Polysaccharides are a major component of carbohydrates, which are a necessary part of the human diet. These large molecules are essentially long chains built from many individual sugar units linked together, hence the name “poly” (many) “saccharide” (sugar). They are found in a wide variety of both plant and animal sources and perform two main functions in food: serving as a source of stored energy or providing structural support. The way the sugar units are linked determines whether the human body can break them down for fuel or if they pass through the digestive system largely intact.
Understanding Polysaccharides in Food
Polysaccharides are classified based on their structure and how they interact with the human digestive system, creating two major functional categories. The first category includes digestible polysaccharides, which are primarily used for energy storage in living organisms. The key examples of these energy sources are starch, found in plants, and glycogen, found in animals.
The second category is composed of non-digestible polysaccharides, commonly known as dietary fiber. These molecules are used for structural support in plants, such as in cell walls. The main types of fiber include cellulose, pectin, and various plant gums.
Digestible Polysaccharides: Energy Sources
The most common polysaccharide in the human diet is starch, which plants use as their main method of storing energy. Starch molecules are composed of two polymers, amylose and amylopectin, both of which are long chains of glucose units that human enzymes can easily break down. This process yields glucose, which the body then absorbs as its primary source of fuel.
Starch is abundant in the seeds, roots, and tubers of plants, providing a dense source of calories. Common food examples include grains like wheat, rice, corn, oats, and barley, which form the basis for staples such as breads and pastas. Starchy root vegetables (potatoes, sweet potatoes, yams, and cassava) and legumes (beans, lentils, and peas) are also rich sources of this polysaccharide.
Glycogen is the animal equivalent of starch and serves as the primary energy storage form in humans and other animals. This highly branched molecule is stored predominantly in the liver and muscle tissue. Glycogen is not a significant dietary source for humans because most of it breaks down after the animal is slaughtered.
Non-Digestible Polysaccharides: Structural and Fiber Sources
Non-digestible polysaccharides, collectively referred to as dietary fiber, are molecules that human digestive enzymes cannot break apart. These structures pass through the small intestine mostly unchanged and are fermented by bacteria in the large intestine. These polysaccharides provide plants with rigidity and form.
Cellulose is the most well-known structural polysaccharide, forming the rigid cell walls of all plants. It is found in the stalks and skins of vegetables, such as celery and the outer layers of whole grains. Although humans cannot digest cellulose, it contributes significantly to the bulk of the stool, promoting healthy bowel movements.
Other non-digestible polysaccharides, which are often soluble in water, include pectin and plant gums. Pectin is abundant in the cell walls of fruits, particularly in the rinds of citrus fruits and in apples, and it is the component that allows jams and jellies to form a gel.
Gums and mucilages are found in the seeds and kernels of plants, such as guar gum from guar beans and the soluble fiber in oats and barley. The slimy consistency of soaked chia and flax seeds comes from these structural polysaccharides. These fibers are important for promoting a healthy gut environment.