Oligosaccharides and polysaccharides are both complex carbohydrates. Their building blocks are the dietary monosaccharides glucose, fructose and galactose, also known as simple carbohydrates, or sugars. Disaccharides such as sucrose, which are composed of two monosaccharide units, are also sugars. The prefix “oligo” means “few.” Most oligosaccharides are composed of three to 10 monosaccharide units. Oligosaccharides are not digestible by human gut enzymes. "Poly” means “many”; polysaccharides are composed of long chains of monosaccharide units. Some are digestible by human enzymes, and some are not.
The Role of Oligosaccharides
The nondigestibility of oligosaccharides provides a health benefit as they help to feed friendly bacteria, also known as probiotics, in the large intestine. Therefore, oligosaccharides are often referred to as prebiotics. Two common oligosaccharides are raffinose, composed of three monosaccharide units, and stachyose, composed of five monosaccharide units, found in beans and other legumes. As gut microbes feast on them, they produce the gas bubbles that beans are famous for. Another oligosaccharide category are the fructo-oligosaccharides, composed of varying numbers of fructose molecules. They are found in artichokes, onions, garlic, bananas and other foods.
The only digestible polysaccharide is starch, which is a large molecule composed simply of glucose. It is broken down first in the mouth and then in the small intestine by the enzyme amylase. All the starch that is absorbed into the body is in the form of individual glucose molecules, which are delivered to the cells by the blood for energy. Extra glucose is reassembled into the starchlike molecule called glycogen to be stored between meals in the liver and the muscles. Excessively high intake will be converted to fat. Food sources are grains, root vegetables and legumes.
Unrefined grains, starchy and nonstarchy vegetables, legumes and fruit provide various nondigestible polysaccharides called fiber, which fall into two main categories. Insoluble fiber, also known as cellulose, is abundant in wheat bran and vegetables. Cellulose, like starch, is composed purely of glucose molecules, but the bonds are such that human enzymes can’t break them. Its main role is to keep undigested food moving through the intestines, preventing constipation. A variety of soluble fibers, such as pectins, gums and mucilages, are abundant in oat bran, beans and many fruits. As its name suggests, soluble fibers absorb water. Fibers add satiety value to meals and additional bulk to stools.
To Your Health
Oligosaccharides and polysaccharides have separate and overlapping functions in maintaining good colon health and energy. Raffinose, stachyose, fructooligosaccharides and soluble fiber have additional benefits as well. For example, by feeding beneficial gut microbes, they help the bacteria to synthesize certain vitamins, such as biotin and K, which can be absorbed. Well-fed gut bacteria can also synthesize beneficial fatty acids that may protect colon health.