医学争论:到底需要多少盐?

A new study published in a respected American medical journal concludes that low-salt diets could lead to heart disease.  U.S. public health officials say Americans are eating too much salt. The cstudy caused a controversy about how much salt is OK and how much is too much.  

The controversy began when a study published in the prestigious Journal of the American Medical Association contradicted long-standing advice from public health officials and medical specialists on heart disease, stroke and recommended salt consumption.

The study involved more than 3,600 men and women and followed them for eight years until the participants were about 49-years-old. When the data was analyzed, those who consumed the lowest amounts of salt turned out to be the most likely to die from cardiovascular disease. The study also reports that salt consumption did not cause high blood pressure for some 2,000 participants whose blood pressure was normal at the start of the study.

The study was hailed by the U.S. salt industry.  Lori Roman is president of the Salt Institute, the industry's research arm.  "The evidence continues to mount...that reducing sodium can cause great harm,” she said. “And that salt reduction as a strategy to reduce blood pressure is not the best choice."

The U.S. Centers for Disease Control and Prevention publicly criticized the salt study, something the federal health agency rarely does.  The CDC's Dr. Peter Briss says one study does not change the evidence against salt.

"Salt increases blood pressure.  More salt leads to higher blood pressure and higher blood pressure leads to worse cardiovascular health," said Dr. Briss. He said the study was too small, and the participants too young.  He added that some of the participants who died were heavy smokers.  Dr. Briss said there is no evidence that reducing salt is harmful, a point echoed by Dr. Stephen Havas, an expert on cardiovascular disease.

"You have to put this study in the context of how much evidence is out there. The World Health Organization referred to the evidence on the harmful effects of sodium as being conclusive," said Dr. Havas.

In medical research, the word 'conclusive' is rarely used.  In the U.S., warnings on packages of cigarettes say they can cause lung cancer and heart disease.  But a majority of doctors agree that eating too much salt will cause cardiovascular disease.

U.S. public health officials say the upper limit of salt consumption should be 2,300 milligrams per day. The American Heart Association says that it should be no more than 1,500 milligrams.

The Salt Institute says 2,300 milligrams is far too little. "People consume in a very normal and natural range of sodium - somewhere between 2,500 and 4,500 milligrams a day is very normal and natural," said Roman.

"You risk having a stroke, you risk having a heart attack if you are consuming as much salt as the Salt Institute is recommending," Dr. Havas stated.

As for the study itself, Dr. Havas says it is weak and its methodology flawed. Public health officials are concerned that it was published in a prominent American medical journal.  "It just confuses the public,” he said. “It shouldn’t have gotten out there.  Certainly should not have gotten out in a prestigious journal."

The Salt Institute says it would like to see the study repeated. The medical community maintains people should limit their intake of salt, and while doing that, follow a healthy diet, exercise and if overweight, loose at least some of the excess.