The Indigestible Polysaccharide: Cellulose
The primary polysaccharide that humans cannot digest is cellulose. This complex carbohydrate is a major component of plant cell walls, giving plants their rigid structure. It is made of long chains of glucose molecules linked by beta-1,4-glycosidic bonds. The crucial difference between cellulose and digestible starches lies in these chemical bonds.
Our digestive system produces enzymes called amylases that are very efficient at breaking the alpha-1,4-glycosidic bonds found in starches and glycogen, which are digestible polysaccharides. However, humans do not produce the enzyme called cellulase, which is required to cleave the beta-1,4-glycosidic bonds of cellulose. Because of this enzymatic deficiency, the vast majority of cellulose we consume is not broken down into absorbable glucose molecules in the small intestine. Instead, it travels through the digestive tract largely untouched, serving a different, yet vital, purpose.
The Crucial Role of Dietary Fiber
Although indigestible, cellulose is a form of insoluble dietary fiber, and it is a cornerstone of a healthy diet. Instead of providing direct energy, it contributes to digestive health in several ways:
- Increases Bulk: As it passes through the intestines, cellulose adds bulk to stool, which helps with regular bowel movements and prevents constipation.
- Promotes Transit Time: It speeds up the movement of waste through the digestive tract. A faster transit time reduces the risk of exposure to potential toxins and reduces the likelihood of conditions like diverticulitis and hemorrhoids.
- Supports Gut Microbiota: The presence of fiber in the large intestine provides a food source for beneficial bacteria, which ferment the fiber into useful compounds.
Foods rich in cellulose include vegetables, fruits, whole grains, nuts, and legumes. Incorporating these items into your diet is essential for maintaining proper digestive function.
Other Polysaccharides Humans Struggle To Digest
Besides cellulose, other polysaccharides also resist digestion in the upper gastrointestinal tract and are classified as dietary fiber. The degree to which they are fermented by gut bacteria varies.
Resistant Starch
Resistant starch is a type of starch that, as the name suggests, resists digestion in the small intestine and proceeds to the large intestine for fermentation. There are several types of resistant starch based on their source and structure:
- RS1: Found in grains and seeds, physically inaccessible to digestive enzymes.
- RS2: Found in raw potatoes and unripe bananas, indigestible due to its granular structure.
- RS3: Forms when cooked starchy foods like rice, pasta, and potatoes are cooled.
- RS4: Artificially made through chemical modification.
Pectin and Hemicellulose
- Pectin: This polysaccharide is a component of plant cell walls found in fruits like apples and citrus. It is a soluble dietary fiber that binds to cholesterol and slows glucose absorption in the small intestine before being fermented by gut microbes in the colon.
- Hemicellulose: This is another component of plant cell walls, and while it is also largely indigestible by human enzymes, our gut microbiota can ferment a significant portion of it in the large intestine.
The Benefits of Fermentation by Gut Bacteria
When these indigestible polysaccharides reach the large intestine, our resident gut microbiota get to work. Through fermentation, they break down the fibers into short-chain fatty acids (SCFAs), such as butyrate, propionate, and acetate. These SCFAs provide numerous health benefits, including:
- Fuel for Colon Cells: Butyrate is a preferred energy source for the cells lining the colon, helping to maintain a healthy intestinal barrier.
- Reduced Inflammation: SCFAs have anti-inflammatory properties that can help protect against gut diseases.
- Improved Immune Function: A healthy gut microbiome supported by fiber contributes positively to the body's overall immune system.
Comparison of Digestible vs. Indigestible Polysaccharides
| Feature | Digestible Polysaccharides (e.g., Starch, Glycogen) | Indigestible Polysaccharides (e.g., Cellulose, Fiber) |
|---|---|---|
| Primary Function | Energy storage | Structural support in plants; promotes digestive health in humans |
| Glucose Linkage | Alpha-glycosidic bonds | Beta-glycosidic bonds |
| Enzyme Requirement | Human amylase enzymes break these bonds | Human body lacks the necessary cellulase enzyme |
| Digestion Site | Broken down in the mouth, stomach, and small intestine | Mostly passes through the digestive system intact |
| Energy Absorption | Glucose is absorbed into the bloodstream for energy | Very little to no energy absorbed by the host |
| Passage in Intestines | Absorbed, leaving little residue | Forms bulk and speeds passage of waste |
| Microbial Fermentation | Little to none (unless resistant starch) | Fermented by colonic bacteria into beneficial SCFAs |
Conclusion: The Importance of Indigestible Polysaccharides
To the human body, indigestible polysaccharides are anything but useless. While lacking the enzymes to break down cellulose and other fibers for energy, we rely on them for essential digestive functions and to maintain a healthy gut microbiome. The fermentation of these fibers by gut bacteria produces vital short-chain fatty acids that nourish our cells and support our immune system. Therefore, consuming a diet rich in a variety of dietary fibers, including those with different digestion properties, is a crucial step towards optimal health.
For more detailed information on resistant starch and gut health, visit the National Institutes of Health website at: https://pmc.ncbi.nlm.nih.gov/articles/PMC6116026/
