All About Sodium
Sodium plays a vital role in our health. Although no one knows for certain, scientists estimate the body requires 250 to 500 milligrams (mg) each day for basic physiologic functions. "We need salt to transport nutrients, transmit nerve impulses, and contract muscles, including your heart," says Anna Di Rienzo, Ph.D., associate professor of human genetics at the University of Chicago. But when sodium levels are too high, the kidneys release more water, increasing blood volume. With more blood flowing through the body, pressure increases. Over time, a sustained pressure increase causes the heart to work harder to pump blood and threatens the stability of blood vessels, which raises the risk of heart disease and stroke.
This physiological chain of events prompted the experts behind the 2005 revised Dietary Guidelines for Americans to reduce the recommendations for sodium intake from 2,400mg per day to an upper limit of 2,300mg for adults. "These recommendations are immensely important for everyone, but especially for those predisposed to develop hypertension [high blood pressure], especially African Americans, obese people, and those with a family history of hypertension, stroke, or heart disease," says Julius Linn, M.D., member of the Cooking Light advisory board and executive director of medical publications at the University of Alabama at Birmingham. Yet the recommended figure is nearly half what the average American consumes daily, according to the Centers for Disease Control and Prevention's National Health and Nutrition Examination Survey.
How Your Daily Sodium Intake Measures Up
Daily sodium needed for basic physiological functions:
scant 1/8 tsp = 250mg sodium
Daily sodium limit for seniors:
1/2 tsp salt = 1,200mg sodium
Daily sodium limit for adults:
scant 1 tsp salt = 2,300mg sodium
Daily sodium the average American consumes:
heaping 1 1/2 tsp = 4,000mg sodium
Salt vs. Sodium
Though we tend to swap "salt" and "sodium" as if the two words were interchangeable, there is a difference. Table salt is actually sodium chloride, explains Ilene Smith, R.D., M.S., associate director of Ketchum's Food and Nutrition Group. It's 40 percent sodium and 60 percent chloride.
Very little of the sodium we consume arrives in our diets via saltshakers. The majority-75 percent-comes from processed foods, where it enhances flavor, stabilizes, or preserves, Smith says. There are the usual high-sodium sources: bacon, ham, sausage and other cured meats; frozen or boxed entrées; frozen and canned vegetables; fast foods; and sauces and salad dressings. But sodium also hides in unexpected places. For example, cottage cheese can contain almost 1,000mg per cup. Read labels to find good choices.[pagebreak]
The easiest way to avoid consuming too much sodium is to choose fresh, whole foods that are as close to their natural state as possible, says Steve G. Aldana, Ph.D., who is professor of health and human performance at Brigham Young University in Provo, Utah. The reason: Although small amounts of sodium are naturally found in whole foods, they are infinitesimal compared to the amounts found in many processed foods. That doesn't mean you have to forgo convenience in the kitchen, however. Many canned vegetables are available in sodium-free or reduced-sodium versions.
Independent of sodium intake, fruit and vegetable consumption also has a positive effect on blood pressure. A diet that is rich in whole, unprocessed foods provides a healthy balance of fiber, vitamins, minerals, and antioxidants. "The closer we get to foods in their natural forms, the better," Aldana says.
Another mineral that plays a key role in a low-sodium diet is potassium. Found in substantial levels in root vegetables, leafy greens, and fruits, potassium helps balance the effects of sodium in the body. In fact, when the USDA lowered the dietary guidelines for sodium, it raised the recommendations for potassium-from 3,500 to 4,700mg per day, a figure most of us miss. (Average potassium intakes for women hover between 2,100 to 2,300mg, while men consume between 2,900 and 3,200mg.) In a review of 33 clinical trials, researchers from Tulane University Health Science Center in New Orleans found that increasing intake of potassium-rich foods may lower systolic blood pressure by an average of 3 points and diastolic by 2 points.
Some of us are physiologically sensitive to salt, and thus respond better and faster to a reduction in sodium intake. "Salt-sensitive people will experience a greater reduction in blood pressure than salt-resistant people," Linn says.
Researchers estimate that 10 to 25 percent of the population may be salt-sensitive. Among those with diagnosed hypertension, the number rises to 60 percent.
For now, there's no way to test for salt sensitivity; the best indication may be heredity. To learn more about a possible genetic connection to salt sensitivity, Di Rienzo tested more than 1,000 people from 52 ethnic groups and discovered a variation in a salt-regulating gene called cyp3a5. The gene was most common in natives of sub-Saharan Africa. As the distance from the equator increased, so did subjects' likelihood of a mutation in the gene that causes salt to be retained.
However, experts warn this discovery shouldn't provide license to load up on salt if you aren't sodium sensitive. "While certain traits may tip us off as to who faces the greatest risk from too much dietary sodium, it's difficult to predict who can get away with excess consumption," says Tedd Mitchell, M.D., medical director for the Cooper Wellness Program at the Aerobics Center in Dallas, Texas.
Other factors, such as age, also play a role. Systolic blood pressure rises an average of four points per decade of life naturally, so all adults should reduce the amount of sodium they consume as they age-regardless of sensitivity levels, Linn says. For people over 50, the 2005 Dietary Guidelines recommend an upper limit of 1,500mg sodium per day. For those over 70, the daily limit drops to 1,200mg. Simply controlling for the natural rise in blood pressure that occurs over a lifetime could have far-reaching effects.
Sources: USDA Dietary Guidelines for Americans 2005; Centers for Disease Control and Prevention's National Health and Nutrition Examination Survey
Peggy J. Noonan is a health and nutrition writer in Denver, Colorado.