A 48-year-old female patient complains of hard tissue defects found on the maxillary incisors. The defects appeared about a year ago and have been enlarging since that time. Objectively: on the convex portion of the vestibular surface of the maxillary incisors there are oval varisized enamel defects. The defect surface is smooth, lustrous, solid. In order to prevent progression of this process, the patient should be advised to limit consumption of: