These often include fermentable oligosaccharides, disaccharides, monosaccharides and polyols (FODMAPs). The saccharides are divided into four chemical groups&colon; monosaccharides, disaccharides, oligosaccharides, and polysaccharides. Both non-structural and structural carbohydrates are hydrolysed to monosaccharides or disaccharides by microbial enzymes. All monosaccharides are reducing sugars, along with some disaccharides, some oligosaccharides, and some polysaccharides. Like monosaccharides, disaccharides are soluble in water. Two, three, several or many monosaccharides thus linked form disaccharides, trisaccharides, oligosaccharides, or polysaccharides, respectively. Scientifically, "sugar" loosely refers to a number of carbohydrates, such as monosaccharides, disaccharides, or oligosaccharides. Examples of disaccharides include sucrose, maltose, and lactose. The four disaccharides digested in the small intestine are lactose, sucrose, maltose, and isomaltose. Lactose, maltose, and sucrose are all compound sugars, disaccharides, with the general formula CHO. Monosaccharides contain one sugar unit, disaccharides two, and polysaccharides three or more. Larger sugars such as disaccharides and polysaccharides can be named to reflect their qualities. However, disaccharides, as well as longer polysaccharides (up to nine glucose units), are also present. Alpha-glucosidase breaks down starch and disaccharides to glucose. Lactulose and lactitol are disaccharides that are not absorbed from the digestive tract.