According to Essentials, people consume one-tenth of a calorie every time they lick a stamp. A standard postage stamp contains around 5.9 calories due to the adhesive on the back. However, not all of these calories are consumed simply by licking the stamp.
Essentials also says that commemorative stamps tend to have more calories, at 14.5 per stamp; nevertheless, people consume approximately the same amount of calories when they lick commemorative stamps as when they lick regular stamps. The adhesive on the back of a stamp is a gum that is made up of dextrin and polyvinyl alcohol. Dextrin comes from starch that is found in potatoes and polyvinyl alcohol comes from petroleum.