When given the choice between a scientifically formulated “supplement” and the alfalfa sprouting all around them, something in the pigs’ brains said, Alfalfa. It was as though mixing vitamins in with carbs flicked a switch inside the pigs. It made them eat differently. When given nutritionally complete feed, the pigs lost interest in greenery and became carb-munching, weight-gaining machines. But if the carbs were presented on their own—rich in calories but nutritionally incomplete—the pigs reverted to their omnivore ways. They foraged on alfalfa and dug up roots and did what pigs are born to
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