This section is from the book "Practical Dietetics With Special Reference To Diet In Disease", by William Gilman Thompson. Also available from Amazon: Practical Dietetics with Special Reference to Diet in Disease.
Strong mental emotion, such as fright, terror, or excessive excitement of almost any kind, inhibits the digestive functions, especially in the stomach, but also in the intestines. Such emotion may be accompanied by vascular disturbances which will react upon the digestive organs, and in addition there seems to be a diversion of nerve currents from their proper course. Pleasurable emotions, however, affect digestion favourably, and the expression "laugh and grow fat" is certainly not without physiological basis.
Hufeland wrote that "laughter is one of the greatest helps to digestion with which I am acquainted, and the custom prevalent among our forefathers of exciting it at table by jesters and buffoons was founded upon true medical principles".
Prolonged anxiety and worry, in almost every instance, result in more or less gastric indigestion and malassimilation, so that, although the appetite may remain good, nervous dyspepsia, constipation, and loss of weight result.