An oligosaccharide made up of ]]galactose]] residues
Galacto-oligosaccharide (GOS) is comprised of galactose residues with a glucose on one end. It is comprised of about three to eight saccharide units making it an example of an oligosaccharide. Oligosaccharide is a type of carbohydrate. They differ from polysaccharides (another type of carbohydrate) based on the number of saccharides that make them up. In essence, an oligosaccharide contains fewer saccharides (about three to ten) than a polysaccharide does. Nevertheless, both of them are considered as complex carbohydrates as opposed to simple carbohydrates (sugars) that are made up of one or two saccharides, readily digested, and serve as a rapid source of metabolic energy. In contrast, complex need more time to be digested and metabolized. They often are high in fiber. Unlike simple carbohydrates, complex carbohydrates are less likely to cause spikes in blood sugar.
GOS is an oligosaccharide made up of residues of galactose. Galactose is one of the most common dietary monosaccharides (the others are glucose and fructose). The chain length may vary in the number of galactose residues that make it up. GOS is an organic compound just as the other carbohydrates due to the presence of carbon and C-C and C-H covalent bonds. It has a sweetish taste; its sweetness is about 0.3 to 0.6 times that of sucrose. Thus, it may be extracted and processed for use in the food industry as alternative sweetener (in place of sucrose, the common table sugar). Apart from that, it is also used to reduce the freezing point of foods. It also has a high moisture retaining capacity, making it a good humectant and in preventing excessive drying.  It is produced synthetically via converting lactose from bovine milk using enzymes, i.e. by transgalactosylation reaction using lactose as the substrate and catalyzed by β-galactosidases.
GOS, similar to frucooligosaccharide (an oligosaccharide of fructose residues), is biosynthesized by certain organisms for use as energy reserves. In particular, beans and certain root vegetables produce GOS to pack their energy reserves into condensed forms for later use. GOS, just as the other carbohydrates, are produced through dehydration synthesis. In essence, galactose residues are linked together via glycosidic bonds, thereby forming a chain.
In humans, GOS is non-digestible. It means that it is neither readily digested nor absorbed in the small intestine. The human salivary amylase also could not cleave dietary GOS. Thus, more than 90% of the ingested GOS will pass into the colon where it will be fermented by colonic bacteria. GOS, together with fructooligosaccharide, is regarded as a dietary fiber. Both of them belong to the group of prebiotics as well. As a prebiotic, it means that they can promote the growth of beneficial colonic bacteria. These bacteria are able to metabolize them via fermentation, breaking them down into gases and short-chain fatty acids (SCFAs), which in such forms can be absorbed and used by the body.
GOS in organisms capable of synthesizing them serve as an essential storage oligosaccharides. As for the other organisms (including humans) that are unable to produce them due to lack of inherent genes for the needed enzymes, GOS serves chiefly as a soluble dietary fiber. GOS is one of the common dietary oligosaccharides; others are fructooligosaccharides and human milk oligosaccharides.
GOS as a soluble dietary fiber can soften the stool and thereby promote bowel movements. Apart from that, GOS may serve as prebiotics to promote a healthy gut. (N.B.: Prebiotics should not be confused with probiotics. Prebiotics are those that promote the growth of beneficial gut microorganisms whereas probiotics are substances containing the beneficial microorganisms.) Most of the consumed GOS is metabolized only upon reaching the colon by the so-called "beneficial bacteria" (e.g. bifidobacteria and lactobacilli). Since GOS is indigestible but fermentable, GOS is attributed with a caloric value of 1 to 2 kcal/g.  The caloric value is based on the absorbable byproducts-- particularly, SCFAs -- from fermentation. SCFAs are one of the many byproducts of bacterial fermentation that are useful to humans. SCFAs are absorbed and used as energy fuel. Furthermore, they are also presumed to have a role in colonic epithelial cell transport. Another way by which these good bacteria are regarded as beneficial is by producing vitamins and isoprenoid precursors that are essential in various biosynthetic pathways. They can also provide the body with conjugated linoleic acid isomers and bioactive peptides. With GOS, these good bacteria used them as substrate for growth and thereby tend to inhibit the growth of other colonic bacteria that are not as beneficial as they are, such as Salmonella typhimurium, Clostridia, etc.
GOS may also be produced semi-synthetically from dairy sources. They are produced for use in various industries, such as in the food sector where they are used as an additive in infant formula, powdered milk, dairy products, fruit drinks, confectionery, cereal bars, biscuits, and other commercial foods. 
The recommended consumption amount of GOS is 20 g per day. Too much GOS in the diet may cause intestinal upset, abdominal bloating, flatulence, and pain or cramps.
Some people may be sensitive to GOS. There were reports that GOS can cause the immune system to become more active. As such, individuals with autoimmune diseases (e.g. multiple sclerosis, systemic lupus erythematosus) are advised to avoid GOS especially as medicine as they could increase the symptoms of autoimmune disease. 
- Torres, D. P. M., Gonçalves, M. do P. F., Teixeira, J. A., & Rodrigues, L. R. (2010). Galacto-Oligosaccharides: Production, Properties, Applications, and Significance as Prebiotics. Comprehensive Reviews in Food Science and Food Safety, 9(5), 438–454. [Link]
- Galacto-oligosaccharides (GOS): Uses, Health Effects. (2014, September 20). Retrieved from [Link]
- GALACTO-OLIGOSACCHARIDES. Retrieved from [Link]
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