We should cut back on salt – but there's still room for reasoned debate on the topic, writes PROF EOIN O'BRIEN
WE ADD too much salt to food and there is too much salt in most processed foods. There is no argument but that excessive salt in the diet raises blood pressure, which is a major cause of stroke, heart attack and kidney disease and that reducing salt lowers blood pressure. It should follow that policies to reduce salt intake in the community should be beneficial in preventing the cardiovascular consequences of elevated blood pressure. There has been general acceptance that governments should be persuaded that the lower the salt intake of the nation, the better will be the health of all.
The first serious challenge to such a policy came with publication of a paper in the Journal of the American Association (JAMA) in May 2011 by Prof Jan Staessen and his colleagues in which excessive reduction of salt seemed to be associated with higher cardiovascular mortality. The authors concluded that caution was needed before instigating recommendations for a generalised and indiscriminate reduction of salt intake. They were careful to emphasise that their results did not negate the blood pressure-lowering effects of a dietary salt reduction in patients with high blood pressure.
The study was dismissed by the Lancet as contributing “little to our understanding of salt and disease” and that “the results of this work should neither change thinking nor practice”.
The grounds for such criticism were that important issues “cannot be answered by small observational studies” and that “it is dangerous to jump to conclusions on the basis of single studies and ignore the totality of evidence”.
The refutation to this intolerant dismissal of scientific evidence came with another publication, in JAMA in November 2011, with a Galway-based scientist, Prof Martin O’Donnell, as the lead author. In this study of 30,000 patients, the association of salt with cardiovascular outcome confirmed that too much salt is associated with increased risk from cardiovascular disease but, in keeping with Staessen’s earlier findings, the study also showed that a low sodium intake is associated with increased risk. Based on their findings, the authors stress the need to establish a safe range for sodium intake by performing randomised controlled trials. An accompanying editorial in JAMA, by yet another Irish scientist, Paul Whelton from Tulane University in New Orleans, while acknowledging “that there is general agreement that sodium reduction is appropriate for persons with hypertension, there are some questions about applying this recommendation to the remainder of the population”. He concluded that “the scientific underpinning for the health benefits from sodium reduction is strong, and the available evidence does not support deviating from the stated goal of reducing the exposure to dietary sodium in the general population”.
O’Donnell, like Staessen, is likely to come under more severe fire from the proponents of an indiscriminate reduction of salt. In assessing both the JAMA papers and their critics, we would do well to remind ourselves that these researchers are scientists of impeccable repute who have been to the fore in conducting the largest epidemiological studies into hypertension and cardiovascular disease in the world. We should be aware, moreover, that the JAMA is one of the world’s most prestigious journals and that any paper it publishes will have been rigorously scrutinised.
It is understandable that proponents of salt reduction fear the new evidence will be manipulated by vested interest groups to undermine the call for salt restriction. However, rational deliberation must not fall victim to emotional rhetoric.
We can conclude that nothing changes for people with high blood pressure who should be encouraged to reduce their salt intake, and for those who do not have high blood pressure and whose salt intake is high, the message should be for a reduction in intake.
What these studies highlight is the importance of establishing how much dietary salt should be reduced in people whose intake of salt is low. Hence the need for rigorous, large-scale, population-based randomised clinical trials. However, many researchers, such as Prof Graham MacGregor at Queen Mary University of London and Prof Ivan Perry at University College Cork, who have led the drive for dietary salt reduction, question the feasibility of establishing such trials that could require randomisation of many thousands of participants to a low or high salt diet for at least five years. It may be that such trials are not feasible; it is up to the proponents of randomised control trials to accept the challenge by showing not only how such trials could be done, but how they would be financed, and how any ethical issues would be overcome.
Eoin O’Brien is professor of molecular pharmacology at the UCD Conway Institute of Biomolecular and Biomedical Research