In the 1970s, investigators were searching for new models of diet-induced obesity. Anthony Sclafani looked at something new, and disarmingly simple -- what came to be known as the "cafeteria diet". He gave rats access to a variety of both palatable human foods and standard rat food. They ignored the rat food and rapidly became obese. The cafeteria diet remains the quickest and most effective way of producing dietary obesity and metabolic syndrome in rodents using solid food.
A number of studies have found similar effects in humans, sometimes when the studies weren’t even looking for it. One such study began simply as an attempt to design a novel way of accurately measuring food intake. Investigators created an "automated food-selection system" consisting of two large vending machines filled with a variety of prepared foods. The study subjects immediately began consuming excessive calories. Over the course of just seven days, the volunteers gained an average of 5.1 pounds.
According o Whole Health Source:
“Just as in rats, exposing humans to a variety of readily accessible, energy dense, palatable foods causes excessive food intake and rapid weight gain. The degree of overeating varies by individual, but nearly everyone overeats to some degree. Whatever the mechanism(s) underlying this may be, the phenomenon has important implications for the commercialization of food and the associated obesity epidemic in affluent nations.”