Salt reduction heart-healthy but only for people with the saltiest diets?

By MIMS Multidisciplinary / Elvira Manzano

CUTTING down on salt is thought to be the holy grail of cardiovascular disease (CVD) prevention, but new evidence from the PURE* study suggests that salt reduction is only good for people with the saltiest diets.

“Sodium is an essential nutrient, lowering too much can actually increase mortality,” said lead author Associate Professor Andrew Mente from the McMaster University in Hamilton, Ontario, Canada.

“Our data showed sodium consumption was fine up to 5 g per day.”

In the study, every 1 g increase in sodium intake led to an additional 0.73 cardiovascular events per 1,000 years (p<0.0001). But the increase in CV risk was only found in communities where mean sodium intake exceeds 5 g daily, mainly in China. [Lancet 2018;392:496-506]

Around 96,000 adults (age, 35–70 years) in 300 communities across 18 countries who had no CVD at enrolment were assessed for blood pressure (BP) and 83,000 for CV outcomes over a median of 8.1 years in PURE. Urine output was measured to determine their sodium intake. Most communities (224 of 266) in other countries had sodium intake of 3–5 g per day, except in China where 82 of the 103 communities sampled exceeded 5 g per day.

The World Health Organization recommends a daily sodium intake of only <2 g per day to reduce BP and subsequently the risk of CV, on the premise that a lower BP translates to a better CV outcome. Yet no country in the world has so far achieved this target.

Deviation in linearity

Overall, mean systolic BP increased by 2.86 mm Hg per 1 g increase in mean sodium intake, but positive associations were only seen for communities in the highest tertile of sodium intake (p<0.0001 for heterogeneity). There was a non-linear association between mean sodium intake and major CV events (p=0.043). The lowest tertile (mean 4.04 g sodium/day) had significantly lower CV risk (-1.00 events per 1,000 years; p=0.0497). There was no association of risk in the middle tertile (mean 4.70 g/day) with sodium intake (+0.24 events per 1,000 years; p=0.8391). The highest tertile (mean 5.75 g/day) had a positive but nonsignificant association for greater risk (+0.37 events per 1,000 years; p=0.0712).

Of note, a strong association was seen between sodium intake and stroke in China (mean sodium intake 5.58 g/day, 0.42 events per 1,000 years; p=0.0020) compared with other countries (4.49 g/day, –0·26 events; p=0.0124), with p<0.0001 for heterogeneity. What was interesting was potassium intake decreased all major CV outcomes in all communities.

Less salt for some, more veggies for all

“Our findings suggest that a population-specific strategy for sodium reduction targeted at countries or communities with sodium intake >5 g/day would be preferable to a population-wide strategy of sodium reduction to reduce CVD and premature deaths. In contrast, there is a stronger case for increasing the consumption of foods that are rich in potassium [eg, fruits and vegetables] population wide,” the authors said.

In an accompanying editorial, Dr Franz Messerli, professor of medicine and cardiology, University Hospital Bern, Switzerland and colleagues took the PURE results with a grain of salt.

“The findings are exceedingly provocative and should be tested in randomized controlled trials,” they wrote. “But if the BP is normal, there seems little reason to restrict salt intake.” [Lancet 2018; 392: 456-458]

*PURE: Prospective Urban Rural Epidemiology

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