IN A significant challenge to current thinking, a major study led by an Irish doctor has questioned whether people with an average salt intake should reduce their intake further.
Published last night in the Journal of the American Medical Association (Jama), the study of almost 30,000 patients confirmed the link between a high sodium intake and an increased risk of cardiovascular disease such as heart attack and stroke.
However, Prof Martin O’Donnell of the HRB-clinical research facility at University Hospital Galway and NUIG, working with colleagues from McMaster University, Hamilton, Canada, also established a link between low salt intake and cardiovascular mortality as well as the risk of being hospitalised with heart failure.
Using 24-hour urinary excretion measurements of sodium as a measure of salt intake, the researchers found that compared with a baseline of 4-5.99 grammes a day, sodium excretion greater than 7gr a day was associated with an increased risk of all cardiovascular events; a sodium excretion of less than 3gr a day was associated with increased risk of death from cardiac disease and hospitalisation for congestive heart failure.
“Our study, together with other recent studies, raises uncertainty about whether those with moderate/average sodium intake should reduce their intake further,” Prof O’Donnell said.
“The only way to resolve this uncertainty is with a large randomised 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 the public health implications.”
An editorial accompanying the research notes: “Although there is general agreement that sodium reduction is appropriate for persons with hypertension, there are some questions about applying this recommendation to the remainder of the population.”
However a strongly worded editorial in the Lancetin July claimed the totality of evidence showed "substantial (population) benefits in reducing the average intake of salt".
Underlining how the research addresses an important population health issue – the association between salt intake and cardiovascular disease – Prof O'Donnell told The Irish Times: "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,” Prof O’Donnell added, “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.”
Most dietary salt is contained in processed food rather than by adding table salt to meals. Avoiding processed foods and not adding table salt is the best way to ensure a salt intake of between 3gr and 6gr a day.
Taking some advice with a pinch of salt
ANALYSIS:Research strengthens evidence for a certain daily minimum consumption of sodium, writes
Dr MUIRIS HOUSTON
CONTROVERSY OVER public health targets for salt intake has been brewing in medical circles for some time. Even the most ardent advocates have admitted research in the area has been inconsistent. And those who have hesitated in their endorsement of an absolute salt reduction policy have raised concerns about reports of metabolic changes such as insulin resistance in people who limit their sodium intake, and have expressed disquiet about a shortage of clinical trials that document the efficacy of sodium reduction as a means to reduce the risk of cardiovascular disease.
Today’s research is the strongest yet to support the thesis that salt has a physiological role in the human body. In other words, we need some salt for our cells to function optimally. By showing harm among people whose salt intake was less than 3 grams per day in the form of increased mortality and a greater chance of hospitalisation for heart failure, NUI Galway’s Prof Martin O’Donnell and his co-authors have strengthened the evidence for some daily minimum level of sodium intake.
They have also strengthened the evidence for the association between high sodium intake and increased risk of cardiac disease and stroke in patients with established cardiovascular disease, which highlights the importance of salt reduction in those consuming high-sodium diets (more than 5-6 grams per day) and the importance of efforts to reduce sodium content of manufactured foods high in salt.
However, for those of us who consume a moderate amount of sodium in our diet, there is now uncertainty about whether we need to reduce our salt intake.
The only way to resolve this uncertainty is to carry out large randomised control trials to determine whether, by reducing population salt intake from moderate to low levels, we can achieve lower rates of heart disease and stroke.
Some experts have argued both cost and complexity mean it would be difficult to carry out a traditional research project in which individuals are divided into groups which follow either a low sodium intake or a moderate salt intake. But these kinds of trials are the gold standard – they may well provide the basis for sensible, middle-ground advice that salt restriction is appropriate for a section of the population, but not for everyone.
In this focus on salt it must be emphasised the appropriate level of sodium is only one aspect of diet, and that other aspects such as a higher intake of fruit, vegetables and fish, and less fried foods, are critical for reducing the risk of heart disease and stroke.
The nub of the issue is that we have yet to complete definitive research into the specific effects of a low-sodium diet. Most of the available evidence is based on observational research, which is good at identifying factors associated with a particular condition but is not designed to identify a cause and effect.
Until we carry out the rigorous trials we would expect to see before any medical intervention receives regulatory approval, health advice to reduce salt intake may only be appropriate for people with established heart disease who consume in excess of 5 grams of salt per day.