Two modern food favorites -- the hamburger and the taco -- could not have been concocted before the great global food swap that occurred after Christopher Columbus visited the Caribbean 500 years ago. Many of the foods now common to both Europe and the Americas were first exchanged by Columbus and other explorers visiting the New World in the late 1400s and early 1500s, according to a nutritional geographer at UC Davis. For example, at the time of Columbus' arrival, cattle were not found in the New World. Likewise, Europe had not yet been introduced to tomatoes, corn and avocadoes. "Some food exchanges led to improved diversity and diet, but other exchanges led to economic and medical-nutritional disasters," says Louis Grivetti, professor of nutrition and geography. Grivetti points out, for example, that while Europeans were quick to adopt maize, or corn, they did so without adding to their diets the complementary beans or other legumes commonly eaten in the New World. As a result, pellagra, a disease related to niacin and protein deficiencies, soon appeared in Europe. Grivetti presented the findings of his study on the blending of Old World and New World cuisine at a recent scientific gathering.