Are all polysaccharides digestible? The science behind starches and fibers

December 4, 2025 •

4 min read

Approximately 50% of the total energy intake in the human diet is supplied by starch, a major polysaccharide. This fact underscores a critical point in human nutrition: not all complex carbohydrates are created equal, prompting the question, are all polysaccharides digestible?

Quick Summary

No, not all polysaccharides are digestible by humans. Digestible starches are broken down into glucose for energy, while indigestible fibers, like cellulose, pass to the large intestine where they are fermented by gut bacteria and contribute to overall health.

Key Points

Digestibility is not universal: Not all polysaccharides are digestible by humans; the ability to digest depends on the specific chemical bonds present in the molecule.
Enzymes are key: Humans can digest starches and glycogen because we produce enzymes like amylase that can break their alpha-glycosidic bonds.
Fiber is indigestible: Indigestible polysaccharides, such as cellulose and pectin, are considered dietary fiber because humans lack the necessary enzymes (like cellulase) to break their beta-glycosidic bonds.
Gut bacteria fill the gap: What humans can't digest, gut bacteria can. Indigestible fibers are fermented by colonic bacteria, producing beneficial short-chain fatty acids.
Health benefits differ: Digestible polysaccharides provide a primary energy source, whereas indigestible polysaccharides improve gut health, regulate blood sugar, and lower cholesterol.
Processing alters resistance: The way food is processed can change its digestibility. For example, cooling cooked starches can create resistant starch, an indigestible form of starch.

Understanding the different types of polysaccharides

Polysaccharides are long chains of monosaccharide units, but their structure dictates how they are processed by the human body. Based on their fate within our digestive system, they can be broadly classified as either digestible or indigestible. The distinction primarily lies in the type of chemical bonds that link the sugar units together and the availability of specific human enzymes to break those bonds.

Digestible polysaccharides

These are the carbohydrates our bodies can break down and absorb for energy. They are characterized by alpha-glycosidic bonds, which human enzymes are equipped to cleave.

Starch: The most common digestible polysaccharide in human diets, found abundantly in plant foods like potatoes, rice, wheat, and maize. Starch is a polymer of glucose, consisting of a mixture of amylose (linear chains) and amylopectin (branched chains).
- The digestion process: Digestion begins in the mouth with salivary amylase. This process pauses in the acidic stomach before resuming in the small intestine, where pancreatic amylase and brush-border enzymes further break down starch into glucose for absorption.
Glycogen: This acts as the primary storage form of glucose in animals and humans, predominantly stored in the liver and muscles. Glycogen is highly branched, which allows for a rapid release of glucose when the body needs quick energy. Digestion is similar to starch, with amylase enzymes breaking the bonds to yield glucose.

Indigestible polysaccharides (dietary fiber)

These complex carbohydrates cannot be broken down by human digestive enzymes. Instead of providing calories, they contribute to gut health and the bulk of stool. Their structure is defined by beta-glycosidic bonds that humans cannot hydrolyze.

Cellulose: The most abundant organic polymer on Earth, forming the structural component of plant cell walls. It is a linear chain of glucose units linked by beta-1,4-glycosidic bonds. Humans lack the enzyme cellulase to break these bonds, which is why it passes through the digestive tract largely intact as insoluble fiber.
Hemicellulose: A diverse group of non-cellulosic polysaccharides also found in plant cell walls. Some forms are soluble, while others are insoluble.
Pectin and Gums: Pectins are found in the cell walls of fruits and vegetables, while gums like guar and xanthan are used as thickeners. Both are soluble fibers that ferment in the large intestine and can increase the viscosity of digesta.
Resistant Starch: A fraction of starch that, for various reasons, resists digestion in the small intestine and functions like dietary fiber. There are five different types, including physically protected starch in whole grains (RS1) and retrograded starch formed after cooking and cooling (RS3).

Comparison of digestible vs. indigestible polysaccharides


Feature	Digestible Polysaccharides (Starch, Glycogen)	Indigestible Polysaccharides (Fiber)
Primary Bond Type	Alpha-glycosidic bonds	Beta-glycosidic bonds
Human Enzymes	Broken down by amylase and other enzymes	Not broken down by human enzymes
Energy Yield	High energy source, broken down into glucose	Not a direct energy source for humans
Digestive Pathway	Digested and absorbed in the small intestine	Pass through the small intestine to the colon
Health Function	Provides readily available energy for cells	Promotes gut motility, feeds beneficial bacteria, produces SCFAs
Example Sources	Potatoes, rice, bread, corn	Whole grains, legumes, vegetables, unripe bananas

The role of gut microbiota

While humans cannot digest certain polysaccharides, the bacteria residing in our large intestine, known as the gut microbiota, can. These microbes possess the necessary enzymes to ferment indigestible carbohydrates, a process that produces short-chain fatty acids (SCFAs) like butyrate. These SCFAs serve as a crucial energy source for the cells lining the colon and have been linked to a wide range of health benefits, including regulating metabolism, boosting immune function, and reducing inflammation. This symbiotic relationship is why dietary fiber is considered so vital for gut and overall health.

Health implications of indigestible polysaccharides

Incorporating a variety of indigestible polysaccharides into your diet has numerous health advantages beyond just providing bulk. For instance, soluble fibers can bind to bile acids, aiding in cholesterol reduction. Different types of fiber also play a role in blood sugar control by slowing down the absorption of glucose. Moreover, the prebiotic effects of many indigestible polysaccharides, such as inulin, promote the growth of beneficial gut bacteria, leading to a healthier microbial community. The fermentation of resistant starch, in particular, has been shown to support colon health and may even guard against genetic damage that can lead to bowel cancer.

How processing affects digestibility

Food processing and preparation methods can significantly alter the digestibility of polysaccharides. For example, cooking starchy foods like potatoes and rice causes the starch granules to swell and gelatinize, making them more accessible to digestive enzymes. However, once cooled, some of this starch can recrystallize into a form that is resistant to digestion (resistant starch type 3). The degree of milling also affects digestibility; coarser milling, which leaves more of the fibrous plant cell walls intact, can reduce the digestibility of starch. Conversely, certain chemical modifications (RS4) and the formation of amylose-lipid complexes (RS5) can also make starches resistant to enzymatic breakdown. Understanding how these factors influence the structure of carbohydrates is crucial for manipulating their digestion properties to achieve specific health outcomes.

For a deeper understanding of the beneficial effects of polysaccharides on intestinal microbiota, consult this detailed article from the National Institutes of Health: Beneficial Effect of Intestinal Fermentation of Natural Polysaccharides.

Conclusion

In summary, the answer to the question "are all polysaccharides digestible?" is a definitive no. The varying chemical structures, particularly the type of glycosidic bonds, determine whether a polysaccharide is broken down for energy or passes through as dietary fiber. Digestible starches and glycogen fuel our bodies with glucose, while indigestible fibers nourish our gut microbiome and contribute to a host of health benefits, from improved digestion to enhanced metabolic health. Incorporating a diverse range of polysaccharides, especially fiber-rich ones, is key to a healthy and balanced diet that supports the complex ecosystem within our digestive tract.

Frequently Asked Questions

Humans cannot digest cellulose because our digestive system does not produce the enzyme cellulase. This enzyme is required to break the beta-glycosidic bonds that link the glucose units in cellulose.

The main difference is the type of chemical bonds. Starch has alpha-glycosidic bonds that human enzymes can break, providing energy. Cellulose has beta-glycosidic bonds, which humans cannot break, making it an indigestible fiber.

Resistant starch is a type of starch that resists digestion in the small intestine and functions like dietary fiber. It provides food for beneficial gut bacteria, leading to the production of beneficial short-chain fatty acids.

Indigestible polysaccharides, or dietary fiber, pass through the stomach and small intestine largely unchanged. They are then fermented by bacteria in the large intestine, a process that produces short-chain fatty acids.

Insoluble fibers, like cellulose, add bulk to stool, promoting regular bowel movements. Soluble fibers form a gel-like substance that can slow down digestion and affect nutrient absorption.

Yes, some herbivores like cows and termites can digest cellulose, but they don't do it directly. They rely on symbiotic microorganisms, which produce the necessary cellulase enzymes in their digestive tracts.

Yes. Cooking typically increases starch digestibility by gelatinizing it. However, cooling a cooked starch, like pasta or potatoes, can cause some of the starch to retrograde and become less digestible, increasing its resistant starch content.