What Are Oligosaccharides?
Oligosaccharides are a type of carbohydrate, classified as saccharide polymers, that consist of a small number of simple sugars (monosaccharides) linked by glycosidic bonds. Unlike simple sugars or starches, many oligosaccharides cannot be digested or absorbed in the human small intestine. Instead, they travel intact to the large intestine where they are fermented by the resident gut microbiota, acting as prebiotics. This fermentation process supports the growth of beneficial bacteria, such as Bifidobacterium and Lactobacillus, which is crucial for digestive and immune health.
Naturally occurring oligosaccharides can be found in a wide variety of plant-based foods, including fruits, vegetables, and legumes, as well as in breast milk. Some are also produced commercially for use as dietary supplements or food additives due to their prebiotic properties.
Functions of Oligosaccharides
Oligosaccharides have several important functions beyond their prebiotic role. They are involved in cell recognition and cell adhesion, especially when linked to lipids (glycolipids) and proteins (glycoproteins) on cell surfaces. These surface-bound oligosaccharides are critical for processes like immune responses and determining blood type. For example, Human Milk Oligosaccharides (HMOs) mimic viral receptors, helping protect infants from infection.
10 Examples of Oligosaccharides
Here are 10 notable examples of oligosaccharides, highlighting their structure, sources, and key functions.
- Fructo-oligosaccharides (FOS): These are short chains of fructose units, typically capped by a glucose molecule. FOS are common plant-derived oligosaccharides found in foods such as onions, garlic, leeks, bananas, and chicory root. They are widely used as low-calorie sweeteners and functional food ingredients due to their potent prebiotic effect, specifically stimulating the growth of Bifidobacterium.
- Galacto-oligosaccharides (GOS): Composed of short chains of galactose units, GOS are primarily found in mammalian milk, including human milk and cow's milk. Commercially, they are synthesized from lactose and added to infant formula to mimic the prebiotic effect of breast milk. GOS are also prebiotics that promote the growth of beneficial gut bacteria.
- Raffinose: A trisaccharide consisting of one molecule each of galactose, glucose, and fructose. Raffinose is a member of the Raffinose Family Oligosaccharides (RFOs) and is commonly found in legumes, cabbage, broccoli, and whole grains. It is indigestible by humans and fermentation by colonic bacteria can sometimes lead to gas and bloating.
- Stachyose: A tetrasaccharide composed of two galactose units, one glucose unit, and one fructose unit. Like raffinose, stachyose is a plant-derived oligosaccharide found in many legumes and beans. As part of the RFOs, it also acts as a storage and transport carbohydrate in plants.
- Maltotriose: This trisaccharide is made up of three glucose units linked together. It is produced during the enzymatic breakdown of starch, for example, by amylase during digestion or industrial processes. It is present in liquid glucose and brown rice syrup.
- Human Milk Oligosaccharides (HMOs): This complex and diverse group of oligosaccharides is found naturally and in high concentration in human breast milk. HMOs are not digestible by infants but play a crucial role in shaping the infant's gut microbiota and provide immune protection by acting as decoy receptors against pathogens.
- Xylo-oligosaccharides (XOS): These consist of short chains of xylose units. XOS are derived from hemicellulose, a component of plant cell walls, and can be produced from materials like corn cobs or birch wood. They are gaining attention as potent prebiotics and are found in fermented foods like miso and soy sauce.
- Isomalto-oligosaccharides (IMOs): A mixture of saccharides, IMOs are produced from starch through enzymatic processes. They possess prebiotic properties and can be found in certain fermented foods and honey.
- Pectic-oligosaccharides: Derived from pectin, a polysaccharide found in plant cell walls. Pectic-oligosaccharides (pAOS) possess prebiotic functions and have shown potential anti-inflammatory effects. They can be extracted from citrus peels and apples.
- Mannan-oligosaccharides (MOS): These are oligosaccharides derived from mannan, a polymer found in the cell walls of yeasts like Saccharomyces cerevisiae. MOS are commonly used as prebiotic supplements, particularly in animal feed, to support gastrointestinal health.
Comparison of Key Oligosaccharides
| Feature | Fructo-oligosaccharides (FOS) | Galacto-oligosaccharides (GOS) | Raffinose & Stachyose | Human Milk Oligosaccharides (HMOs) |
|---|---|---|---|---|
| Primary Source | Plants (chicory, onion, garlic) | Dairy (human milk, cow's milk) | Legumes, broccoli, cabbage | Human breast milk |
| Structure | Chains of fructose units | Chains of galactose units | Raffinose (trisaccharide), Stachyose (tetrasaccharide) | Complex and diverse chains of simple sugars |
| Key Benefit | Strong prebiotic effect, low-calorie sweetener | Promotes infant gut flora, prebiotic | Seed storage, prebiotic fermentation causes gas | Crucial for infant immunity and gut flora development |
| Digestion | Not digested by human enzymes | Not digested by human enzymes | Not digested by human enzymes | Not digested by infants, acts as prebiotic |
The Role of Oligosaccharides in Functional Foods
The unique properties of many oligosaccharides have made them popular functional food ingredients. Functional foods are those that provide health benefits beyond basic nutrition. Oligosaccharides like FOS and GOS are frequently added to products such as yogurt, cereals, and infant formula to boost their prebiotic content. This provides consumers with an easy way to increase their intake of beneficial fiber and support their gut health.
For example, manufacturers add prebiotic oligosaccharides to some infant formulas, aiming to replicate some of the benefits of breast milk, which is naturally rich in HMOs. Other applications include using them as low-calorie sweeteners or fat replacers due to their bulking properties, allowing for healthier product alternatives without compromising taste or texture.
Conclusion
Oligosaccharides are a diverse and important class of carbohydrates with a wide range of functions, particularly as prebiotics that feed beneficial gut bacteria. The 10 examples discussed—FOS, GOS, Raffinose, Stachyose, Maltotriose, HMOs, XOS, IMO, pAOS, and MOS—illustrate the variety in their structure and origin. Whether naturally occurring in foods like legumes and breast milk or produced commercially for functional foods, these compounds play a significant role in supporting human and animal health by promoting a healthy and balanced gut microbiota. Including a variety of these compounds in your diet can be a valuable strategy for enhancing digestive and immune function.
For further reading on the science behind prebiotics, explore the article "Prebiotics and synbiotics: Recent concepts in nutrition" on ScienceDirect.
