The term oligosaccharide comes from the Ancient Greek words olígos ('few') and sákkhar ('sugar'), literally meaning "a few sugars". These carbohydrates are composed of a relatively small number of monosaccharides (simple sugars) linked together. While the name oligosaccharide is widely used in biochemistry and nutrition, context dictates several other common names for this versatile biomolecule.
The Many Names for Oligosaccharides
The specific name of an oligosaccharide often depends on its composition and function. From general biochemical classifications to dietary roles, here are some of the most common alternative names.
Glycans and Oligose: The General Terms
In a broad biochemical context, a glycan is another name used for any carbohydrate chain, which can be linked to other molecules like proteins or lipids to form glycoproteins and glycolipids. Because oligosaccharides function as these chains in many biological processes, they are a specific type of glycan. Another, less common name derived from its Greek roots is oligose.
Specific Functional Oligosaccharides
Beyond the general terms, more specific names are used for distinct types of oligosaccharides based on their monomeric units, source, and function. Prominent examples include:
- Fructooligosaccharides (FOS) / Oligofructans: These are composed of short chains of fructose units and are found in plants like chicory root, onions, and asparagus. FOS are often used in the food industry as food additives and sweeteners.
- Galactooligosaccharides (GOS): As the name suggests, these are short chains of galactose molecules and are naturally found in legumes and dairy products.
- Human Milk Oligosaccharides (HMOs): This is a complex mixture of oligosaccharides present in human breast milk. HMOs are vital for infant gut health and immune system development.
- Maltodextrins: These oligosaccharides are produced from the partial breakdown of larger polysaccharides, such as starch.
- Raffinose Family Oligosaccharides (RFOs): A group of oligosaccharides, including raffinose and stachyose, commonly found in beans and other vegetables.
Prebiotics and Dietary Fiber
Many oligosaccharides are indigestible by human enzymes in the small intestine and instead travel to the large intestine where they are fermented by beneficial bacteria. Because they selectively feed these microbes, they are often referred to as prebiotics, a type of dietary fiber.
How Oligosaccharides Compare to Other Carbohydrates
Oligosaccharides represent an intermediate category of carbohydrates, falling between simple sugars and complex starches. Understanding their properties relative to monosaccharides and polysaccharides helps clarify their unique role.
| Feature | Oligosaccharides | Monosaccharides | Polysaccharides |
|---|---|---|---|
| Structure | 3 to 10 (sometimes up to 20) simple sugar units linked together. | A single sugar unit, the basic building block of all carbohydrates. | Long chains of more than 10 simple sugar units, often hundreds or thousands. |
| Common Examples | Raffinose, Stachyose, FOS, GOS, HMOs. | Glucose, Fructose, Galactose. | Starch, Cellulose, Glycogen. |
| Digestion | Poorly digested by human enzymes; fermented by gut bacteria. | Readily absorbed in the small intestine; serves as a rapid energy source. | Digestion depends on the type of glycosidic bond; some are digested, while others act as dietary fiber. |
| Physiological Role | Acts as prebiotics, supports gut microbiome, cell recognition. | Basic energy source for cells. | Energy storage or structural support. |
| Sweetness | Mildly sweet or not sweet at all; sweetness decreases with increasing chain length. | Very sweet. | Not sweet. |
Key Sources and Functions
Oligosaccharides play a diverse range of biological roles and are derived from various natural sources.
Sources:
- Legumes: Beans, lentils, and chickpeas are rich sources of galactooligosaccharides (GOS).
- Vegetables: Onions, garlic, leeks, and asparagus contain fructooligosaccharides (FOS) and other types.
- Fruits: Bananas and watermelon are examples of fruits containing oligosaccharides.
- Grains: Wheat, rye, and barley are sources of fructans.
- Dairy: Human breast milk is a significant source of human milk oligosaccharides (HMOs).
Functions:
- Cell Recognition and Adhesion: As glycans attached to cell membrane proteins and lipids, oligosaccharides are crucial for cell-cell communication and recognition. This is famously illustrated by the role of oligosaccharides in determining human blood types.
- Prebiotic Activity: Non-digestible oligosaccharides feed beneficial bacteria in the gut, which ferments them into short-chain fatty acids (SCFAs). These SCFAs are important for intestinal health and nutrient absorption.
- Immune System Support: The fermentation of oligosaccharides in the gut promotes a healthy gut environment, which supports the overall immune system.
- Energy Storage: In some plants, oligosaccharides like those in the raffinose family serve as storage or transport carbohydrates.
Conclusion
In conclusion, while the term oligosaccharide refers to short-chain carbohydrates in general, its specific name varies depending on its function, structure, and source. Common alternate names include glycan, oligose, and the functional designations prebiotics, FOS, and GOS. This diverse terminology reflects the molecule's wide range of biological activities, from providing structural stability and aiding cell communication to promoting a healthy gut microbiome. For further reading on the biological roles of carbohydrates, consider resources from the National Institutes of Health.
