A new study suggests that in people with heart disease, eating too much salt is linked to a higher risk of heart-related hospitalizations and deaths – but so is eating too little.
The findings are the latest addition to the cloudy picture of the role of salt in heart risks, but the study's lead researcher said he believes they actually help make it clearer.
"What we're showing is that the association between sodium intake and cardiovascular diseases appears to be J-shaped," said Martin O'Donnell, from McMaster University in Hamilton, Ontario.
The J-shaped line depicting heightened risk at very low salt levels and at high levels, with low risk in the middle, could explain why studies in different groups of people have come to different conclusions on the effects of eating more or less salt, he added.
In theory, based on the new findings eating less salt could be linked to a reduced risk of heart problems in a group that generally eats lots of salty food – but may be associated with an increase in those risks if salt consumption is already on the low end.
"We would hope earnestly that our study would not add to the controversy, but would provide some clarity to what appeared to be conflicting findings of previous studies," O'Donnell said.
Still, he cautioned, the new findings don't prove that people can change their risks by tinkering with the salt in their diets, since salt is only one component linked to heart health and this type of study isn't designed to tease out cause-and-effect.
US guidelines recommend that people consume less than 2,300 milligrams of sodium daily, and less than 1,500 mg if they're at extra risk for high blood pressure or heart disease. The World Health Organization also recommends keeping sodium under 2,000 mg a day.
In the new analysis, published in the Journal of the American Medical Association, the lowest risk of stroke, heart attack and other hospitalizations and deaths came with an estimated daily sodium intake of 4,000 to 6,000 mg – which was about what people were getting on average to begin with.
O'Donnell and his colleagues used data from two drug trials involving close to 30,000 people who already had cardiovascular disease or diabetes.
At the start of those trials, participants had their full-day sodium intake estimated through a morning urine test. They were then followed for an average of four to five years for any cardiovascular-related hospitalizations and deaths, including strokes and heart attacks.
During that time, there were about 4,700 such events, including just over 2,000 deaths due to cardiovascular disease.
Compared to people who ate between 4,000 and 6,000 mg of sodium in a day, those who got more than 8,000 mg were 50 to 70% more likely to suffer a heart attack or stroke or to be hospitalized or die from heart disease.
That was after taking into account what medications the participants were taking, as well as other aspects of health and lifestyle known to affect heart problems, such as weight, smoking, fruit and vegetable consumption and cholesterol levels.
Lower sodium intake
Between 2,000 and 3,000 mg per day – was also linked to a 20% higher risk of cardiovascular-related death as well as hospitalizations for congestive heart failure.
"In people with heart risks, very high sodium intake causes harm. What is emerging now is that going too low might also cause harm," said Dr Jan Staessen, who studies salt and cardiovascular disease at the University of Leuven in Belgium but wasn't involved in the new study.
"There's a lot of evidence accumulating that going as low as (1,500 mg) of sodium might cause harm."
Staessen's own research has suggested that in the general population, lower levels of sodium are linked with a higher risk of death. Another recent review of past studies didn't find an overall health benefit associated with cutting back on salt.
In an interview with, Staessen emphasized that the people in this study were very high-risk, and the findings don't necessarily apply to everyone.
O'Donnell agreed that the optimum amount of salt in the diet may be different in people without heart disease or diabetes, but he suspects that the J-shaped relationship between salt intake and heart risks will generally hold up.
"Clearly the message that we don't want lose in this observation (is) a strong association between high sodium intake and cardiovascular events," O'Donnell said.
While sodium recommendations currently give an upper limit for daily intake, "it makes sense that the recommendations should probably be a safe range," he concluded.
Still, gold standard randomised studies are needed to figure out what exactly that range is, he said.
(Reuters Health, Genevra Pittman, November 2011)