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To the Editor: Indirect evidence suggests that gum chewing may have greater metabolic effects than has been appreciated. The thermic effect of food is reduced when nutrition bypasses the mouth.1 In cows, chewing increases energy expenditure by approximately 20 percent.2,3 We measured how energy expenditure changes with gum chewing in humans.Energy expenditure was measured in a temperature-controlled, darkened, silent laboratory with an indirect calorimeter (model 229, SensorMedics, Yorba Linda, Calif.) that was calibrated before each measurement with two primary-standard gases (a combination of 4 percent carbon dioxide and 16 percent oxygen and a combination of 26 percent oxygen . . .