The statement that swallowed food reaches the stomach via peristalsis regardless of body position (such as being upside down) is a matter of everyday physiological understanding and common knowledge, so no citation is strictly needed.
The retrieved papers discuss esophageal motility disorders, swallowing exercises, and body positions like the bridge position, but do not directly test or address the specific textbook physiological fact about swallowing while upside down. Because the claim represents a well-established physiological mechanism known to the public and basic biology, it is classified as common knowledge while noting that the specific retrieved sources do not bear directly on it.