A chalky suspension of barium sulfate swallowed before X-ray imaging of the esophagus, stomach, and upper intestine, used to reveal swallowing disorders, ulcers, tumors, and structural abnormalities. Barium is opaque to X-rays, so as the meal travels it outlines the digestive tract in sharp contrast, and radiologists watch its progress in real time on fluoroscopy. The preparation tastes unpleasant and the procedure requires fasting, but it remains valuable for evaluating motility and anatomy that endoscopy cannot always assess. Afterward, patients drink plenty of fluids to flush the inert barium through, since it can constipate. Nutritionally it is a diagnostic tool rather than a food, notable mainly for the brief period when eating and drinking are suspended around it.