New study questions conventional wisdom on health threat from salt
Controversial new study suggests sodium isn't the health risk it has been made out to be, and that too little can actually be bad for you
A large international study questions the conventional wisdom that most people should cut back on salt, suggesting that the amount most people consume is OK for heart health - and too little may be as bad as too much.
But the new findings came under immediate attack by other scientists.
Limiting salt is still important for people with high blood pressure - and in fact, a second study estimates that too much sodium contributes to up to 1.65 million deaths each year. The studies both have strengths and weaknesses, and come as the US government is preparing to nudge industry to trim sodium in processed and restaurant foods.
The first study's leader, Dr Salim Yusuf of McMaster University's Population Health Research Institute in Hamilton, Ontario, urged keeping an open mind.
"There are those who have made a career out of promoting extreme sodium reduction that will attack us," he said, adding it was better to focus on a healthy lifestyle and overall diet instead of a single element.
No-one should view the new study as permission to eat more salt, he said. "Most people should stay where they are."
The studies were published in yesterday's New England Journal of Medicine.
Yusuf's study was observational, rather than a strict experiment, and had big limitations in its methods. But its size lends strength - more than 100,000 people in 17 countries, the largest on this topic. It is also from a general population, not just people at high risk of heart disease, as many past studies have been.
Researchers found that sodium levels generally correlate with the risk of high blood pressure. But this link is strongest when sodium intake is high, and was not seen at all when consumption is low. The link is also stronger as people age.
Meanwhile, a different nutrient - potassium, found in vegetables and fruits - seems to lower blood pressure and heart risks, and offsets sodium's effect.
People who consume three to six grams of sodium a day (about eight to 15 grams of salt) had the lowest risk of heart problems or death from any cause during the nearly four-year study. More or less sodium raised risk. About three-quarters of the world's population is in the ideal range.
Guidelines from various groups for heart-disease prevention recommend 1.5 to 2.4 grams of sodium a day. The American Heart Association advises no more than 1.5 grams.
"These are now the best data available," Dr Brian Strom said of the new study. Strom, the chancellor of Rutgers Biomedical and Health Sciences, led a US Institute of Medicine panel last year that found little evidence to support very low sodium levels.
"Too-high sodium is bad. Too low also may be bad, and sodium isn't the whole story. People should go for moderation."
About 40 per cent of participants had high blood pressure.
Sodium levels were estimated from a single urine test instead of the preferred method of over 24 hours at multiple times, which Yusuf said was impractical in such a big group.
That drew criticism from a host of scientists.
"This is a fundamental flaw" that undermines confidence in the results, said Dr Elliott Antman, a Brigham and Women's Hospital cardiologist.
The second study in the journal was led by Dr Dariush Mozaffarian of Tufts University. Researchers looked at dozens of studies around the world on sodium intake, calculated its relationship to high blood pressure, and then the relationship of high blood pressure to cardiovascular deaths.
There were 1.65 million deaths from intake of over two grams of sodium a day, they estimate, and half a million deaths based on current average worldwide consumption of four grams a day, Mozaffarian said.