By Anne Harding
People with heart disease may increase their risk of heart attack, stroke, heart failure, and dying from heart-related causes even more if they consume a diet high in sodium, according to a new study that followed nearly 30,000 people for more than four years.
At the other end of the spectrum, heart patients who consumed very little sodium were also more likely than those with average intake to die of heart disease or be hospitalized for heart failure. But the study authors and other experts on sodium and health downplayed this finding, noting that the group was small and may have had health problems that led them to cut their sodium intake.
Several recent studies have suggested that below-average sodium intake may actually increase the risk of heart problems. The authors of the new study say their results, which appear this week in the Journal of the American Medical Association, validate the well-known risks associated with consuming too much sodium.
"The most important message is that our study confirms the robust association between high sodium or salt intake and the risk of cardiovascular disease," says Martin J. O'Donnell, Ph.D., a coauthor of the study and a stroke specialist at the National University of Ireland, in Galway.
O'Donnell and his team looked at 28,880 people participating in one of two clinical trials for heart-disease drugs. (The trials, as well as the new study, were funded by the drug manufacturer Boehringer-Ingelheim, but the company had no role in the design, data collection, analysis, or writing of the new study.)
The researchers used urine samples collected at the study's outset to estimate the participants' sodium intake. This measure, known as 24-hour urinary sodium excretion, provides a rough estimate of the amount of sodium a person consumed in the previous day.
The average sodium excretion for the group was 4,770 milligrams. (To put that in perspective, U.S. health officials recommend that adults limit their daily sodium intake to 2,300 milligrams, or 1,500 milligrams for African-Americans, people 50 and older, and people with high blood pressure, since those groups are generally more sensitive to the health effects of sodium.)
During the study period, 16 percent of the study participants had a heart attack or stroke, required hospitalization for heart failure, or died of heart disease.
The participants who excreted the most sodium -- more than 8,000 milligrams -- were roughly 50 percent more likely than the group with average excretion to experience one of these problems. Meanwhile, the people who excreted the least sodium -- less than 2,000 milligrams -- were 37 percent more likely than the average group to die of heart-related causes and 29 percent more likely to be hospitalized for heart failure.
"Our study would suggest that moderate sodium intake -- average sodium intake -- is associated with the lowest risk of cardiovascular events," O'Donnell says. "The key question now is, [if] you're somebody who consumes an average amount of sodium, should you reduce your sodium further? And we think this question is not answered."
Given the recent studies linking low-sodium diets to increased illness and mortality, a clinical trial to address this question is "essential," O'Donnell says. "You can appreciate how much confusion this area has caused to the general public."
The current study helps explain the apparent contradictions in recent sodium research, O'Donnell says. The relationship between sodium intake and heart risk seen in the study was "J-shaped": Risk is somewhat elevated in people with the lowest sodium intake, drops to its lowest point in people with average intake, and then climbs steadily in groups with higher intake. (Researchers have found similar patterns for cholesterol levels, body mass index, and alcohol consumption.)
The "hook" of the J -- in this case, the people with the lowest sodium intake -- should be interpreted very carefully, says Paul Whelton, M.D., a research professor at the Tulane University School of Public Health and Tropical Medicine, in New Orleans, who wrote an editorial accompanying the study. Fewer than 3 percent of the study participants fell into this low-intake group, and many of them may have changed their diet due to illness, Whelton says.
"The low sodium group ... is very problematic," says Lawrence Appel, M.D., a professor of medicine, epidemiology, and international health at Johns Hopkins University, in Baltimore. "It's impossible to get to that low level unless you are really, really trying -- and if you're really, really trying, you're probably sick," he says. "It's not the sodium, it's something else."
An important weakness of the study, Whelton says, is that the researchers used just one urine sample from the study participants. "The amount we take in varies from day to day, so it's hard to get a fix on sodium," he says.
And because the study included only people with preexisting heart disease, the findings may not necessarily apply to people at lower risk of heart problems. People with heart disease may be more "vulnerable to the extremes of sodium intake" than the general population, the authors note.