Two newly-published studies have added grains of salt to the conventional wisdom that you should consume as little sodium as possible, but the findings are leaving a bad taste in the mouths of some experts.
In one study that looked at death and heart disease, researchers found that extremely low-salt diets may not be as beneficial as many experts believe, and might even pose a hazard.
The other, focused on blood pressure effects, found that people with a moderate salt intake didn't benefit from reducing their consumption as much as people in the high-salt group.
Read: 10 salty food culprits
"Previously it was believed that the lower you go the better. What these studies show collectively is that there is an optimal level, and lower is not necessarily better," Dr. Andrew Mente of McMaster University in Hamilton, Ontario, chief author of the blood pressure study, told Reuters Health by phone.
On the other hand, a third international study supports the conventional wisdom that there's a direct link between less salt and better health, and found no evidence that extremely-low levels of sodium are dangerous.
All three reports are published in the New England Journal of Medicine.
Too much salt is bad for you
One thing all the studies confirm: too much salt is bad for you. High amounts contribute to high blood pressure, stroke, heart attack, kidney problems and heart failure.
That third study concluded that high salt intake was responsible for 1.65 million heart disease deaths worldwide in 2010. It's based on computer models, surveys of salt use in 66 countries and 107 published studies.
"These are staggering numbers when you actually think about it," Dr. Elliott Antman, president of the American Heart Association, told Reuters Health.
Read: Too much salt could kill you
Worldwide, daily consumption is usually 3 to 6 grams of sodium, which translates to 7.5 to 15.0 grams of salt. That's well above the limit of 1.5 to 2.4 grams of sodium each day recommend by the World Health Organisation, the American Heart Association and other organisations.
"Even at 4 grams per day, we found half a million deaths per year," chief author Dr. Dariush Mozaffarian, dean of the Friedman School of Nutrition Science and Policy at Tufts University, told Reuters Health.
Is this the same for everyone?
Mozaffarian said his analysis uncovered no danger at low levels or below, and he argued that even Mente's blood pressure study showed no risk there.
But Mente said the relationship seems to be more complicated than previously thought, and the health benefits of lowering salt intake don't fall at the same pace for everyone.
"If people are eating a very high level of sodium and they reduce their intake, you get a large reduction in blood pressure," he said. "But if you're eating a moderate level of sodium – about what most North Americans eat – and you reduce it to a lower level, you're not really getting much in return as far as blood pressure reduction is concerned."
Sodium intake and death
The most dramatic evidence was the study that looked at the link between sodium intake and death, heart attacks and strokes using urine samples to estimate sodium consumption.
Led by Mente's colleague at McMaster, Dr. Martin O'Donnell, it found that consuming less than 3 grams of sodium per day increased the risk of death or major cardiovascular events by 27 percent compared to people who consumed 4 to 6 grams daily.
The optimum sodium level was 3 to 6 grams per day. "Both higher and lower levels of estimated sodium excretion were associated with increased risk," the O'Donnell team concluded.
That study shows "there are unintended consequences of going lower" Mente said in a telephone interview.
Currently only about 1 percent of the people worldwide "were consuming less than 2 grams per day," he said. "What we're recommending right now is what almost nobody around the world eats, apart from maybe a few hunter-gatherer societies. We're making recommendations where the human experience is extremely sparse. These findings strongly question our current recommendation."
Read: Current salt guidelines may be too low
Yet Mozaffarian argued that they don't. Both the O'Donnell and Mente studies derived their results from the same research project, known as the PURE study.
"When you have a single study like the PURE study, that's one study in the context of many," said Dr. Mozaffarian. Some have suggested a risk for very-low salt intake while others have not.
Not only does the weight of the evidence suggest that there is no risk in aggressive salt reduction, Mozaffarian said. "You don't have any plausible biology to explain why you'd get an uptick in risk with low sodium intake."
Urine sample unreliable for assessing sodium consumption
There's another reason for scepticism. The PURE study assessed sodium consumption based on a single urine sample collected each morning. Antman called that a "very unreliable method" for measuring salt intake.
"The gold standard is 24-hour urine collection, and they didn't use it. And even then your sodium intake may vary a lot from day to day," he told Reuters Health.
"The bottom line is, the American Heart Association position has not changed in response to the PURE paper," said Antman, a cardiovascular expert at Brigham and Women's Hospital in Boston. The association recommends less that 1.5 grams of sodium daily.
Read: Heavier teens eat more salt
Nonetheless, Dr. Suzanne Oparil of the University of Alabama at Birmingham contends in a Journal editorial that, "Both high and low levels of sodium excretion may be associated with an increased risk of death and cardiovascular-disease outcomes and (the tests suggest) that increasing the urinary potassium excretion counterbalances the adverse effect of high sodium excretion."
That idea should be confirmed in a large study, she said, but "in the absence of such a trial, the results argue against reduction of dietary sodium as an isolated public health recommendation."
High sodium remains bad
"The big picture is that high sodium is bad and should be reversed, and there's just some controversy over how low you should go," said Mozaffarian, adding that doctors and patients should be pushing for a strong national policy to reduce the sodium content of processed food.
"Whether it should be 2 grams or 1.5 grams or 2.5 grams per day, that's all theory." he said. "Right now it's close to 4 grams per day. Let's get it down below 3, and then we can argue how low it should go. But right now it's clearly way too high."
The truth about salt
Are you eating too much salt?
Shake your salt habit
Image: Salt from Shutterstock