A lack of salt in the diet can increase the risk of a major cardiovascular event and death, just as consuming too much salt does, research has shown.

Previous studies have suggested a “J shaped” association between sodium intake and cardiovascular disease or death, but many of these studies included participants at high cardiovascular risk who were not representative of the general population. But in the Prospective Urban Rural Epidemiology (PURE) study, published in the New England Journal of Medicine last month, researchers examined how urinary sodium and potassium excretion was associated with death and major cardiovascular events in a population group who largely had no history of cardiovascular disease.

The researchers obtained morning fasting urine samples from 101 945 people in 17 high and middle income countries, 42% of whom lived in China. Estimated 24 hour sodium and potassium excretion was calculated for each person, and this was used as a surrogate for sodium and potassium intake. The mean estimated 24 hour sodium excretion was 4.93 g, and the mean estimated 24 hour potassium excretion was 2.12 g.

Patients were followed up for an average of 3.7 years. Over the study period 3317 people (3.3%) died or had a major cardiovascular event: of these, 1976 participants died, 650 from cardiovascular causes; 857 had myocardial infarction; 872 had stroke; and 261 had heart failure (some participants experienced more than one cardiovascular event). The researchers set reference ranges based on the estimated excretion of sodium and potassium: for sodium the reference range was set at 3.00 g to 5.99 g a day, and for potassium it was set at less than 1.50 g a day.

Higher than estimated sodium excretion (≥7 g a day) was associated with an increased risk of death or major cardiovascular event (odds ratio 1.15 (95% confidence interval 1.02 to 1.30)) compared with the reference range, and this association was strongest in patients with hypertension. But patients with estimated sodium excretion of less than 3 g a day were also found to have a higher risk of death or major cardiovascular event (1.27 (1.12 to 1.44)) than those in the reference range.

Potassium excretion higher than the reference level of less than 1.5 g a day was associated with a lower risk of death or major cardiovascular event.

Current guidelines for preventing cardiovascular morbidity recommend a maximum sodium intake of 1.5 g to 2.4 g a day, and they are based on evidence from largely short term clinical trials showing that reducing sodium intake from a moderate to a low level resulted in modest reductions in blood pressure.

The authors of the PURE study said, “The projected benefits of low sodium intake with respect to cardiovascular disease are derived from models of data from these blood pressure trials that assume a linear relationship between sodium intake and blood pressure and between blood pressure and cardiovascular events.

“Implicit in these guidelines is the assumption that there is no unsafe lower limit of sodium intake. However, sodium is known to play a critical role in normal human physiology, and activation of the renin-angiotensin-aldosterone system occurs when sodium intake falls below approximately 3.0 g per day.”

They added, “An increased potassium intake may reduce the risk of death and cardiovascular disease through its effects on blood pressure, or it may simply be a marker of healthy dietary patterns that are rich in potassium (eg, high consumption of fruit and vegetables).”

After adjustment for fruit and vegetable intake and blood pressure, the association between potassium excretion and reduced risk of death and major cardiovascular event was attenuated, the authors noted.

Reference

BMJ 2014;349:g5507