The research - published in JAMA - confirmed previous research findings reporting salt intake at high levels (over 8000 mg of sodium per day) is associated with an increased risk of heart attack and stroke, or hospitalisation or death from heart disease.
However, the research team also found that low sodium intake - of between 2,000 and 3,000 mg per day - was also linked to a 20% increase in the risk of cardiovascular-related death, in addition to hospitalisation for congestive heart failure.
“Compared with moderate sodium excretion, we found an association between high sodium excretion and cardiovascular events and low sodium excretion and cardiovascular death and hospitalization for congestive heart failure, which emphasizes the urgent need to establish a safe range for sodium intake,” said the researchers, led by Professor Martin O'Donnell, of McMaster University, Canada.
“Higher urinary potassium excretion was associated with lower stroke risk and is a potential intervention that merits further evaluation for stroke prevention,” they added.
O'Donnell and his colleagues examined the association between sodium and potassium excretion – markers of intake – and cardiovascular events and mortality using data from more than 28,000 who took part in the ONTARGET and TRANSCEND trials.
They estimated 24-hour urinary sodium and potassium excretion from a morning fasting urine sample. Analysis models were then used to determine any association between urinary sodium and potassium with cardiovascular events including myocardial infarction (heart attack), stroke, and hospitalization for congestive heart failure (CHF) and mortality.
The researchers found that higher and lower baseline sodium excretion, were both associated with an increased risk of the composite of cardiovascular death, heart attack, stroke, and hospitalization for CHF compared to the average intake.
Compared to an average intake of between 4,000 and 6,000 mg of sodium in a day, those who consumed more than 8,000 mg were between 50 and 70% more likely to suffer a from the cardiovascular events measured.
"What we're showing is that the association between sodium intake and cardiovascular diseases appears to be J-shaped," said O'Donnell.
He explained that a J-shaped line, which shows heightened risks at very low and at high salt levels, but a low risk in the middle, could offer an explanation as to why studies in different groups of people have reported conflicting conclusions on the effects of eating more or less salt.
"Our findings emphasise the burden of cardiovascular disease associated with excess sodium intake and the importance of population-based programs to reduce sodium intake in populations consuming high-sodium diets," added the researchers.
They noted, however, that risk was not elevated until sodium levels exceeded 6,500 mg per day – far higher than the upper limits established by the World Health Organisation (2,000 mg/day) and the American Heart Association (1,500 mg/day).
“Our study, together with other recent studies, [also] raises uncertainty about whether those with moderate or average sodium intake should reduce their intake further," O'Donnell said.
Volume 306, Issue 20, Pages 2229-2238, doi:10.1001/jama.2011.1729
“Urinary Sodium and Potassium Excretion and Risk of Cardiovascular Events”
Authors: M.J. O'Donnell, S. Yusuf, A. Mente, P. Gao, J.F. Mann, K. Teo, et al