High-salt diet linked to 1.6 million heart deaths

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"Salty diet 'causes 1.6 million deaths worldwide each year'," reports The Daily Telegraph. It goes on to quote a researcher saying this is "nearly 1 in 10 of all deaths from cardiovascular causes worldwide".

This scary-sounding headline has a grain of truth in it, but the science it's based on doesn't prove that salt is causing these deaths. In fact, the news is based on a modelling study.

To estimate the effect of current sodium intake on cardiovascular mortality worldwide, researchers used available data on:

Globally, 1.65 million deaths from cardiovascular causes in 2010 were attributed to people consuming more than 2g of sodium per day. That's roughly 5g of salt a day. Currently, UK advice is for adults to eat no more than 6g of salt a day.

But this study could not prove that sodium restriction reduces cardiovascular mortality. This means the findings are generally in keeping with current salt recommendations that adults consume no more than 6g of salt a day.

The study was carried out by researchers from Tufts University, the Harvard School of Public Health, Brigham and Women's Hospital, Harvard Medical School, and the University of Washington in the US, and the Cambridge Institute of Public Health and Imperial College London in the UK.

It was funded by the Bill and Melinda Gates Foundation.

The study was published in the peer-reviewed New England Journal of Medicine. This article was open access, which means it's free to view online.

The media coverage is generally representative of this research, but it's worth bearing in mind that the study's results are estimates only. Also, the link between sodium and death has only been indirectly assessed by examining sodium's effect upon blood pressure, and then blood pressure's effect upon cardiovascular death.

This was a modelling study that aimed to estimate the effects of sodium intake on cardiovascular deaths around the world.

This modelling study can estimate how many deaths from cardiovascular disease can be attributed to a sodium intake over 2g.

However, it does not prove that sodium consumption of more than 2g a day caused any of these deaths, or that sodium restriction reduces cardiovascular mortality.

Elsewhere in this week's New England Journal of Medicine, a separate study analysed sodium in the urine of more than 100,000 people. It found the association between the intake of sodium and blood pressure was greatest in those with high blood pressure, the elderly, and those with high sodium diets (more than 5g of salt a day).

And a further salt study, again looking at more than 100,000 people, found a sodium intake of more than 6g a day (about 15g of salt) is associated with an increased risk of death and cardiovascular events. The risk of death and cardiovascular events was lowest in people who ate between 3g and 6g of sodium a day (about 7.5g to 15g of salt). Eating less than 3g of sodium a day was associated with an increased risk.

This was reported as "no need to cut back on salt" by the Mail Online as current UK guidelines state people should consume no more than 6g of salt a day. The Mail provided a good summary of the research, and mentioned some of its limitations, including some concerns over how sodium intake was measured.

The researchers modelled the effects of sodium intake on cardiovascular mortality around the world. They estimated the fraction and numbers of deaths estimated to be attributable to sodium intake above a reference level of 2g of sodium a day.

To do this, the researchers needed estimates of global sodium consumption, the effect of sodium intake on blood pressure, and the effect of blood pressure on cardiovascular deaths.

Previously conducted national or subnational surveys on individual level sodium consumption were systematically traced by the researchers. These surveys were based on measurements of sodium in urine, or estimates of sodium intake in the diet, or both. The researchers quantified consumption according to age, sex and country.

The researchers carried out a meta-analysis of all randomised controlled trials identified in two prior systematic reviews that had evaluated the effect of reduced sodium on blood pressure. They looked at the effects according to age, race and the presence or absence of hypertension.

The effect of blood pressure levels on deaths as a result of cardiovascular disease was assessed by combining results from two large international projects (including 99 cohorts, comprising a total of 1.38 million participants, among whom there were 65,000 cardiovascular events) that pooled individual level data. The researchers looked at the effects according to age.

The number of people who die from cardiovascular disease was estimated from the Global Burden of Disease Study 2010.

The researchers estimated the average level of consumption of sodium worldwide was 3.95g a day and regional averages ranged from 2.18g to 5.51g a day. From their meta-analysis of randomised controlled trials, they found reducing sodium intake reduced blood pressure.

Each reduction of 2.30g of sodium a day was associated with a reduction of 3.82mmHg in blood pressure, although the effects depended on population characteristics such as age and race.

They also found lower blood pressure was associated with a reduced risk of cardiovascular death.

The researchers calculated nearly 1 of every 10 deaths from cardiovascular causes (1.65 million deaths a year, 9.5% of all cardiovascular deaths) is attributed to a sodium intake of more than 2g a day.

Four of every five deaths (84.3%) occurred in low- and middle-income countries, and two of every five deaths (40.4%) were premature (before 70 years of age).

The rate of death from cardiovascular causes associated with sodium intake above the reference level was highest in Georgia and lowest in Kenya.

The researchers concluded that: "In this modelling study, 1.65 million deaths from cardiovascular causes that occurred in 2010 were attributed to sodium consumption above a reference level of 2.0g per day."