Reducing daily salt intake by a moderate amount reduces blood pressure but not people’s chances of dying or getting cardiovascular disease, said UK researchers who systematically reviewed evidence available from published trials. Trials involving much larger groups cutting their salt intake by more than a moderate amount could tell a different story, they noted.

“We believe that we didn’t see big benefits in this study because the people in the trials we analyzed only reduced their salt intake by a moderate amount, so the effect on blood pressure and heart disease was not large,” lead author Professor Rod Taylor who works at the Peninsula College of Medicine and Dentistry at the University of Exeter, told the press.

Taylor and colleagues wrote about their findings in the latest issue of the The Cochrane Library.

An earlier Cochrane review concluded in 2004 there wasn’t sufficient evidence to say what effect reducing salt intake had on mortality or cardiovascular events, so Taylor and colleagues set out to find more recent studies to pool data from. They found seven studies covering a total of 6,489 participants.

When they analyzed the pooled data they found evidence that moderate reductions in salt intake reduces blood pressure:

“Intensive support and encouragement to reduce salt intake did lead to a reduction in salt eaten and a small reduction in blood pressure after more than six months,” said Taylor.

But what they wanted to see was evidence that this change in diet also reduced people’s risk of dying or having cardiovascualr disease, he explained. But they did not find this.

However, as the old saying goes, “absence of evidence does not equal evidence of absence” and it could simply be that the studies done so far have not been big enough.

Taylor said while 6,500 or so was enough participants to start drawing some conclusions, he believes you need at least 18,000 participants before you start seeing impact on health.

Many countries now have government-supported recommendations calling for reductions in dietary sodium. For example, NICE (short for National Institute of Health and Clinical Guidance, an independent body that gives national guidance on managing health and disease), says the UK should accelerate reduction of dietary salt from a maximum of 6g per day per adult by 2015 to 3g a day by 2025.

Taylor said against a backdrop of governments calling for these accelerated salt intake reductions, and food companies reducing it in their products, “it’s really important that we do some large research trials to get a full understanding of the benefits and risks of reducing salt intake”.

He also thinks we need to find ways of reducing salt in our diets that are both practicable and inexpensive.

Meanwhile, some of the authors of the studies that Taylor and colleagues included in their review have criticized their methods. They spoke about it to Heartwire.

Dr Lawrence Appel, from Johns Hopkins University Medical School in Baltimore, said the review “adds little to our knowledge about the health effects of sodium reduction”.

He said none of the studies were designed to look at the effect of sodium reduction on cardiovascular events and risk of death, and “even in aggregate, the number of events is small and the statistical power is limited”.

Dr Nancy R Cook, from the Harvard Medical School in Boston, agrees. She described the analysis as “crude” and “therefore loses information”. She said the review misrepresents several of the studies, so the results are not reliable. For example, Taylor and colleagues just counted the number of events and didn’t use individual patient data, which is what Cook and colleagues did in their study and that is why they found signficant results, she added.

“They are saying there’s no evidence for an effect, but the analyses are different,” said Cook, adding that if they had used a different method, Taylor and colleauges would have got different results.

“Reduced dietary salt for the prevention of cardiovascular disease.”
Taylor RS, Ashton KE, Moxham T, et al.
Cochrane Database of Systematic Reviews 2011;
DOI: 10.1002/14651858.CD009217
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Sources : Peninsula College of Medicine and Dentistry, Heartwire 5 July 2011.

Written by: Catharine Paddock, PhD