TORONTO - Excess sodium consumption has been linked to higher rates of heart attack, stroke and heart failure, but a new study suggests having too little dietary salt may also be detrimental, at least in people already at risk for cardiovascular disease.
The international study of almost 29,000 individuals, published Tuesday in the Journal of the American Medical Association, found that high consumption of sodium-rich foods is indeed associated with a greater incidence of cardiovascular events like heart attack.
"But what was surprising -- and we didn't expect it -- is that very low sodium intake actually was also associated with a higher risk of these events," said co-principal author Dr. Salim Yusuf, a cardiologist and epidemiologist at McMaster University.
The researchers found that people with a low salt intake had higher rates of cardiovascular death and hospitalization for congestive heart failure.
"There is the theory in which people who are promulgating a policy of extreme salt reduction are assuming that it will keep on tracking down" in terms of reducing cardiovascular events, Yusuf said from Hamilton. "And we weren't able to show that."
It was only among people who had moderate sodium intake of about three to six grams a day -- the case for a third to half of Canada's population -- that researchers found the lowest rates of heart attacks, strokes and heart failure. "Above that and below that we had higher rates.
"This is important because if we go out and say every food should aggressively go down in salt content, we have to be careful with making that a national law or a national policy. Because we're assuming that the lower the sodium, the better it is for people -- and our study's challenging that."
Many researchers and several groups, including the Heart and Stroke Foundation and the Stroke Network, are calling for a national policy to reduce sodium content in foods.
The average Canadian consumes about 3,400 mg of sodium a day, the vast majority of which is contained in processed foods.
Following a report by the Sodium Working Group last year, the federal government endorsed an interim goal of reducing sodium intake to 2,300 mg per day by 2016, and the implementation of voluntary industry targets and monitoring for sodium reduction.
But some people are lobbying for even lower consumption targets, less than 1,500 mg a day.
"That is a big jump," which could only be achieved with a substantial and costly alteration to the way in which food is processed, said Yusuf, noting that neither he nor his co-authors have any financial ties to the food industry.
"If that is justified, we should do it. The issue is it's uncertain that it is justified."
"If, in fact, there is harm by lowering the sodium too much, then we're going to harm people by a super aggressive policy."
Dr. Michel Joffres, a professor of health sciences at B.C.'s Simon Fraser University, suggested the ideal sodium intake is about 1,000 mg to 1,200 mg per day, which he said is still five times more than what the body requires.
There is no logical explanation for low sodium levels being detrimental, added Joffres, noting that clinical trials and other data have shown that the lower a person's salt intake, the better it is for cardiovascular health -- in healthy people.
"The problem is you don't have healthy people," he said of the study.
Joffres, a spokesman for the Heart and Stroke Foundation, said the subjects could have had low sodium levels because they were already in poor cardiovascular health and had been advised to reduce salt intake in their diets.
"The people were already sick ... They are at a higher risk anyway, even if they have lower sodium now," he said Tuesday. "And those, therefore, do not represent the true association."
Yusuf acknowledged that might have been the case, so the research team adjusted for that possibility in their analysis and still came up with a link between low intake and a higher rate of adverse events.
"This is challenging the concept that lower is better," he said. "What this is reaffirming is that there's an optimal level, and if you go above it, it's bad. If you go below it, it may not be better and it may even be bad."
However, Dr. Paul Whelton of Tulane University in New Orleans writes in an accompanying editorial that most adults consume sodium far in excess of physiologic need, most of which comes from processed foods.
"The scientific underpinning for the health benefits from sodium reduction is strong, and the available evidence does not support deviating from the stated goal of reducing the exposure to dietary sodium in the general population."
