As sweat pours from the pores of many Americans enduring summer heat waves, it might be time to do a little checkup on salt.
Most research over the past few decades indicates that too much salt in the diet overloads the kidneys and cranks up blood pressure, increasing the risk for heart disease and stroke.
Just this week, researchers working with federal health survey data reiterated the dangers of too much sodium on the circulatory system.
The new study published in the Archives of Internal Medicine, along with several other recent reports, also suggest that salt alone may not be the culprit.
The Archives study showed that the greatest risk to the heart comes not only from a diet high in sodium, but one that's also low in potassium. Specifically, those who had the highest salt intake and the lowest potassium intake were 50 percent more likely to die than those with the reverse numbers.
The findings come from an analysis of data from more than 12,000 adults who took part in a health and nutritional survey in 1998 and were tracked in the ensuing years for mortality. A total of 2,270 died, including more than 1,250 who died of cardiovascular disease (heart attack or stroke) or ischemic heart disease (poor blood supply to the heart muscle).
Researchers from the Centers for Disease Control and Prevention and several other institutions point out that nutrition data came from a one-time self-report when the surveys were first done, rather than from more objective measurements.
Still, they showed that men were eating an average of 4,223 milligrams of salt and 3,373 milligrams of potassium a day; women were consuming 2,916 milligrams of sodium and 2,444 milligrams of potassium. Dietary guidelines call for adults to consume no more than 1,500 milligrams of salt and no less than 4,700 milligrams of potassium daily.
Dr. Elena Kuklina, the CDC nutritional epidemiologist who led the study, noted that a diet heavy on fruits and vegetables will be naturally low in salt and high in potassium, while people eating more processed foods are more likely to have the opposite ratio. But she also cautions that other micronutrients besides salt and potassium may influence blood pressure and disease risk.
Still, salt is vital to life. This time of year, athletes, outdoor workers and the elderly and very young (who have less efficient kidneys) need to be careful to match salt and potassium intake with the amount of water they're drinking to avoid a condition known as water intoxication. It can cause nausea, muscle cramps, confusion and even seizures and death.
Sports drinks or a salty snack and fresh foods like bananas and celery, high in potassium, need to accompany heavy water intake.
Low sodium levels have been linked to depression or anxiety, several recent studies have shown. Some of the same brain pathways tied to a craving for salt have been tied to drug addiction, which may explain tendencies to take solace in a bag of chips or large order of fries.
Research also raises questions about how the body responds when denied salt.
A recent European study, published in May in the Journal of the American Medical Association, showed that people who were excreting the least amounts of salt in their urine (based on a one-time, 24-hour measurement) were more than 50 percent more likely to die from cardiovascular causes than were those with the highest salt excretion levels.
Dr. Jan Staessen, a hypertension researcher at the University of Leuven in Belgium and senior author of the study, said this is the latest in a number of studies his team has done that don't show much of a tie between salt excretion levels and blood pressure. He conceded there were relatively few heart deaths (84) in a sample of about 3,600 individuals whose salt habits may have changed in the eight years following the one-time measurement.
Still, he thinks it's possible that cutting back too sharply on salt "can activate some of the systems that conserve sodium and they are known to have a negative effect on cardiovascular outcomes." That casts doubt on whether it's a good idea to set one-amount-fits-all guidelines for salt intake by everyone, he argues.