Arguments over salt and health have broken out on both sides of the Atlantic, with dissenting opinions among individual experts and even professional societies. What’s going on?

New IOM report

The latest round of the salt controversy was triggered by the Institute of Medicine (IOM) in the United States, a very conservative and scientific organisation. Their latest report had two key messages, the first confirming the positive association between high sodium intakes, high blood pressure and the risk for heart disease. No drama there. The contentious finding was that there was not a strong scientific case for shifting from moderate to low salt intake.

Just to get your bearings, a high sodium intake is considered to be about 5000 mg/day – the average intake in China. Intakes in many western countries are moderate by comparison, about 3300-3700 mg/day. However, the upper limit of sodium intake recommended in Australia’s Nutrient Reference Values is only 2300mg/day, the same figure recommended in the Dietary Guidelines for Americans. Just last week the European Society of Hypertension and the European Society of Cardiology published new guidelines for the management of hypertension, recommending sodium intakes in the range of approximately 2000-2300 mg/day. The American Heart Association goes even further, recommending just 1500 mg/day.

Response from American Heart Association

The American Heart Association immediately fired off a press release, Chief Executive Officer Nancy Brown stating “While the American Heart Association commends the IOM for taking on the challenging topic of sodium consumption, we disagree with key conclusions.” The AHA described the IOM report as ‘incomplete’, with too little emphasis on the extensive scientific evidence that links excess sodium consumption and high blood pressure. According to the AHA, 90% of all Americans are expected to develop high blood pressure in their lifetime. Less dietary sodium would significantly reduce the rise in blood pressure that occurs with age.

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The heart of the matter

Dissenting opinions among professional societies is never a good look so why has the IOM apparently stuck its neck out?

Everyone agrees that lowering salt intake from moderate to low intakes in clinical trials lowers blood pressure. The key question is whether this translates into lower risk for heart attack, stroke and death. Prospective cohort studies do not provide a clear picture – associations between sodium intake and cardiovascular outcomes have been inconsistent and some recent studies have even reported increased risk for cardiovascular disease at low sodium intake. Randomised controlled trials into the effect of sodium intake on cardiovascular events would provide the answer but no large-scale trials have been conducted. For a good review of the issues see O’Donnell and colleagues (2013).

To be fair, the Institute of Medicine’s brief was not to look at risk factors; it was to look for direct effects of sodium intake on health outcomes such as heart attack, stroke and death. The Institute found that both the quality and the quantity of the evidence of direct links between sodium and health outcomes were limited. The committee concluded that ‘more randomized controlled trials will be needed, as these represent the highest quality study design for determining the effect of sodium on health outcomes … Further research may shed more light on the association between lower—1,500 to 2,300 mg—levels of sodium and health outcomes’.

So, this would appear to be a case of ‘no evidence of effect’ rather than ‘evidence of no effect’.

Debate in Europe

A debate on the pros and cons of sodium restriction was held between Professor Salim Yusuf from McMaster University in Canada and Professor Pasquale Strazzulo from Federico II University in Italy at the European Society of Hypertension conference held earlier this month. According to heartwire, Yusuf said there is “absolutely no rationale for the guidelines” and that the targets were simply “numbers pulled out of a hat … we are making guidelines to levels that nobody in the world consumes … How sensible is that?” Yusuf said “Extreme sodium reduction is not practical or desirable … let’s not make the whole world miserable … Let us enjoy a moderate amount of salt in our food and have fun.”

Strazzulo returned fire stating that lowering sodium intake is not a futile exercise and that the targets are not arbitrary. He cited evidence from the Northern Manhattan Study showing that individuals who consumed more than 4000 mg/day of sodium had more than twice the risk of stroke compared with those who consumed less than 1500 mg/day. He highlighted that the DASH study had showed a stepwise reduction in sodium resulted in stepwise reduction in blood pressure.

Strazzulo said sodium reduction was a crucial objective for disease prevention. Exposure to sodium is a lifelong process that starts in childhood and policy makers need to do something to create a global low-sodium environment. He noted that the United Nations is currently aiming for a 30% reduction in global sodium intake in the next decade.

“We have to do this for the young generations, for our children, so that they do not become dependent on high-salt intakes as their parents and grandparents have,” he said.

Conclusion?

We haven’t heard the end of this one.

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