Consuming less sodium has been shown to lower blood pressure, but for many people the health benefits this provides may be offset by a slight uptick in their cholesterol levels, a new analysis of previous research suggests.
The analysis, which was conducted by Danish researchers and published this week in the American Journal of Hypertension, found that sharply reducing sodium intake increases a person's total cholesterol by about 2.5 percent, on average -- potentially upping the risk of heart disease and heart attacks, the researchers say.
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But several experts on sodium and heart health who were not involved in the study say the findings should be taken with a grain of salt -- if not a heaping tablespoon.
Many of the studies included in the analysis were too small or too short to offer reliable data, even when pooled together, says Lawrence Appel, M.D., a professor of epidemiology at the Johns Hopkins School of Public Health, in Baltimore. The duration of the studies, most of which were less than 30 days long, is a "key issue," Appel says, because "when there is a large, abrupt reduction in sodium, it takes time [for the body] to acclimate."
Sodium-related increases in cholesterol would most likely diminish over time, Appel says. Moreover, he says, LDL (or bad) cholesterol -- the type most closely linked to heart attacks -- accounted for just a fraction of the overall increase seen in the analysis.
"We eat a lot of sodium -- way too much -- and I don't think it's going to hurt anybody to lower sodium in the current American diet," says Penny Kris-Etherton, Ph.D., a spokesperson for the American Heart Association and a distinguished professor of nutrition at Pennsylvania State University, in State College.
Federal health officials recommend limiting daily sodium intake to 2,300 milligrams, or 1,500 milligrams for certain populations that are especially sensitive to sodium, such as black people, people over 50, and those with high blood pressure (hypertension). But the average intake in the United States is well over 3,000 milligrams for both groups, according to a recent report from the Centers for Disease Control and Prevention.
The new analysis, led by Niels A. Graudal, M.D., a researcher at Copenhagen University Hospital, in Denmark, is an update of a similar study (known as a Cochrane Review) published by the same authors in 2003. Graudal and his colleagues, who report no financial ties to the salt or food industries, re-analyzed data from 167 studies that each compared a group of people who slashed their sodium intake with a group who maintained their normal diet.
Overall, the average sodium reduction in the studies was dramatic: 2,162 milligrams per day in people with hypertension and 3,358 milligrams per day in people with normal blood pressure.
Among people with hypertension, switching to a low-sodium diet reduced systolic blood pressure (the top number in a reading) by five to 10 points and diastolic blood pressure (the bottom number) by two to three points, depending on race. Asians experienced the biggest decreases, followed by black people and white people.
In people with normal blood pressure, the results followed a similar pattern but the reductions were generally smaller. In white people, for instance, systolic blood pressure declined by just one point and the changes in diastolic blood pressure were negligible.
The researchers also found, however, that low-sodium diets led to small increases in hormones that cause the kidneys to hold on to salt (such as renin), and increased total cholesterol by 2.5 percent and triglycerides by 7 percent, on average.
"We know that a decrease in blood pressure would probably improve or decrease the risk of cardiovascular death but, on the other hand, an increase in [cholesterol] would increase the risk of cardiovascular death," Graudal says. "It's likely that these two antagonistic effects will out-balance each other, so there will be no net effect of sodium reduction on people with normal blood pressure."
Based on the results, he says, people who eat "reasonable" amounts of sodium -- which Graudal estimates at 2,400 to 3,600 milligrams per day -- shouldn't worry about eating less, but should instead focus on quitting smoking, reducing alcohol consumption if they drink excessively, and losing weight if they are too heavy. "They should focus on everything else," he says.
Appel, however, questions the true heart risk posed by the small increases in cholesterol. If the authors' conclusions were accurate, one could expect to see spikes in heart-attack rates in people who start low-sodium diets, but so far that has not been borne out in studies, he says.
Keith C. Ferdinand, M.D., the chief scientific officer of the Association of Black Cardiologists, says a modest drop in blood pressure among people with normal blood pressure is to be expected, and shouldn't be interpreted to mean that cutting back on sodium isn't worthwhile.
In industrialized nations, Ferdinand says, the percentage of people with hypertension climbs steadily with age -- something that doesn't happen in parts of the world where people eat less salt. What the findings suggest, he says, is that reducing salt intake might help keep blood pressure within the normal range as people get older.
And the cholesterol increases Graudal and his team report would not be large enough to wipe out the "profound" benefits of reducing sodium intake in people with hypertension, Ferdinand says.
"This study does nothing to defer the recommendation that across the general population, sodium restriction would have a huge benefit in terms of decreasing cardiovascular disease, and perhaps lives saved," he says.