Study calls sodium intake guidelines into question

The study, published in the Journal of the American Medical Association (JAMA), found that moderate salt intake is associated with the lowest risk of cardiovascular events, whereas higher intake of sodium was associated with an increased risk of stroke, heart attack and other cardiovascular events while low intake was associated with an increased risk of cardiovascular death and hospitalisation for congestive heart failure.

The research was co-led by Professor Martin O'Donnell, Professor of Translational Medicine, NUI Galway and Dr Salim Yusuf, Population Health Research Institute (PHRI) at McMaster University in Canada and Hamilton Health Sciences. Professor O'Donnell is also Associate Director of the HRB Clinical Research Facility at NUI Galway and University Hospital Galway.

"This research addresses an important population health issue -- the association between sodium (salt) intake and cardiovascular disease," said NUI Galway's Professor O'Donnell. "This area has become topical again, with the recent publication of another paper in JAMA reporting an association between low-sodium intake and cardiac death. In general, previous observational studies have either reported a positive association, no association or an inverse association between sodium intake and heart disease and stroke. This has resulted in a lot of controversy. Our study is the first to report a J-shaped association between sodium intake and cardiovascular disease, which may explain why previous studies have found different results."

Compared with moderate sodium excretion (between 4 to 5.99 grams per day), the researchers found that sodium excretion of greater than 6-7 grams per day was associated with an increased risk of all cardiovascular events, and sodium excretion of less than 3 grams per day was associated with an increased risk of cardiovascular death and hospitalisation for congestive heart failure.

The findings call into question current guidelines for salt intake, which recommend less than 2.3 grams (or 2,300 mg) per day. The guidelines are mostly based on previous clinical trials that found blood pressure is lowered modestly when sodium intake is reduced to these levels (which was also found in the present study), but there are no large studies looking at whether such low levels of sodium intake reduce the incidence of heart attacks and stroke. Clarifying the optimal daily intake of sodium is particularly important in patients with established heart disease, as they may be especially vulnerable to the cardiovascular effects of very high- and low-salt intake and are most likely to receive recommendations on restricting sodium in their diets, the authors concluded.

"Our research confirms the association between high sodium intake and increased risk of cardiovascular disease, which emphasizes the importance of salt reduction in those consuming high-sodium diets (over 6-7g per day) and the importance of efforts to reduce sodium content of many high-salt manufactured foods. However, our study, together with other recent studies, raises uncertainty about whether those with moderate/average sodium intake should reduce their intake further. The only way to resolve this uncertainty is with a large randomized controlled trial that determines whether reducing moderate sodium intake to lower levels results in lower rates of heart disease and stroke. While we accept there are challenges to conducting such trials, they are required urgently given their public health implications' said Professor O'Donnell.

For the observational study, the researchers examined 28,880 people at increased risk of heart disease from the ONTARGET and TRANSCEND trials, which were conducted from 2001-2008. The researchers estimated 24-hour urinary sodium and potassium excretion from a morning fasting urine sample. During follow-up, over 4,500 cardiovascular events occurred making this the most powerful study examining the relationship between sodium excretion (which is a surrogate measure of sodium consumption), as well as potassium excretion and cardiovascular events. Extensive and careful statistical analytic methods were used to determine the association of urinary sodium and potassium with cardiovascular events -- heart attack, stroke, hospitalisation for congestive heart failure and death.

In addition to the sodium findings, the researchers found higher urinary potassium excretion was associated with lower stroke risk. They concluded this is a potential intervention that merits further evaluation for stroke prevention.