Starch: The Staple of the Digestible Polysaccharides
Starch, a complex carbohydrate produced by most green plants, is unquestionably the most common digestible polysaccharide in foods. Plants produce and store starch as a reserve energy source, which is then consumed by humans and other animals. Starch is a significant component of the human diet, accounting for a large percentage of our total energy intake. Pure starch is a white, tasteless, and odorless powder that is insoluble in cold water. It is stored in plants in the form of granules within chloroplasts and in storage organs like roots, tubers, and seeds.
The Dual-Component Structure of Starch
Starch is not a single molecule but rather a mixture of two distinct polymers of glucose: amylose and amylopectin. The proportion of these two polymers varies depending on the botanical source, but most starches contain 20–25% amylose and 75–80% amylopectin.
- Amylose: This is a linear, unbranched polymer consisting of glucose units joined by α-1,4-glycosidic bonds. Its structure forms a helical shape, which makes it less soluble in water and more resistant to rapid digestion. Starches with a higher amylose content, such as certain varieties of corn, are often more slowly digested.
- Amylopectin: This is a highly branched polymer of glucose units. It contains the same α-1,4-glycosidic bonds as amylose but also features occasional α-1,6-glycosidic bonds that create the branching points. The branched structure makes amylopectin more soluble in water and significantly easier for digestive enzymes to access and break down. This is why waxy starches, which are high in amylopectin, are rapidly digestible.
The Digestion of Starch
For humans and other animals, starch from plants is broken down into its constituent glucose molecules to supply energy to the body's tissues. The digestion of starch begins in the mouth and continues in the small intestine through the action of enzymes called amylases.
- Oral Digestion: The process begins in the mouth where salivary α-amylase, an enzyme in saliva, starts to break down the complex starch molecules into smaller polysaccharides and the disaccharide maltose. Chewing increases the surface area of the food, allowing the enzyme to work more effectively.
- Gastric Processing: In the stomach, the acidic environment inhibits the activity of salivary amylase, and mechanical contractions continue to break down the food into a semi-liquid called chyme. While starch digestion is minimal here, the process of gastric emptying, which delivers the chyme to the small intestine, is a critical factor influencing the overall digestion rate.
- Intestinal Digestion: The main phase of starch digestion occurs in the small intestine. The pancreas secretes pancreatic α-amylase, which continues the process of hydrolyzing starch into maltose, maltotriose, and limit dextrins. At the surface of the small intestine's lining, or brush border, additional enzymes like maltase and isomaltase further break down these smaller carbohydrate units into absorbable glucose monomers.
Once converted to glucose, the simple sugar is absorbed through the small intestinal wall into the bloodstream. From there, it is transported to the body's cells for energy or stored in the liver and muscles as glycogen for future use.
Common Sources of Dietary Starch
Numerous foods, particularly plant-based ones, are rich in starch. These include:
- Grains and Grain Products: Wheat, rice, corn, oats, barley, and products like bread, pasta, and cereals are primary sources of starch globally.
- Tubers and Root Vegetables: Potatoes, sweet potatoes, cassava (tapioca), and yams are well-known starchy foods.
- Legumes: Dried beans, lentils, peas, and chickpeas are excellent sources of starch, often containing types that are more slowly digestible.
Comparison of Starch and Other Polysaccharides
To understand why starch is the most common digestible polysaccharide, it's helpful to compare it to other prominent polysaccharides, such as cellulose and glycogen.
| Feature | Starch | Cellulose | Glycogen |
|---|---|---|---|
| Source | Plants (seeds, roots, tubers) | Plants (cell walls) | Animals (liver and muscles) |
| Chemical Linkages | Primarily α-1,4 and α-1,6 glycosidic bonds | β-1,4 glycosidic bonds | Primarily α-1,4 and highly frequent α-1,6 glycosidic bonds |
| Structure | A mixture of linear (amylose) and branched (amylopectin) chains | A linear, fibrous structure | A very highly branched structure |
| Digestibility in Humans | Digestible. Enzymes (amylases) can break the α-linkages. | Indigestible. Lacks the enzymes (cellulase) to break the β-linkages. | Digestible. Broken down to glucose via glycogenolysis. |
| Function | Primary energy storage in plants | Provides structural support for plants (dietary fiber) | Primary energy storage in animals |
The Importance of Starch Digestibility
The digestibility of starch is not a constant value and can be influenced by several factors. Cooking, for instance, significantly increases starch digestibility by causing the granules to swell and burst, a process called gelatinization. Conversely, retrogradation—the recrystallization of starch chains after a cooked starchy food has cooled—can create resistant starch (RS), which behaves more like dietary fiber. Resistant starch can have positive effects on blood sugar control and gut health. Different types of starch (rapidly digestible, slowly digestible, and resistant) also affect metabolic responses differently. For example, slowly digestible starches, found in whole grains and legumes, lead to a more gradual release of glucose and a more stable blood sugar level compared to rapidly digestible starches in white bread or processed foods.
Conclusion
Starch, the most common digestible polysaccharide in the human diet, is a critical energy source that originates from a wide range of plant-based foods. Its two main components, amylose and amylopectin, have different structures that influence their digestibility. The body's amylase enzymes are specifically adapted to break down the α-glycosidic bonds in starch, a capability that sets it apart from other common polysaccharides like cellulose, which is indigestible to humans. Understanding the nature of starch and how factors like cooking and cooling can alter its digestibility is key to appreciating its central role in nutrition and health.
The Role of Starch in the Global Food System
Beyond its nutritional significance, starch is an industrially vital compound. Most commercial starch is extracted from abundant crops like corn, wheat, and potatoes. It is used extensively in food processing as a thickening agent, binder, and stabilizer in everything from sauces and puddings to baked goods and confections. The ability to modify starches has further expanded their applications, allowing them to withstand various processing conditions and deliver specific functional properties. These applications underscore the ubiquity and versatility of starch, solidifying its status as not only the most common digestible polysaccharide in our diet but also a cornerstone of the modern food industry.
Food and Agriculture Organization of the United Nations (FAO)
Starch vs. Fiber: A Matter of Linkage
The difference in digestibility between starch and indigestible polysaccharides like cellulose comes down to the specific chemical bonds that link their glucose units. Starch has α-1,4 and α-1,6 glycosidic linkages, which can be broken by human digestive enzymes. Cellulose has β-1,4 glycosidic linkages, and humans lack the enzyme (cellulase) needed to break them, meaning it passes through the digestive system largely intact. This fundamental chemical distinction is the reason why one provides energy and the other provides dietary fiber.
