Polysaccharides Definition and Structure
Polysaccharides [Greek poly = many; sacchar = sugar] are complex carbohydrates, composed of 10 to up to several thousand monosaccharides arranged in chains. The most common monosaccharides that appear as parts of polysaccharides are glucose, fructose, galactose and mannose.
Picture 1. A polysaccharide example:
starch, made of glucose molecules
Three Main Polysaccharides
Three main polysaccharides related to the human nutrition include:
- Starch ─ an energy source obtained from plants
- Cellulose ─ a structural polysaccharide in plants; when consumed, it acts as a dietary fiber
- Glycogen ─ a storage form of glucose in the human liver and muscles
Chart 1. List of Polysaccharides and Food Examples
|Starch||Glucose||4.2||Cereal grains (wheat, oats, barley, corn, rice…) and their products (bread, pasta, pastries, cookies), potatoes, tapioca, yam, legumes|
|Dextrin (starch gum)||Glucose||3.8||An artificially produced food additive|
|Glycogen||Glucose||4||Shellfish, animal liver|
|NON-DIGESTIBLE (DIETARY FIBER)|
|Cellulose||Glucose||0||Whole grains, green leafy vegetables, beans, peas, lentils|
|Hemicellulose||Arabinose + xylose||0||Cereals bran|
|Polydextrose||Glucose||1.2||A food additive|
|Inulin||Fructose + glucose||1-2||Wheat, onions, chicory root, leeks; a food additive|
|Beta-glucan||Glucose||~2||Barley, whole oats, supplements|
|Pectin||Various monosaccharides||3.3||Fruits, carrots, sweet potatoes; a food additive|
|Psyllium husk mucilage||Various monosaccharides||Psyllium seed husk|
|Galactomannans or gums: beta-mannan, carob, fenugreek, guar and tara gum||Galactose + mannose||1-4||A food additive derived from beans and seeds|
|Glucomannan or konjac gum||Glucose + mannose||0||A food additive extracted from konjac plant|
|Other natural gums: gum acacia (arabic), karaya, tragacanth||Various monosaccharides||1.7||Food additives|
|Artificially produced gums: arabinoxylan (soluble), gellan, xanthan||Various monosaccharides||<2||Food additives|
|Seaweed polysaccharides: agar-agar, alginate, carrageenan||Galactose||Food additives derived from marine algae|
|Chitin and chitosan||Glucosamin||Dietary supplements, derived from shells of crustaceans|
Polysaccharides Types, Digestion, Function and Benefits
Digestible polysaccharides, such as starch, are digested (broken down) in the mouth and small intestine in several steps that eventually yield glucose, which is absorbed. They are a source of energy; they provide about 4 Calories (kilocalories) per gram. They also provide carbon atoms for the synthesis of fats, proteins and other substances in your body.
Non-digestible polysaccharides or dietary fiber, such as cellulose, promote the passage of food through the gut and thus help maintain bowel regularity. Some non-digestible polysaccharides, such as inulin, may also promote the growth of beneficial intestinal bacteria.
None of the polysaccharides are essential nutrients; you do not need to consume them in order to be healthy.
- Dietary fiber (cellulose, inulin, pectin, gums…)
Plant and Animal Sources of Polysaccharides
Plant foods are by far the commonest source of polysaccharides:
- Starch is in cereal grains (wheat, oats, rye, barley, buckwheat, rice, etc.), potatoes and legumes (beans, peas, lentils).
- Fiber is mainly in whole grains (whole-grain bread, brown rice, etc.), legumes, vegetables and fruits.
Animal foods are a poor source of polysaccharides:
- A small amount of glycogen is in shellfish and animal liver.
- An indigestible fiber chitin and its derivative chitosan are in shells of crustaceans (crabs, shrimps).
Polysaccharides as Food Additives
Naturally-occurring or artificially produced polysaccharides added to commercial foods as thickeners or fibers include various types of starches, dextrin, polydextrose, inulin and gums.
Storage vs Structural Polysaccharides
Storage polysaccharides are a storage form of energy, for example cellulose in plants and glycogen in animals and humans.
Structural polysaccharides give structure to plants; examples include cellulose in plants and chitin in the shells of crustaceans.