This section is from the book "The Hygienic System: Fasting And Sun Bathing", by Herbert M. Shelton. Also available from Amazon: The Hygienic System Vol III Fasting and Sun Bathing.
The secretion of gastric juice is continuous throughout most of the fast, but in a greatly diminished quantity and is of a weakly acid character. At times its secretion may be stimulated by the usual factors responsible for "psychic secretion."
In cases of gastric hyperacidity, gastric distress continues and may even increase during the first three to four days of the fast. The hyper-secretion, however, soon ends, discomfort wanes and finally ceases entirely, and after a few more days of fasting, eating may be resumed without the previous distress. No other measure will so speedily or so surely end hyperacidity. There are fasting experts of large experience who hold that regularly the secretion of gastric juice ends with the expulsion of the last morsel of food in the stomach and does not recommence until the next meal is taken. On the other hand, I have seen a few cases that regurgitated and expelled gastric juice after the fast had progressed two, three and more weeks.