As mentioned in Carbohydrates: Dogs' Doodads or Spawn of Satan?, there are three common disaccharides, sucrose (table sugar), lactose (milk sugar) and maltose (beer sugar). These all contain a molecule of glucose linked by a glycosidic bond to molecules of fructose, galactose and glucose respectively. In our bodies, our guts contain the enzymes sucrase, lactase & maltase which hydrolyse the glycosidic bond breaking the disaccharides into their constituent monosaccharides. People who lack any of these enzymes are unable to break the disaccharide into monosaccharides. As disaccharides are not absorbed, they pass along the gut until they reach the colon, where colonic bacteria ferment the disaccharides into short-chain fatty acids and gas. This increases ammonia levels in the colon which attracts water. End result: Lots of gas, soft poo and the excretion of ammonia (nitrogenous waste).
There's a synthetic disaccharide called lactulose, which is a disaccharide of fructose and galactose. As our bodies don't contain the enzyme lactulase, lactulose is fermented, producing lots of gas & soft poo. I thought I'd post about this as I'm taking lactulose to prevent the constipation caused by high dose Co-Codamol, the pain-killer I'm on while my 4-inch scar heals. It works!
An interesting but otherwise useless fact: The artificial sweetener sucralose that's made from & is about 600 times sweeter than sugar has the chemical name 1,6-Dichloro-1,6-dideoxy-β-D-fructofuranosyl-4-chloro-4-deoxy-α-D-galactopyranoside (I'm nerdy enough to know that by heart!). Sucralose has a structure like lactulose rather than sucrose, which has the chemical name β-D-fructofuranosyl-(2→1)-α-D-glucopyranoside. As the amount of sucralose used is about one 600th that of sucrose, it doesn't cause the above effects.