A sensitivity to salt increases the risk of death as much as high blood pressure, according to a study supported by the National Heart, Lung, and Blood Institute (NHLBI), part of the National Institutes of Health.
The study is the first to show that salt sensitivity increases the risk of death even for those with normal blood pressure. Prior studies had found that salt sensitivity increases the risk of death, as well as heart attack or other cardiovascular events, among those who already have high blood pressure, also called hypertension.
The new study’s findings were true for African Americans and whites, as well as men and women.
The study, "Salt Sensitivity, Pulse Pressure, and Death in Normal and Hypertensive Humans," appears in the Council Supplement issue of Hypertension: Journal of the American Heart Association, published on February 16, 2001. The study was conducted by researchers at the Indiana University School of Medicine in Indianapolis.
"This study provides yet more evidence that Americans should be careful about their daily salt intake," said NHLBI Director Dr. Claude Lenfant. "People who have normal blood pressure but are salt sensitive should take action to protect their health.
"Unfortunately," he continued, "there is no easy way to test for salt sensitivity. So, it’s advisable for all Americans with normal blood pressure to follow the Federal recommendation of having no more than 2,400 milligrams of sodium a day."
"Salt sensitivity increases the risk of death, whether or not a person has high blood pressure," stated Dr. Myron Weinberger, Director of the Hypertension Research Center at the Indiana University School of Medicine in Indianapolis and the study’s principal investigator. "We also found in a prior study that those who were salt sensitive were more likely to develop high blood pressure as they aged. The United States is a ‘salt-abundant’ environment, which makes this problem more likely. Those who are salt sensitive need to take steps to reduce their salt intake and, thus, lower their risk of death and cardiovascular disease."
Weinberger noted that some Americans are more likely than others to be salt sensitive. These include older persons, African Americans, and those with a family member who is salt sensitive or who have a parent, sibling, or child with hypertension. Based on the researchers’ earlier studies, he estimates that about 26 percent of Americans with normal blood pressure and about 58 percent of those with hypertension are salt sensitive.
Salt sensitivity is a measure of how blood pressure responds to a decrease in salt intake. Besides hypertension, salt sensitivity increases the risk of developing such conditions as left ventricular hypertrophy, in which the heart’s main pumping chamber is enlarged and does not function properly, and the likelihood of kidney problems.
In the new research, investigators followed up on persons who had participated in a hypertension study done more than 25 years ago. The original group included 708 persons aged 18 to 80. Investigators located 596 of the group and re-examined those still living. For those who had died, the investigators identified the cause.
At the time of the initial study, about 40 percent of the group had hypertension, about 45 percent were salt sensitive, about 44 percent were women, and about 25 percent were African American. About 26 percent of the original group had normal blood pressure and a sensitivity to salt.
Researchers determined salt sensitivity by giving participants a saline solution followed by a diuretic and testing blood pressure and volume over 2 days as salt in the body was increased and then decreased.
They found that, after about 25 years, 123 (about 21 percent) of the original group had died from a cardiovascular disease or other cause. Participants who had normal blood pressure but were salt sensitive at the outset fared no better than those who were hypertensive at the outset. Only those with normal blood pressure who were not salt sensitive at the outset had a significantly better survival rate.
The findings were true for various measures of blood pressure-systolic blood pressure (the top number in a blood pressure reading), diastolic blood pressure (the bottom number), and pulse pressure (the difference between systolic and diastolic pressures.)
Additionally, the study found that such long-recognized risk factors as body mass index and all measures of blood pressure-systolic, diastolic, and pulse pressures-also increased the likelihood of death.
"When we began the study more than 25 years ago," said Weinberger, "we wanted to learn more about the mechanisms involved in how the body controls blood pressure. Now, we’ve found that some of these mechanisms can be life-threatening.
"But Americans can take advantage of this finding. They don’t need to drastically cut back on salt intake to reduce their risk of death or developing hypertension as they age. They’ll reap a considerable benefit if they reduce their salt intake to 2,400 milligrams a day. The benefit will be even greater if they reduce their salt intake to 1,500 milligrams a day, as was shown in the DASH-Sodium study. In that study, the lower the sodium intake, the lower the blood pressure level.
"Only 10 percent of dietary sodium comes from salt added to food at the table," he continued. "So, to reduce their salt intake, Americans should be careful about the sodium content in prepared, preserved, and processed foods. People should check food labels and watch out for high sodium items, including such processed foods as luncheon meats, prepared cheeses, canned vegetables, and baked products."