Previous work has strongly indicated that apple and pectin exert their favorable effect on diarrhea by way of their local action in the intestine.1 The cellulose and lignin contained in apples must act as an absorbing colloid or mechanically, since they are not appreciably decomposed. The pectin, which is decomposed only in the colon,2 could act similarly in the small intestine, but not in the colon, unless, as in the fasting dog, considerable pectin is not decomposed.3 The local effect of pectin in the colon, therefore, is due to the action either of the intact molecule or of its decomposition products on pathogenic flora or their toxic or irritating products.
The chief products found to be formed in the decomposition of pectin were formic and acetic acids; either the quantity of galacturonic acid produced (pectin is a galacturonic acid complex) is small or it is decomposed as
WERCH SC, JUNG RW, PLENK H, DAY AA, IVY AC. PECTIN AND GALACTURONIC ACID AND THE INTESTINAL PATHOGENS. Am J Dis Child. 1942;63(5):839-846. doi:10.1001/archpedi.1942.02010050003001