One patient with chronic diarrhea noticed improvement in symptoms after prolonged fasting. He is most probably suffering from:

The demonstration of the effect of prolonged (>24 h) fasting on stool output can be very effective to differentiate osmotic and secretory diarrhea. A secretory (enterotoxin-induced traveler's) diarrhea is not affected by prolonged fasting. Diarrhea secondary to lactose malabsorption would cease during a prolonged fast. Ref: Harrison, E-18, P-2460.