Can Polysaccharides Be Broken Down by the Body?

November 17, 2025 •

3 min read

While some polysaccharides like starch are readily broken down by human digestive enzymes, others like cellulose are indigestible by the body. This difference depends entirely on the specific chemical bonds linking the sugar units together and the enzymes available to break them down. This distinction is crucial for understanding how our bodies process complex carbohydrates and the vital role of our gut microbiome.

Quick Summary

This article explains how different polysaccharides, like starch and cellulose, are processed in the human body. It details the enzymatic hydrolysis process, the stages of digestion in the gastrointestinal tract, and the critical role of gut bacteria in fermenting indigestible fibers.

Key Points

Enzyme Specificity: The breakdown of polysaccharides, like all enzyme-catalyzed reactions, is highly specific, depending on the enzyme's ability to recognize and cleave particular glycosidic bonds.
Two Primary Fates: Polysaccharides are either broken down by human enzymes for energy, as with starch and glycogen, or fermented by gut bacteria for other health benefits, as with cellulose and inulin.
Role of Amylase: Amylase, secreted in the saliva and pancreas, is the key human enzyme responsible for hydrolyzing the $\alpha$-glycosidic bonds in starches into smaller sugars.
Fiber is Undigestible: Humans cannot digest dietary fibers such as cellulose because they lack the necessary enzyme, cellulase, to break its $\beta$-glycosidic bonds.
Gut Bacteria are Essential: Indigestible polysaccharides serve as prebiotics, nourishing beneficial gut microbes that ferment them into short-chain fatty acids (SCFAs).
Not all Digestion is Human-Driven: Breakdown of complex dietary polysaccharides in the colon is a microbial process, producing beneficial metabolites that the host can then utilize.
Viscosity Affects Digestion: The viscosity created by certain soluble fibers can inhibit or slow down the enzymatic digestion of other nutrients, impacting absorption rates.

The Chemical Nature of Polysaccharide Breakdown

Polysaccharides are long chains of monosaccharides (simple sugars) linked by glycosidic bonds. The breakdown of these complex carbohydrate chains into their individual sugar units is a process called hydrolysis, which requires the addition of water to break the bonds. The digestibility of any given polysaccharide hinges on two primary factors: the type of monosaccharide units and the specific configuration of the glycosidic bonds.

In humans, digestion of carbohydrates like starch begins in the mouth with the enzyme salivary amylase, which starts breaking down the $\alpha$-1,4 glycosidic bonds. However, this enzyme is inactivated by the acidic environment of the stomach, and significant breakdown resumes in the small intestine with pancreatic amylase. The final breakdown to monosaccharides for absorption is completed by enzymes like maltase and sucrase in the small intestine's brush border.

Digestible vs. Indigestible Polysaccharides

Not all polysaccharides are created equal when it comes to digestion. The human body is equipped to handle certain types, while others pass through largely intact. The primary difference lies in the configuration of the glycosidic bonds.

Digestible Polysaccharides

These are complex carbohydrates that can be broken down by human enzymes to release energy. The two main types are:

Starch: The energy storage form in plants, composed of amylose and amylopectin, which are long chains of glucose units linked by $\alpha$-glycosidic bonds. Humans produce amylase enzymes specifically to cleave these alpha bonds. Sources include potatoes, rice, and wheat.
Glycogen: The storage form of glucose in animals, stored primarily in the liver and muscles. It is a highly branched polysaccharide composed of glucose units linked by $\alpha$-1,4 and $\alpha$-1,6 glycosidic bonds. Its structure is readily broken down by human enzymes to supply the body with glucose.

Indigestible Polysaccharides (Dietary Fiber)

These are polysaccharides that the human body cannot break down due to a lack of the necessary enzymes. Instead, they provide bulk and support the health of the digestive system.

Cellulose: The main structural component of plant cell walls, composed of glucose units linked by $\beta$-1,4 glycosidic bonds. Humans lack the enzyme cellulase needed to break these bonds, so cellulose passes through the digestive tract largely undigested, acting as insoluble fiber.
Inulin: A fructan found in plants that serves as energy storage. Human digestive enzymes cannot completely break it down, and it acts as a prebiotic, feeding beneficial gut bacteria.

The Role of Gut Microbes in Polysaccharide Fermentation

While humans cannot digest certain polysaccharides, our gut microbiota can. The colon is home to trillions of bacteria, including species of Bacteroides and Firmicutes, that possess the necessary carbohydrate-active enzymes (CAZymes) to ferment these complex, indigestible fibers.

Fermentation is the metabolic process by which these gut bacteria break down polysaccharides into valuable byproducts, primarily short-chain fatty acids (SCFAs) like acetate, propionate, and butyrate. SCFAs are a crucial energy source for the cells lining the colon and play a significant role in maintaining gut health, regulating metabolism, and influencing immune function. This symbiotic relationship highlights that even indigestible polysaccharides are not without function; they play a vital role in nourishing the gut ecosystem.

A Comparison of Polysaccharide Digestion


Polysaccharide	Digestibility by Human Enzymes	Site of Breakdown	Mechanism	Primary End Products	Ultimate Fate in Body
Starch	High	Mouth, Small Intestine	Enzymatic Hydrolysis (Amylase)	Maltose, Glucose	Absorbed for energy
Glycogen	High	Small Intestine	Enzymatic Hydrolysis	Glucose	Absorbed for energy
Cellulose	None	Colon (by microbiota)	Fermentation	Short-Chain Fatty Acids (SCFAs)	Excretion, used by colon cells
Inulin	Limited	Colon (by microbiota)	Fermentation	Short-Chain Fatty Acids (SCFAs)	Excretion, used by colon cells

Conclusion: The Spectrum of Polysaccharide Breakdown

In conclusion, the question, "Can polysaccharides be broken down?" doesn't have a single answer. The digestibility of these complex carbohydrates exists on a spectrum, determined by their molecular structure. Digestible polysaccharides like starch and glycogen are efficiently broken down by human enzymes to provide rapid energy, while indigestible fibers like cellulose are processed by our gut microbiome. This bacterial fermentation yields beneficial short-chain fatty acids, underscoring the complexity and sophistication of our digestive system and its reliance on symbiotic microorganisms for complete nutritional processing. The intricate interplay between human enzymes, the gut microbiota, and the chemical composition of polysaccharides dictates their ultimate fate and their contribution to overall health.

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Frequently Asked Questions

Humans can digest polysaccharides such as starch and glycogen. This is possible because our digestive system, specifically the salivary glands and pancreas, produces amylase enzymes that are capable of breaking the $\alpha$-glycosidic bonds found in these carbohydrates.

Humans cannot digest cellulose because our bodies do not produce the enzyme cellulase. Cellulose is a linear chain of glucose units linked by $\beta$-glycosidic bonds, and our enzymes are specific to the $\alpha$-bonds found in starch and glycogen.

Indigestible polysaccharides, or dietary fibers, pass through the stomach and small intestine largely unchanged. They enter the large intestine where they are fermented by gut bacteria. This process produces beneficial short-chain fatty acids (SCFAs), which support colon health and overall metabolism.

The digestion of polysaccharides is minimal in the stomach. While salivary amylase from the mouth may have some initial action, it is quickly inactivated by the stomach's low pH (high acidity). The stomach's gastric juice does not contain carbohydrase enzymes.

Starch is primarily broken down by amylase. Salivary amylase begins the process in the mouth, and pancreatic amylase continues the hydrolysis in the small intestine. Other intestinal enzymes, such as maltase, further break down the resulting smaller sugars.

Yes, gut bacteria play a crucial role in breaking down certain polysaccharides, especially dietary fibers that humans cannot digest. Microbes in the colon ferment these carbohydrates, producing short-chain fatty acids and other compounds beneficial to the host.

The complete digestion of digestible polysaccharides like starch and glycogen results in monosaccharides, primarily glucose, which are then absorbed into the bloodstream for energy. For indigestible polysaccharides like fiber, the end products are short-chain fatty acids from microbial fermentation.