A large worldwide study has found that, contrary to popular thought, low-salt diets may not be beneficial and may actually increase the risk of cardiovascular disease (CVD) and death compared to average salt consumption.
In fact, the study suggests that the only people who need to worry about reducing sodium in their diet are those with hypertension (high blood pressure) and have high salt consumption.
The study, involving more than 130,000 people from 49 countries, was led by investigators of the Population Health Research Institute (PHRI) of McMaster University and Hamilton Health Sciences.
They looked specifically at whether the relationship between sodium (salt) intake and death, heart disease and stroke differs in people with high blood pressure compared to those with normal blood pressure.
The researchers showed that regardless of whether people have high blood pressure, low-sodium intake is associated with more heart attacks, strokes, and deaths compared to average intake.
"These are extremely important findings for those who are suffering from high blood pressure," said Andrew Mente, lead author of the study, a principal investigator of PHRI and an associate professor of clinical epidemiology and biostatistics at McMaster's Michael G. DeGroote School of Medicine.
"While our data highlights the importance of reducing high salt intake in people with hypertension, it does not support reducing salt intake to low levels.
"Our findings are important because they show that lowering sodium is best targeted at those with hypertension who also consume high sodium diets."
Current intake of sodium in Canada is typically between 3.5 and 4 grams per day and some guidelines have recommended that the entire population lower sodium intake to below 2.3 grams per day, a level that fewer than five per cent of Canadians and people around the world consume.
Previous studies have shown that low-sodium, compared to average sodium intake, is related to increased cardiovascular risk and mortality, even though low sodium intake is associated with lower blood pressure.
This new study shows that the risks associated with low-sodium intake -- less than three grams per day -- are consistent regardless of a patient's hypertension status.
Further, the findings show that while there is a limit below which sodium intake may be unsafe, the harm associated with high sodium consumption appears to be confined to only those with hypertension.
Only about 10 per cent of the population in the global study had both hypertension and high sodium consumption (greater than 6 grams per day).
Mente said that this suggests that the majority of individuals in Canada and most countries are consuming the right amount of salt.
He added that targeted salt reduction in those who are most susceptible because of hypertension and high salt consumption may be preferable to a population-wide approach to reducing sodium intake in most countries except those where the average sodium intake is very high, such as parts of central Asia or China.
He added that what is now generally recommended as a healthy daily ceiling for sodium consumption appears to be set too low, regardless of a person's blood pressure level.
"Low sodium intake reduces blood pressure modestly, compared to average intake, but low sodium intake also has other effects, including adverse elevations of certain hormones which may outweigh any benefits. The key question is not whether blood pressure is lower with very low salt intake, instead it is whether it improves health," Mente said
Dr. Martin O'Donnell, a co-author on the study and an associate clinical professor at McMaster University and National University of Ireland Galway, said: "This study adds to our understanding of the relationship between salt intake and health, and questions the appropriateness of current guidelines that recommend low sodium intake in the entire population."
"An approach that recommends salt in moderation, particularly focused on those with hypertension, appears more in-line with current evidence." The study was funded from more than 50 sources, including the PHRI, the Heart and Stroke Foundation of Canada and the Canadian Institutes of Health Research.