"Salt [added to food] is the major cause of high blood pressure, and high blood pressure is the major cause of death worldwide," said Graham MacGregor, professor of cardiovascular medicine at Queen Mary, University of London. "The evidence for salt [in causing premature death] is overwhelming, equivalent to the onus of smoking."

Salt, or sodium chloride, is essential for human health; we need about 350 milligrams of sodium daily, according to the Centers for Disease Control and Prevention (CDC). That's about the weight of half a raisin. Unfortunately, Americans consume on average about 3,500 milligrams of sodium daily.

Most of that sodium comes not from the saltshaker but rather from processed foods, in amounts unrealized by most consumers. A bag of potato chips might be an obvious culprit; one serving contains more than 250 milligrams of sodium. But so does a single slice of store-bought bread or a bowl of breakfast cereal. The bigger offenders are canned vegetables, canned soups and frozen dinners, each with around 1,000 milligrams of sodium.