Too Much or Too Little Salt May Be Harmful

Two reports from a global collaborative study involving hundreds of investigators from 18 countries published in the New England Journal of Medicine are shaking up conventional wisdom around salt consumption.

The Prospective Urban & 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, andstrokes.

Current sodium intake in Canada is typically between 3.5 and 4 g per day, and some guidelines have recommended that the entire population lower its sodium intake to below 2.3 g per day, a level that fewer than 5% of Canadians (and people around the world) now consume.

Researchers have shown that the effects of increasing sodium intake on raising blood pressure—a risk factor for heart attack, heart failure, stroke, and other problems—become dramatically worse as intake rises above 5 g per day, especially among people who already have high blood pressure, who are older than 55, or both.

But the blood pressure effects are more modest at average levels of sodium consumption (3 to 5 g per day) and not evident at low levels of intake below 3 g of sodium per day (a level that is higher than the maximum currently recommended by many guidelines), says Andrew Mente, PhD, the leadauthor of one report, and an assistant professor of clinical epidemiology and biostatistics at McMaster University.

“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 islikely to have the greatest benefit in lowering blood pressure,” Mente says. “This can be achieved by moderation in salt intake, combined with eating lots of fruits and vegetables.”

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

In fact, the lead author of the second report, Martin 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 adverse 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,” says O’Donnell, an associate clinical professor at McMaster University and the National University of Ireland Galway. “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 g 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 andhealth outcomes and adds considerable strength to the contention that moderate sodium intake is optimal.”

“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, MD, 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.

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 g is optimal. The good news is that most people in the world already consume an amount in that range, the researchers found.

Source: McMaster University

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