Low levels of salt consumption are associated with a higher rate of cardiovascular disease and deaths, European researchers reported Tuesday, but U.S. experts promptly criticized the study, which contradicts the prevailing dietary wisdom. "This study might need to be taken with a grain of salt," Dr. Peter Briss of the Centers for Disease Control and Prevention told the New York Times. Dr. Ralph Sacco of the University of Miami, president of the American Heart Assn., also criticized the study's design and conclusions, noting that the association would continue to stand by its guideline that Americans should consume no more than 1,500 milligrams of salt per day, well below the current average of about 3,500 mg per day. He argued that a vast amount of literature supports the current recommendation and that one study is not sufficient to make any changes in the guidelines.
The level of salt in the diet has been a highly controversial topic for at least two decades. Opponents of salt argue, and several studies have shown, that higher levels of salt increase blood pressure and are associated with higher rates of cardiovascular disease and deaths from heart attacks. Proponents, however, argue that only a small proportion of the population, at best, is susceptible to the deleterious effects of salt and that the rest of the populace should not have to give up the flavor-enhancing effects of the food additive. In that group, some have even suggested, low salt intake might even be deleterious.
Despite this controversy, there have been growing efforts to decrease the amount of salt in food. The biggest sources of salt are processed foods purchased in grocery stores and prepared food served in restaurants and, particularly, fast-food establishments. In those places, the additive is favored for enhancing the taste of the food. Only about a quarter of salt consumption occurs in the home, for most people.
In the new study, reported in the Journal of the American Medical Assn., a European team headed by Dr. Jan Staessen of the University of Leuven in Belgium studied 2,856 people who did not have hypertension or cardiovascular disease at the beginning of the study. The subjects collected all their urine for exactly 24 hours at the beginning and end of the study, and researchers measured the amount of salt they excreted, a common measure of salt consumption. The group was then divided into thirds based on salt excretion and monitored for as long as 7.9 years.
The researchers observed 50 deaths in the third of the subjects with the lowest salt consumption, 24 in the third with mid-range consumption and 10 in the third with the highest consumption. The risk of cardiovascular disease was 56% higher in the third of the group with the lowest salt intake. No association between salt intake and hypertension was observed. Increased intake of sodium was associated with an increase in systolic blood pressure, but all the subjects had a low enough blood pressure at the beginning of the study that the increase did not boost them into a hypertensive category.
The results do "not support the current recommendations of a generalized and indiscriminate reduction of salt intake at the population level," the authors wrote.
But experts noted that the participants in the study were all white and young and that they were not monitored for long enough. Moreover, there were very few cardiovascular events observed, making it difficult to draw conclusions. Hypertension tends to be a disease of the elderly, the experts noted, and it takes a long time for its effects to produce cardiovascular disease. Blacks and other ethnic groups are also much more susceptible to hypertension and the effects of salt, so the results may not be generalizable to the population at large. Also, subjects who had the lowest sodium excretion produced the least amount of urine during the 24-hour period, suggesting that they may not have collected all of their urine, which would mean that the measured salt excretion was incorrect.
"One message that you might take out of this is that, for people who are young and healthy and have normal blood pressure, there is not a compelling reason for them to restrict sodium," said Dr. Jerome Fleg, a medical officer and cardiologist at the National Heart, Lung and Blood Institute. "Perhaps we should target the groups that would benefit the most: those who are at high risk for cardiovascular disease, those who are sodium sensitive, blacks, older people and those who are already hypertensive.... Targeting those groups would give the biggest bang for the buck."
But Michael F. Jacobson, executive director of the Center for Science in the Public Interest, argued that we should begin intervening while people are still healthy: "My view is that one flawed, but new, paper shouldn't distract people from the enormous evidence from epidemiology, animal, clinical and intervention studies that, as the Institute of Medicine affirmed last year, reducing sodium levels in packaged and restaurant food would save thousands of lives a year and should be required by the Food and Drug Administration."