Two reports from a global collaborative study involving hundreds of investigators from 18 countries, published today in the New England Journal of Medicine, are shaking up conventional wisdom around salt consumption. NUI Galway’s Professor Martin O’Donnell played a lead role in the study, and is first author on one of the reports.

The Prospective Urban and Rural Epidemiological (PURE) study, led by investigators from the Population Health Research Institute, McMaster University and Hamilton Health Sciences, followed more than 100,000 people for nearly four years. The study assessed sodium and potassium intake and related them to blood pressure as well as to deaths, heart disease and strokes.

The salt debate
Professor Martin O’Donnell, formerly of McMaster University, is now Professor of Translational Medicine at NUI Galway and Associate Director of the HRB Clinical Research Facility Galway.

An expert in cardiovascular health, he explained the debate around salt intake: “A much debated question is how low should we go with our salt intake? Most of the current guidelines recommend very low salt intake, which the vast majority of people don’t achieve.”

While too much salt has long been recognised as a serious health risk, the researchers have also found that there may be a risk from eating too little.

In fact, Professor O’Donnell suggests that what is now generally recommended as a healthy daily ceiling for salt consumption appears to be set too low. “Low sodium intake does reduce blood pressure modestly, compared to moderate (or average) intake, but low sodium intake also has other effects, including elevations of certain hormones that are associated with an increase in risk of death and cardiovascular diseases. The key question is whether these competing physiologic effects result in net clinical benefit or not,” O’Donnell said.

Current intake of sodium is typically between 3.5 and 4 grams per day and some guidelines have recommended that the entire population lower its sodium intake to below 2.3 grams per day, a level that fewer than 5 per cent of people now consume.

Professor O’Donnell continued: “In the PURE study, we found the lowest risk of death and cardiovascular events in those who consumed moderate amounts of sodium intake (3 to 6 grams per day), with an increased risk above and below that range. While this finding has been reported in previous smaller studies, PURE is the largest international study to study sodium intake and health outcomes, and adds considerable strength to the contention that moderate sodium intake is optimal.”

However, he also emphasised: “Our findings do not mean that people should become complacent about salt intake, high salt intake is a health risk especially in those with high blood pressure. People should avoid adding salt to their foods, as many foods already contain salt, and should reduce or avoid processed foods that contain high salt. Moderate salt intake is only one part of a healthy diet, other aspects of a balanced diet are important, such as increasing fruit and vegetable intake.”

The blood pressure effect
The researchers showed that the effects of increasing sodium intake on raising blood pressure – a risk factor for heart attack, heart failure, stroke and other problems – become most apparent as intake rises above 5 grams per day, especially among people who already have high blood pressure, or who are older than 55, or both.

“But the blood-pressure effects are more modest at average levels of sodium consumption (3 to 5 grams per day) and not evident at low levels of intake below 3 grams of sodium per day (a level that is higher than the maximum currently recommended by many guidelines), says Andrew Mente, Assistant Professor in the Department of Clinical Epidemiology and Biostatistics at McMaster University, and lead author of one of the new reports. The study also found an association between increasing potassium intake and lower blood pressure. 

“While there has been much focus on reducing salt in the diet, an important and ignored approach to lowering blood pressure is increasing the amount of potassium consumed. A balanced approach is what is likely to have the greatest benefit in lowering blood pressure,” says Mente. “This can be achieved by moderation in salt intake, combined with eating lots of fruits and vegetables.”

The studies were funded from more than 50 sources, including the PHRI, the Heart and Stroke Foundation of Ontario and Canada and the Canadian Institutes of Health Research.

“The findings of both studies are robust, globally applicable and collectively question established dogma and recommended policies. This also means that salt reduction should be primarily targeted at those who have high blood pressure and those who consume a lot of salt,” says Salim Yusuf, the Principal Investigator of the global PURE study, senior author of both reports, and Director of the Population Health Research Institute, which designed and coordinated the study.

Sweet Spot
Taken together, the papers show there is a ‘sweet spot’ for sodium consumption, where too much or too little can be damaging, while a moderate amount between 3 and 6 grams is optimal for the general population The good news is that most people in the world consume an amount in the optimal range, the researchers found.

In an accompanying editorial in the NEJM, Professor Suzanne Oparil from the University of Birmingham, Alabama, urged reconsideration of current guidelines and recommended randomized trials comparing clinical outcomes in people who consume usual salt intake to low intake.

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