Your Stomach Produces Acid Whether You Eat or Not
The stomach secretes hydrochloric acid in anticipation of food, driven by the vagus nerve and by gastrin, a hormone released partly in response to the smell and thought of eating. During a normal eating pattern, food buffers that acid. During a long fast, the acid accumulates against an unprotected mucosal lining. The longer the gap, the longer the lining is exposed without the neutralising effect of food or saliva.