Intestinal pH and propulsion: an explanation of diarrhoea in lactase deficiency and laxation by lactulose.

Subjects deficient in lactase may experience bloating, cramps and diarrhoea after ingesting milk, due to the unhydrolysed and poorly-absorbed lactose. The diarrhoea may result from an osmotic effect of the lactose itself or its poorly-absorbed acidic products of fermentation (Weijers, van de Kamer & others, 1961; Christopher & Bayless, 1971), possibly together with an alteration of sodium and water absorption due to the lowered colonic pH (Rousseau & Sladen, 1971). Laxation by lactulose (1-4-beta-galactosidofructose) may operate through an analogous mechanism. The drug is a synthetic dissaccharide which, in oral doses of 10-20 g, relieves chronic constipation (Wesselius-de Casparis, Braadbaart & others, 1968). It is neither hydrolysed by intestinal dissaccharidase (Dahlqvist & Gryboski, 1965) nor absorbed in the gut, but it is converted in the colon mainly to lactic and acetic acids by various bacteria including Lactobacillus acidophilus. Apart from the increased osmotic effect, the pH in the proximal colon falls markedly (Bown, Gibson & others, 1974), and larger doses may reduce stool pH. Weijers & others (1961) inferred that the acidic products formed from lactose in the colon stimulate propulsion, and K.S. Liem (Philips-Duphar) suggested to us that lactulose may relieve constipation partly by stimulation of propulsion due to the lowered pH. The experiments described below support this view.