Doctors and nutritionists say the average American is getting two to three times the amount of salt necessary each day, often hidden in canned and boxed prepared foods or brimming from that cardboard envelope of shoestrings that go with fast-food lunches. People who suffer from kidney problems, high blood pressure or congestive heart failure have no choice - cut salt out now, doctors say - but why do the rest of us need to stop with the salt? "Salt by itself is not bad; it doesn't cause disease, but it exacerbates disease processes," says Stephanie Broderson, a family doctor with Sioux Valley Clinic at Sycamore Avenue and 26th Street.