An oligosaccharide (from the Greek oligos, a few, and sacchar, sugar) is a saccharide polymer containing a small number (typically three to nine) of simple sugars (monosaccharides). Oligosaccharides can have many functions; for example, they are commonly found on the plasma membrane of animal cells where they can play a role in cell–cell recognition.
Fructo-oligosaccharides (FOS), which are found in many vegetables, consist of short chains of fructose molecules. (Inulin has a much higher degree of polymerization than FOS and is a polysaccharide.) Galactooligosaccharides (GOS), which also occur naturally, consist of short chains of galactose molecules. These compounds can be only partially digested by humans.
Oligosaccharides are often found as a component of glycoproteins or glycolipids and as such are often used as chemical markers, often for cell recognition. An example is ABO blood type specificity. A and B blood types have two different oligosaccharide glycolipids embedded in the cell membranes of the red blood cells, AB-type blood has both, while O blood type has neither.
Mannan oligosaccharides (MOS) are widely used in animal feed to improve gastrointestinal health, energy levels and performance. They are normally obtained from the yeast cell walls of Saccharomyces cerevisiae. Research at the University of Illinois has demonstrated that mannan oligosaccharides differ from other oligosaccharides in that they are not fermentable and their primary mode of actions include agglutination of type-1 fimbrae pathogens and immunomodulation 
When oligosaccharides are consumed, the undigested portion serves as food for the intestinal microflora. Depending on the type of oligosaccharide, different bacterial groups are stimulated or suppressed.
Role in human reproduction system
The Sialyl-Lewisx Oligosaccharide identified in a 2011 study, seems to be the most abundant carbohydrate receptor present in the outer coating (zona pellucida (ZP)) of the human ova, and is implicated as a large factor of Sperm-ZP binding leading to fertilization. These sialyl-Lewisx branches are much more common on the ovum than on any other kind of body cell; they completely coat it. The discovery of the largest receptor on the human ovum may help improve infertility research. 
Role in mother-to-child transmission of HIV-1
Breast-feeding is the highest prevalence factor of postnatal transmission of HIV-1. However, most breast-fed infants do not contract the virus from the infected mother despite the continuous exposure. The Lewis antigen glycans in human milk compete with HIV-1 glycoprotein-120 and binds to the dendritic cell-specific ICAM3-grabbing non-integrin (DC-SIGN) on human dendritic cells. The glycans inhibit the transfer of the virus to the CD4 T lymphocytes. Because the human milk oligosaccharides carry one more of the Lewis antigen glycan, it's hypothesized that they do compete with gp120 for binding. 
FOS and inulin are found naturally in Jerusalem artichoke, burdock, chicory, leeks, onions, and asparagus. FOS products derived from chicory root contain significant quantities of inulin, a fiber widely distributed in fruits, vegetables and plants. Inulin is a significant part of the daily diet of most of the world’s population. FOS can also be synthesized by enzymes of the fungus Aspergillus niger acting on sucrose. GOS is naturally found in soybeans and can be synthesized from lactose (milk sugar). FOS, GOS, and inulin are available as nutritional supplements in capsules, tablets, and as a powder.
Not all natural oligosaccharides occur as components of glycoproteins or glycolipids. Some, such as the raffinose series, occur as storage or transport carbohydrates in plants. Others, such as maltodextrins or cellodextrins, result from the microbial breakdown of larger polysaccharides such as starch or cellulose.
Carrots are an excellent source of oligosaccharides but only after they have been cooked for an hour. This cooking breaks down the starches into the shorter chains needed for bioavailability.
- Oligosaccharide synthesis
- Oligosaccharide nomenclature
- Mannan Oligosaccharides, MOS
- Isomaltooligosaccharide, IMO
- Oligosaccharides at the US National Library of Medicine Medical Subject Headings (MeSH)
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