PUBLISHED 10/12/2005
Even Freud would concede that a runner who finishes a marathon with bananas on the brain is not thinking impure thoughts. There's a simple, physiological reason for the water-and-bananas combo that's become a staple in almost every postrace recovery tent: Water hydrates and bananas supply potassium.
Potassium is a mineral that works with sodium (also a mineral) to balance the fluids and electrolyte levels in your body. And since steady fluid levels help to regulate your heartbeat and prevent muscles from cramping, potassium is of particular importance to runners. "Think of it as the gatekeeper for fluid movement in and out of the body's cells," says Lisa Dorfman, R.D., a sports nutritionist at the University of Miami's athletic department. Most of the sodium in your body is stored outside your cells, while most of the potassium is stored within. Yet because of their different concentration levels, potassium constantly wants to get out and sodium wants to get in. The transfer of these two crucial minerals in and out of the cells--the "sodium-potassium pump"--comprises 20 to 40 percent of an adult's resting energy expenditure.
Put that adult in motion, running, and studies have shown that he or she will finish a marathon with more potassium outside his or her cells than inside. That's why you feel weak, your legs might start to cramp, and you may begin to feel bloated. But thanks in part to that unoriginal (albeit useful) banana and water foisted upon you at the finish line, the imbalance returns to normal in about an hour.
Although potassium's main job is maintaining fluid and electrolyte balance within the body, it may also protect against heart disease. In the definitive 1997 study called Dietary Approaches to Stop Hypertension (DASH), conducted by the National Heart, Lung, and Blood Institute, volunteers who took in 4,700 milligrams of potassium per day through a well-rounded diet that was loaded with fruits, vegetables, whole grains, and low-fat dairy products, plus some fish, poultry, nuts, and beans, lowered their blood pressure in just two weeks. "Potassium can act as a diuretic," says Leslie J. Bonci, R.D., director of the sports-medicine nutrition program at the University of Pittsburgh Medical Center. "This is helpful because it gets rid of extra fluid and decreases the pressure forced through the cardiovascular system."
