EXPERT REACTION: Moderate salt intake best for cardiovascular health

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Both low and high intake of sodium (below 2g/day or above 5g/day) is associated with an increased risk of cardiovascular disease, with no increased risk identified for moderate sodium intake (2-5 g/day) according to a new observational study in 18 countries including over 90,000 people. Current WHO guidelines recommend reducing sodium intake to 2g/day in order to reduce the risk of cardiovascular disease and mortality. However, this has not been achieved in any country and the new study suggests that sodium reduction strategies should instead target countries of communities with very high levels of sodium intake (above 5 g/day), rather than a population-wide approach.

Journal/conference: The Lancet

Organisation/s: The Lancet

Media Release

From: The Lancet From: McMaster University, Canada

Peer reviewed / Observational Study / People

The Lancet: Sodium reduction programmes may only be appropriate for communities with very high salt intake

A new study shows that for the vast majority of communities, sodium consumption is not associated with an increase in health risks except for those whose average consumption exceeds 5g/day (equivalent to 12.5g of salt, or 2 ½ teaspoons). Communities with high average levels of sodium intake (above 5g/day) were mostly seen in China, with only about 15% of communities outside China exceeding this level of consumption.

WHO guidelines recommend a global approach to reducing sodium intake in all populations to below 2g/day, but this has not been achieved in any country. The authors say that sodium reduction strategies should instead target communities with high average levels of sodium consumption (above 5g/day).

The findings come from a new observational study of over 90000 people in more than 300 communities in 18 countries, published in The Lancet.

“No country has managed to reduce levels of sodium consumption from moderate to very low (below 2g/day), and our study shows we should be far more concerned about targeting communities and countries with high average sodium intake (above 5g/day, such as China) and bringing them down to the moderate range (3 to 5g/day),” says Professor Andrew Mente, Population Health Research Institute (PHRI) of Hamilton Health Sciences and McMaster University (Canada). [1]

Data from the ongoing Prospective Urban Rural Epidemiology (PURE) study was used in the analysis, and 95,767 participants aged 35-70 years in 369 communities in 18 countries [2] were included in the study. A morning fasting midstream urine sample was collected from every participant and was used to estimate 24h urinary sodium and potassium intake. Information about demographic factors, lifestyle, health history, and medication use were recorded and height, weight and blood pressure were measured.

Average follow-up was 8.1 years, during which time 3695 people died, 3543 had major cardiovascular events (1372 myocardial infarctions; 1965 strokes; 343 heart failures; 914 cardiovascular deaths). The analysis was based on the number of people who suffered a cardiovascular event or death (6281).

The analysis was done at a community level: 255 communities (all with over 100 participants) for cardiovascular disease and mortality, and 369 (all with over 50 participants) for blood pressure.

80% (82/103) of the communities in China has a mean sodium intake greater than 5g/day, whereas in other countries, 84% (224/266) communities had a mean intake of 3-5g/day. No communities in the study had a mean sodium intake below 3 g/day.

Higher sodium intake was associated with increased blood pressure and increased incidence of stroke, but the association was found in communities with very high sodium intake (mostly in China) and not others. Higher sodium intake was associated with lower rates of myocardial infarction and total mortality.

“Our study adds to growing evidence to suggest that, at moderate intake, sodium may have a beneficial role in cardiovascular health, but a potentially more harmful role when intake is very high or very low. This is the relationship we would expect for any essential nutrient and health. Our bodies need essential nutrients like sodium, but the question is how much. The recommendation to lower sodium consumption to 2g/day is based on short-term trials of sodium intake and blood pressure, and the assumption that any approach to reduce blood pressure will necessarily translate into a lower risk of cardiovascular disease with no unintended consequences. While low sodium intake does reduce blood pressure, at very low levels it may also have other effects, including adverse elevations of certain hormones associated with an increase in risk of death and cardiovascular diseases,” adds Professor Mente. [1]

Furthermore, rates of stroke, cardiovascular death, and total mortality decreased with increasing potassium intake in these communities. Diets rich in fruit and vegetables are high in potassium. However, it is not known whether potassium itself is protective, or whether it might simply be a marker of a healthy diet.

Professor Martin O’Donnell, McMaster University, co-author on the study, adds: “Our findings support other research recommending an all-round healthy diet with an emphasis fruit and vegetables, dairy foods, potatoes, nuts and beans. Very high sodium consumption (above 5g/day) is harmful, but the amount that is consumed by the majority of people does not appear to be linked to an increased risk of cardiovascular disease or death.” [1]

The study published today follows a paper published in The Lancet in 2016 [3], which used the same cohort but the analyses were performed at an individual level, rather than community. Compared with moderate sodium intake, the study found that high sodium intake (above 7g/day) was associated with an increased risk of cardiovascular events and mortality in hypertensive populations, and low sodium (below 3g/day) intake was associated with an increased risk of cardiovascular events and mortality in people with or without hypertension. By including the community level analyses, and additional years’ follow-up, the new study adds additional evidence and approaches to prevention for communities and countries.

Writing in a linked Comment, Franz H Messerli and Louis Hofstetter, University Hospital, Bern (Switzerland) and Sripal Bangalore, New York University School of Medicine (USA), note: “A cursory look at 24h urinary sodium excretion in 2010 and the 2012 UN healthy life expectancy at birth in 182 countries, ignoring potential confounders, such as gross domestic product, does not seem to indicate that salt intake, except possibly when very high, curtails life span… Before we change recommendations, let us remember, that Mente and colleagues’ findings are observational data in a predominately Asian population, and base 24 h sodium excretion calculations on overnight fasting urine measurements. It does not necessarily follow that active intervention, such as decreasing salt intake in patients at risk of stroke or increasing salt intake in patients at risk of myocardial infarction, will turn out to be beneficial. Nevertheless, the findings are exceedingly interesting and should be tested in a randomised controlled trial. Indeed, such a trial has been proposed in a closely controlled environment, the federal prison population in the USA… The simple fact that a trial looking at salt restriction has to be done in the federal prison population indicates that curtailing salt intake is notoriously difficult. Incentivising people to enrich their diets with potassium through eating more fruit and vegetables is likely to need less persuasion.”

NOTES TO EDITORS
The study was funded by the Population Health Research Institute, Canadian Institutes of Health Research, Heart and Stroke Foundation of Ontario, and European Research Council.

The labels have been added to this press release as part of a project run by the Academy of Medical Sciences seeking to improve the communication of evidence. For more information, please see: http://www.sciencemediacentre.org/wp-content/uploads/2018/01/AMS-press-release-labelling-system-GUIDANCE.pdf if you have any questions or feedback, please contact The Lancet press office pressoffice@lancet.com

[1] Quote direct from author and cannot be found in the text of the Article.
[2] The PURE study includes data from three high-income countries (Canada, Sweden, and United Arab Emirates), 11 middle-income (Argentina, Brazil, Chile, China, Colombia, Iran, Malaysia, occupied Palestinian territory, Poland, South Africa, and Turkey) and four low-income countries (Bangladesh, India, Pakistan, and Zimbabwe).
[3] https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(16)30467-6/fulltext

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Expert Reaction

These comments have been collated by the Science Media Centre to provide a variety of expert perspectives on this issue. Feel free to use these quotes in your stories. Views expressed are the personal opinions of the experts named. They do not represent the views of the SMC or any other organisation unless specifically stated.

Professor John Funder is a Senior Fellow at the Hudson Institute of Medical Research and also works with Obesity Australia

The paper in the Lancet by Mente et al needs to be viewed in three different ways. 

First, whether or not potassium intake is a marker of better diet (fruit and vegetables) it clearly is a positive in terms of cardiovascular outcomes, so the authors' suggestion of a potassium-based salt substitute on the table is a good one.

Second, the finding that with a sodium intake of 2-5 gm/day (salt intake of 5-13 gm/day) there is no increase in cardiovascular mortality comes to some extent as a surprise; on the other hand the authors' suggestion that attention focus on communities with an average sodium intake of greater than 5 gm, rather than exhorting those on a lower intake to limit sodium even further, makes good sense.

Third, the J-shaped curve (an intake below 2 gm/d not as safe as 2-5 gm/d) will certainly stir the possums/provoke controversy. 

What it should do is to provoke those in the field to think laterally, and seek mechanisms that might account for such a finding.

Two caveats. First, the estimates of daily sodium and potassium were done on a morning midstream urine sample. 

The authors cite several studies supporting this being a good index of daily intake levels, which is still relatively open to debate. 

The other is that sodium is only one player in cardiovascular mortality, as documented in the accompanying editorial commentary. 

Women in Hong Kong have an average sodium intake of 8 - 9 gm/d, and the highest life expectancy (87.3 years) in the world.

Last updated: 09 Aug 2018 3:15pm
Declared conflicts of interest:
None declared.

Dr Jacqui Webster is the Head of Public Health Advocacy and Policy Impact, and the Director of the World Health Organization Collaborating Centre on Population Salt Reduction

Global experts in salt and hypertension have been working together to develop quality criteria for salt reduction studies to ensure that the evidence is robust.

The PURE study published today would not meet these criteria for a number of reasons. Comprehensive systematic reviews of the evidence that use agreed criteria for study quality repeatedly conclude that the benefits of reducing salt below current levels are clear.

The World Health Organization’s (WHO) Global Action Plan for the Prevention and Control of Noncommunicable diseases recommends a 30 percent relative reduction in mean population salt intake by 2025.

WHO’s recommendation is less than 5 grams of salt or 2 grams of sodium per person per day. Many countries are already taking action but further concerted efforts are going to be required to achieve the global targets.

Last updated: 09 Aug 2018 1:08pm
Declared conflicts of interest:
None declared.
Professor Merlin Thomas is from the Department of Diabetes at Monash University

Outside of populations with excessive sodium intake (>5g/day), there is no solid basis for recommending low sodium intake for the long-term prevention of cardiovascular disease in adults or entire populations. 

We have previously demonstrated that reducing sodium intake can activate pathways that contribute to the development and progression of cardiovascular disease.

Moreover, in many prospective cohort studies and in some specific settings, such as in people with diabetes or kidney disease, a J-shaped relationship is observed such that a low sodium intake (as well as an excessive sodium intake) are associated with an increased risk of cardiovascular events and premature mortality.  

This new observational study is entirely consistent with this previous data.

Last updated: 09 Aug 2018 12:33pm
Declared conflicts of interest:
None declared.
Bruce Neal is a Senior Director at The George Institute for Global Health, Professor of Medicine at the University of Sydney and Chair of the Australian Division of World Action on Salt and Health.

The PURE study reported essentially the same findings, in the same journal using the same data just a couple of years ago. While the repeat publication of the data is surprising, the results are not. The methods used for the analysis are a little different but the weaknesses inherent in the original study remain. Only data from new, large-scale, randomised trials will provide further insight into the effects of salt reduction on health.  Further analyses of the same don’t really help.

There is nothing in this report to suggest any need to revise current recommendations to reduce salt intake. It remains the case that the totality of the available evidence provides a strong argument for significant harms from excess salt consumption all around the world. And there is a very strong likelihood that reducing salt intake will benefit the great majority of the world’s population. The likelihood that salt reduction will be harmful is small.

Last updated: 09 Aug 2018 12:28pm
Declared conflicts of interest:
None declared.

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