What Exactly Are Oligosaccharides?
Oligosaccharides are carbohydrates composed of a small number (typically 3 to 10) of monosaccharide units linked together. Many dietary oligosaccharides are not digested in the human small intestine because our bodies lack the necessary enzymes. They pass intact to the large intestine where they are fermented by gut bacteria, acting as prebiotics that selectively promote beneficial microorganisms. This fermentation, however, can also produce gas, potentially causing bloating. These indigestible oligosaccharides are a type of dietary fiber.
The Two Most Common Oligosaccharides
The two most common oligosaccharides in food sources are raffinose and stachyose. Both belong to the raffinose family of oligosaccharides (RFOs), found commonly in plants.
Raffinose: The Trisaccharide
Raffinose is a trisaccharide consisting of galactose, glucose, and fructose.
Key Characteristics:
- Composition: $\alpha$-D-galactopyranosyl-(1$\rightarrow$6)-$\alpha$-D-glucopyranosyl-(1$\rightarrow$2)-$\beta$-D-fructofuranoside.
- Digestion: Humans cannot digest raffinose due to the lack of the enzyme $\alpha$-galactosidase.
- Food Sources: Prevalent in legumes, cruciferous vegetables, and whole grains.
Stachyose: The Tetrasaccharide
Stachyose is a tetrasaccharide containing two galactose units, one glucose, and one fructose.
Key Characteristics:
- Composition: $\alpha$-D-galactopyranosyl-(1$\rightarrow$6)-$\alpha$-D-galactopyranosyl-(1$\rightarrow$6)-$\alpha$-D-glucopyranosyl-(1$\rightarrow$2)-$\beta$-D-fructofuranoside.
- Digestion: Indigestible by human enzymes and fermented in the large intestine.
- Food Sources: Found in many legumes like soybeans and lentils.
Comparison of Raffinose and Stachyose
| Feature | Raffinose | Stachyose |
|---|---|---|
| Classification | Trisaccharide (3 sugar units) | Tetrasaccharide (4 sugar units) |
| Monosaccharide Units | Galactose, Glucose, Fructose | Two Galactose, Glucose, Fructose |
| Common Sources | Legumes, broccoli, cabbage, whole grains | Legumes, chickpeas, lentils |
| Digestibility | Indigestible by human enzymes; fermented by gut bacteria | Indigestible by human enzymes; fermented by gut bacteria |
| Primary Role | Storage and transport sugar in plants; prebiotic in humans | Storage and transport sugar in plants; prebiotic in humans |
Other Significant Oligosaccharides
Other notable oligosaccharides include Fructooligosaccharides (FOS) and Galactooligosaccharides (GOS), which are often used as prebiotics in the food industry. FOS are fructose chains found in foods like chicory root and onions. GOS are galactose chains, naturally present in breast milk and produced commercially from lactose.
The Impact on Gut Health
The indigestibility of these oligosaccharides means they reach the colon and are fermented by beneficial bacteria, producing short-chain fatty acids (SCFAs) like butyrate. SCFAs are important for colon cell health and gut barrier function. However, the fermentation can cause symptoms like bloating, particularly in individuals with IBS, as oligosaccharides are part of the FODMAP group. Tolerance varies, and the prebiotic benefits for the gut microbiome are well-established.
Common Food Sources of Oligosaccharides
- Legumes: High in raffinose and stachyose.
- Vegetables: Contain varying oligosaccharides, including raffinose and FOS.
- Grains: Whole wheat contains fructans and other oligosaccharides.
- Dairy: GOS is found naturally in milk and is commercially produced.
Conclusion
Raffinose and stachyose are the two most common oligosaccharides found in many plant-based foods. Along with others like FOS and GOS, they function as prebiotics, supporting beneficial gut bacteria. While offering health benefits, their fermentation can lead to gas and discomfort for some individuals. Understanding their sources is important for managing digestive health. For more on the health benefits, see this review: Oligosaccharides: a boon from nature’s desk.
