Potatoes are a good source of potassium, as are bananas, grapes and carrots. (Eric Boyd, Los Angeles Times )
For decades now, we've heard that too much sodium can cause hypertension and raise the risk of cardiovascular disease.
People have paid far less attention to potassium, a mineral that has opposite effects on health: Get enough of it, and it can actually lower your blood pressure and protect your heart.
Now a study of more than 12,000 adults has underscored something that doctors and nutritionists have been saying for years: If you watch your sodium but ignore potassium, you're missing an important part of the picture.
The study, published in the July 11 issue of the journal Archives of Internal Medicine, found that people whose diets had the lowest ratio of sodium to potassium (translating to roughly equal amounts of the two nutrients) were at the lowest risk of dying from heart attack and stroke. Those who consumed the highest amounts of sodium relative to potassium — 50% more, on average — had a 46% higher risk of dying from cardiovascular-related illness.
However, the study did not prove a cause-and-effect relationship, said coauthor Dr. Elena Kuklina, a nutritional epidemiologist at the U.S. Centers for Disease Control and Prevention in Atlanta. "We found some relationship between diet and mortality, but since it was not a clinical trial, we can't say for sure that diet is a cause of mortality." To show cause and effect, scientists would have to put people on set diets, randomly assigned, for a long period of time and follow them until they died — an inordinately difficult undertaking.
Though doctors know that potassium plays a significant role in heart health, many are reluctant to take any attention away from sodium, said Dr. Gordon Tomaselli, president of the American Heart Assn. and chief of cardiology at Johns Hopkins University in Baltimore. "Sodium is important," he reiterated. People can improve their cardiovascular health simply by eating less of it, he said, and any benefit from high potassium foods would be a bonus.
The new study followed 12,267 adults for an average of 14.8 years. Researchers used dietary surveys to estimate the potassium and sodium intakes at the start of the study. As expected, people who consumed the most sodium were also the most likely to die during the study — a 73% increase over those who consumed the least sodium — while people who consumed the most potassium had relatively low death rates — a 39% lower risk than those who consumed the least.
But the balance between sodium and potassium mattered, too. Those participants who got high sodium and low potassium had the highest death rates of all: a 46% higher risk of dying from any cause than those who ate equal proportions of the nutrients. They were especially vulnerable to death from heart attack, for which the risks doubled.
Tomaselli said the study was noteworthy because of the large number of participants representing a cross-section of Americans and because they were followed for long enough to include a significant number of deaths. However, dietary intake was not directly measured but was estimated based on each subject's memory. And even assuming that the estimates are accurate, Tomaselli noted that diets heavy in sodium and light in potassium may be unhealthful in ways that have little to do directly with the two minerals.
A previous study, published in the Archives of Internal Medicine in 2009, also found an association between cardiovascular disease and the balance between sodium and potassium. Rather than estimating dietary intake, researchers measured actual levels of sodium and potassium in the urine of 2,275 subjects with prehypertension (diastolic blood pressure between 80 and 89) and followed them for 10 to 15 years.
Again, higher sodium seemed to increase the risk of heart attack and stroke, and potassium seemed to have the opposite effect. But the only association that passed muster statistically was the balance between sodium and potassium. "The size of the effect was very similar to the CDC study," noted study coauthor Nancy Cook, a researcher in preventative medicine at Brigham and Women's Hospital in Boston.
Focusing on the ratio between sodium and potassium makes biological sense because the minerals are known to have opposite effects on blood pressure, Kuklina said. Sodium generally increases blood pressure and signals the body to retain fluids. Potassium, however, relaxes blood vessels, lowers blood pressure and helps rid the body of excess fluids.
The U.S. dietary guidelines recommend limiting sodium intake to 2,300 milligrams per day and even lower — 1,500 mg — for those 51 and older and people of any age who are African American or have high blood pressure, kidney disease or diabetes. (The American Heart Assn. recently switched to a target of 1,500 mg per day for everyone.)
The average daily intake of sodium by Americans is much higher than that — more than 3,400 mg per day, according to CDC estimates.
Recommended potassium intake is 4,700 mg per day, but average U.S. intake is in the range of only 2,000-2,500 mg per day, Cook said.
For those whose eyes already glaze over when told to read nutrition labels, there's a simpler way to reduce sodium and increase potassium in your diet: Choose fresh, whole foods over packaged, processed ones.
More than 75% of American sodium intake comes in the form of processed foods, Kuklina said. And the best potassium sources are fruits and vegetables such as potatoes, bananas, grapes, carrots, greens and citrus fruits. Simply by eating fewer processed foods you can decrease your sodium intake and increase your potassium intake in one fell swoop.
"The message is to eat a healthy balanced diet," Kuklina said. "It's good for health in general and for cardiovascular health."