EXPERT REACTION: Moderate salt intake best for cardiovascular health

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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.

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[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