What causes tooth decay? Sugar doesn't cause tooth decay. You can put a tooth in a bowl of sugar or breast milk for years and the tooth will not decay. Acid need to be present in order for tooth decay to occur along with sugar. The acid is produced when the sugars in foods or drinks react with bacteria present on the tooth surface. The acid production leads to the loss of calcium from the enamel causing deminralization which is the begining process of tooth decay. Keeping your mouth clean by brushing twice and flossing once daily your risks of tooth decay will go down significantly.