Experts Respond: Moderate salt reduction benefits questioned

6 July 2011

A review of scientific literature by The Cochrane Library suggests moderate reductions in salt intake don't lead to reduced risk of a person suffering cardiovascular events or dying.

The paper, Reduced Dietary Salt for the Prevention of Cardiovascular Disease: A Meta-Analysis of Randomized Controlled Trials (Cochrane Review)' by R.S. Taylor et al. will be published in the American Journal of Hypertension today.

The Cochrane Library's "plain language" summary of the research:

"Cutting down on the amount of salt has no clear benefits in terms of likelihood of dying or experiencing cardiovascular disease. Cardiovascular disease includes heart attacks, strokes, and the need for heart surgery and is a major cause of premature death and disability. This review set out to assess whether advice to cut down on salt in foods on altered our risk of death or cardiovascular disease.

"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. There was not enough information to understand the effect of these changes in salt intake on deaths or cardiovascular disease. Further research in needed to confirm our finding that dietary advice to reduce salt may increase deaths in people with heart failure."

The SMC gathered reaction to the research from experts. Feel free to use the quotes below. Contact the SMC if you would like to interview the quoted experts.

Robert Beaglehole, Emeritus Professor, University of Auckland and former World Health Organisation Director of the Department of Chronic Disease and Health Promotion comments:

"I agree that there is not yet sufficient evidence on hard outcomes, though there is plenty of evidence that salt reduction reduces blood pressure. A decent mortality study is still required. In the meantime, we should be doing all we can to reduce salt intakes - current high levels serve no useful purpose and are probably very harmful. Further, the real problem is that giving advice to reduce salt intake doesn't work. We need to reduce the amount of salt in manufactured food".

Professor Robert Walker, Head of Department, School of Medicine, University of Otago, comments:

"An interesting analysis. I think the important issue is that it is from a relatively small number of studies 7 in total but only 2 were in groups that would be deemed at moderate risk - the more hypertensive group.

"Lowering salt intake has clear cut evidence for reducing blood pressure. However blood pressure is not the only cardiovascular risk and therefore it has to be seen in the global context of reducing risk not as the sole intervention to reduce heart attacks.

"In addition, the impact of salt is not solely on blood pressure, elevated salt intake also has direct effects on blood vessel function independent to that of blood pressure. This may be more critical to the risk of stroke or kidney damage, rather that heart attack, which the meta-analysis did not address.

"Therefore in the context of general good health, it is not appropriate to go out and reload the salt shaker. Dietary reduction in salt for those at risk of cardiovascular disease should still be encouraged and placed in the same context as exercise, healthy diet and smoking cessation."

Elaine Rush, Professor of Nutrition, Auckland University of Technology, comments: "Cutting down on salt does not reduce the likelihood of dying or experiencing cardiovascular disease.

Facts: "Both the elements of "common salt", sodium and chloride, are essential for animal life. Sodium is also found in food as sodium bicarbonate, monosodium glutamate and food additives. The daily intake of sodium is exquisitely balanced by the body through excretion in urine and sweat.

"Higher blood pressure is associated with high intakes of sodium and also obesity and existing high blood pressure. There is a genetic predisposition for risk high blood pressure in some families and ethnic groups. Across the lifecourse all the environmental pressures that drive obesity also drive hypertension

What we do not know: "The authors state that there is insufficient data to exclude clinically important effects of reduced dietary salt on mortality or cardiovascular morbidity.

"For the studies reported ethnicity, socioeconomic status, gender, menopausal status, age were not included in the overall analysis. Most of the participants were white and male. The percentage female is not reported.

What we should consider: "Hypertension does not just happen, as we age blood pressure increases. Relatively high blood pressure in young children and adolescents is a risk factor for high blood pressure in later life. Reducing salt intake may help, but increased physical activity, less stress, losing extra weight and improving the nutrient quality of the foods eaten also help reduce blood pressure and improve health

"Putting the spotlight on single nutrient trials and generalizing dietary advice for a single nutrient, in this case to reduce salt intake, is not helpful. What is helpful is for the food industry to reformulate products to reduce sodium AND increase the nutrient quality of foods by using real ingredients.

"Junk food and marketing to children are elephants in the room. Changes in our attitudes to these would markedly reduce salt consumed, shift consumption to real foods and be more friendly to health and the environment - long term."

Delvina Gorton, National Nutrition Advisor, Heart Foundation, comments:

"The Heart Foundation welcomes the Cochrane Collaboration's robust review of the effect of moderate salt reduction. As the author's acknowledge, however, there was simply not enough data to allow them to draw firm conclusions around whether moderate salt reduction has an effect on risk of heart attack or death.

The authors estimated they needed 18,000 participants to draw firm conclusions - substantially more than the 6,489 participants in the seven studies included in the review. Reduced risk ratios suggested a beneficial effect from moderate salt reduction but the sample size was too small for the trend to be conclusive.

Other types of research have conclusively linked moderate salt reduction with reduced risk. The Heart Foundation maintains its position of advocating salt reduction for all New Zealanders and will continue to support the food industry in reducing salt levels in pre-prepared foods."

Additional comments gathered by our colleagues at the Science Media Centre in the UK:

Simon Capewell, Professor of Clinical Epidemiology at the University of Liverpool comments:

"This is a disappointing and inconclusive meta-analysis, with mixed data and small numbers of events. It is fronted by a potentially misleading press release from the American Journal of Hypertension.

"The main Cochrane message is that advice to individuals only succeeds in getting them to reduce their salt intake a little.

"This is not a new finding.

"Crucially, it does not change the public health consensus from the WHO, PAHO, NICE etc, which is that: 1. Dietary salt raises blood pressure (this is bad), 2. Reducing dietary salt intake lowers blood pressure (this is good), 3. Government actions are far more effective and cost saving at reducing dietary salt intake, than is advice to individuals."

ends

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