Doctors and health experts have warned us for years that too much salt intake is bad for our health, but a new study from researchers at McMaster University in Hamilton, Ontario, Canada suggests that too little salt intake may be just as bad for our heart health as higher doses.
The new study suggests that in people with heart disease, eating too little salt is linked to a higher risk of heart-related hospitalizations and deaths nearly as much as too much salt intake. It also suggests that clarification on what amounts are appropriate is sorely needed, especially for those with existing heart disease.
However, the study´s lead researcher said he believes the findings of his study, published in the Journal of the American Medical Association (JAMA), paints a clearer picture of how much is too much, as well as how little is too little.
“What we´re showing is that the association between sodium intake and cardiovascular diseases appears to be J-shaped,” said Martin O´Donnell, from McMaster University.
The J-shaped line depicting heightened risk at very low salt levels and at high levels, with low risk in the middle, could explain why studies in different groups of people have come to different conclusions on the effects of eating more or less salt, he added.
In theory, based on the findings, eating less salt could be linked to a reduced risk of heart problems in a group that generally eats a lot of salty foods, but also may be associated with an increase in those risks if salt consumption is already on the low end.
“We would hope earnestly that our study would not add to the controversy, but would provide some clarity to what appeared to be conflicting findings of previous studies,” O’Donnell told Reuters Health.
O´Donnell´s study looked at sodium excretion in the urine (measured for sodium intake in one´s diet) of 28,000 adults who were at high risk for heart disease or diabetes or who had heart disease. Both sodium and potassium excretion were measured for 24 hours, which added to a more clear picture of the impact of dietary salt on health.
The study, which was co-lead by Dr. Salim Yusuf, a professor of medicine and executive director of the Population Research Health Institute (PHRI) at McMaster, found that moderate salt intake (4.0 to 5.9 grams per day) was associated with the lowest risk of cardiovascular events, while higher intake (more than 7.0 grams per day) was associated with an increased risk of stroke, heart attack and other cardiovascular diseases, and a low intake (less than 3.0 grams per day) was linked to an increased risk of cardiovascular death and hospitalization for congestive heart failure.
The investigators cautioned, however, that the new findings do not prove that people can change their risks by changing the amount of salt in their diets, since salt is only one component linked to heart health and this type of study was not designed to extract cause-and-effect.
Still, the research does address “an important population health issue — the association between salt intake and cardiovascular disease,” said O´Donnell, who is also appointed at the Health Research Board Clinical Research Facility, National University of Ireland.
“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,” he added.
Dietary guidelines recommend a sodium intake of no more than 1,500 milligrams (1.5 grams) per day for most adults. But, on average, most adult consume 2,300 milligrams per day, according to past studies.
In the new study, participants were followed for an average of four to five years for any cardiovascular-related hospitalizations and deaths, including strokes and heart attacks. During the course of the study, the researchers recorded about 4,700 such events, including just over 2,000 deaths due to cardiovascular disease.
Compared to those who ate between 4.0 and 6.0 grams of sodium per day, those who got more than 8.0 grams were 50 to 70 percent more likely to suffer a heart attack or stroke or be hospitalized or die from heart disease. That was after the team took into account what medications the participants were on, and other aspects of health and lifestyle known to affect heart health, such as weight, smoking, and amount of fruits and vegetables consumed, as well as cholesterol levels.
Those who had low sodium intake, between 2 and 3 grams per day, also showed a 20 percent higher risk of cardiovascular-related death as well as hospitalization for congestive heart failure.
In people with heart risks, “very high sodium intake causes harm. What is emerging now is that going too low might also cause harm,” Dr. Jan Staessen, who studies salt and cardiovascular disease at the University of Leuven in Belgium, told Reuters.
“There´s a lot of evidence accumulating that going as low as (1,500 mg) of sodium might cause harm,” said Staessen, who was not involved in the new research.
His own research has suggested that in the general population, lower levels of sodium are linked to higher risk of death. And another recent review of past studies didn´t find an overall health benefit associated with cutting back on salt.
Staessen, in an interview with Reuters Health, emphasized that the people in the McMaster study were very high-risk, and the findings don´t necessarily apply to everyone.
O´Donnell agreed that the optimum amount of salt in the diet may be different in people without heart disease or diabetes, but he suspects that the J-shaped relationship between salt intake and heart risks will generally hold up.
“Our study confirms the association between high-sodium intake and cardiovascular disease. Our findings highlight the importance of reducing salt intake in those consuming high-salt diets and the need for reducing sodium content in manufactured foods that are high in salt,” said Yusuf, who is also vice-president of research at Hamilton Health Sciences.
“However, for those with moderate intake, whether further reduction of salt in the diet will be beneficial is an open question. We believe that large clinical trials are the most reliable way to determine if reducing sodium intake to lower levels is of benefit,” he concluded.
O´Donnell and Yusuf agree that more research is needed to refine the guidelines for sodium and potassium intake. But, they wrote in an accompanying editorial, most salt intake comes from processed foods, and those are the foods that should be avoided.
“This shift to a more natural diet would concurrently lead to an absolute increase in dietary potassium content and also lead to an improved sodium-potassium ratio, which may be more desirable than a change of either electrolyte [salt or potassium] on its own,” they wrote.
The research was conducted at the Michael G. DeGroote School of Medicine at McMaster University and the Population Health Research Institute (PHRI) at McMaster and Hamilton Health Sciences.
On the Net:
- McMaster University
- Journal of the American Medical Association
- Population Research Health Institute
- Hamilton Health Sciences