4.1: Classification of Carbohydrates

Carbohydrates are categorized based on the number of sugar units they contain (Figure 4.2). Simple carbohydrates include monosaccharides and disaccharides. Monosaccharides are single sugar units and include glucose, fructose, and galactose. Disaccharides are two sugar units linked together through covalent glycosidic bonds and include sucrose, lactose, and maltose. Complex carbohydrates include oligosaccharides and polysaccharides. Oligosaccharides are chains of 3-10 monosaccharides, including raffinose and stachyose. Polysaccharides are long chains of monosaccharides, typically more than 10 units long. These include starch, glycogen, and dietary fiber.

Figure 4.2 Classification of carbohydrates. Simple carbohydrates include monosaccharides and disaccharides, while complex carbohydrates include oligosaccharides and polysaccharides. Image by Author.

Simple Carbohydrates

Monosaccharides

Glucose is the most abundant monosaccharide in the body. It is produced through photosynthesis, which is a multi-step process that requires sunlight, carbon dioxide, and water as substrates (Figure 4.3). When chlorophyllcontaining plants combine carbon dioxide and water in the presence of sunlight, oxygen is released and the energy from sunlight is stored as chemical energy in simple carbohydrate molecules. These sugar molecules contain energy and the energized carbon that all living things need to survive. This provides an energy source for plants, and when plant foods are consumed, the body breaks down and digests large carbohydrates into glucose, and the body’s cells use it for energy.

Figure 4.3 Photosynthesis uses solar energy, carbon dioxide, and water to produce carbohydrates. Oxygen is generated as a waste product of photosynthesis. Modified by Author from Photosynthesis gif by Riyasachdeva250 licensed by CC BY-SA 4.0 International license.

The primary function of glucose is to provide cells with energy in the form of ATP. It is found naturally in many carbohydrate-rich foods like fruits, vegetables, and grains. Glucose is absorbed into the bloodstream following digestion. The hormone insulin, produced by the pancreas, helps transport glucose from the blood into cells, where it is used for energy. Unused glucose gets stored as glycogen (a complex carbohydrate) in the liver and skeletal muscles for future energy use. It can also be converted into amino acids and fat for energy storage. Maintaining balanced blood glucose levels is essential for overall health, as this monosaccharide is the preferred energy source for the nervous system and red blood cells.

Fructose and galactose are also simple sugars like glucose, though they differ in structure and function (Figure 4.4). Fructose, commonly referred to as "fruit sugar," is naturally found in fruits, honey, and some root vegetables. It is the sweetest naturally occurring carbohydrate and is also a component of sucrose (table sugar), which is a disaccharide composed of one molecule of glucose and one molecule of fructose. Fructose is often consumed in higher amounts through processed foods and beverages sweetened with high-fructose corn syrup, which has raised concerns about its potential contribution to metabolic disorders when consumed in excess.

Galactose has a similar chemical structure to glucose (Figure 4.4), and while less sweet than fructose, it is equally important in the human diet. It is most commonly consumed as part of lactose, the disaccharide found in milk and dairy products, which consists of glucose and galactose. During digestion, the enzyme lactase breaks down lactose into its monosaccharide components, which are then absorbed in the small intestine. The body uses galactose to make components of cell membranes and synthesize lactose, but the majority of it is transported to the liver, where it is converted into glucose to be used for energy or storage.

Figure 4.4 Chemical structures of glucose, galactose, and fructose, each having a 6-carbon structure. Figure 3.5 by Mary Ann Clark, Matthew Douglas, Jung Choi, OpenStax from Biology 2e (2018) is licensed under Creative Commons Attribution 4.0 International license.

Especially when considering overall carbohydrate intake, understanding these sugars' sources and how they are processed by the body is essential for evaluating their roles in health and disease.

Disaccharides

Disaccharides are defined as two monosaccharides joined by glycosidic linkage. Glycosidic linkages are covalent bonds that form between two monosaccharides during a dehydration synthesis (or condensation) reaction. In this process, a hydroxyl group (–OH) from one monosaccharide reacts with a hydrogen atom (–H) from another, resulting in the release of a water molecule (H₂O) and the formation of an oxygen bridge between the two sugar units (Figure 4.5). The position and orientation of the glycosidic bond—such as α(1→4) or β(1→4)—determine the structural and functional properties of the resulting disaccharide (or polysaccharide). For example, an α(1→4) glycosidic bond between two glucose molecules forms maltose, while a β(1→4) linkage between glucose and galactose forms lactose. The human digestive system has specific enzymes that recognize and break certain glycosidic bonds; for example, amylase breaks α(1→4) linkages in starch, while lactase is required to break the β(1→4) bond in lactose. Glycosidic linkages are ultimately important in determining whether a carbohydrate is digested or not, and how it is digested.

Figure 4.5 A glycosidic bond forms between glucose and fructose (monosaccharides) to form the disaccharide sucrose. Dehydration Synthesis by Mary Ann Clark, Matthew Douglas, Jung Choi, OpenStax from Biology 2e (2018) is licensed under Creative Commons Attribution 4.0 International license.

The most common disaccharides include lactose, maltose, and sucrose (Figure 4.6). Lactose consists of glucose bonded to galactose with a β(1→4) linkage and is sometimes referred to as “milk sugar” because it is the most abundant carbohydrate found in milk and most products made from milk. Lactose serves as an important energy source, particularly for infants. During lactation, enzymes in the mammary glands combine glucose and galactose to produce lactose.

Figure 4.6 Three important disaccharides are sucrose, lactose, and maltose. Sucrose and maltose are bonded with alpha glycosidic bonds, while lactose is formed with a beta glycosidic bond. Figure 3.8 by Mary Ann Clark, Matthew Douglas, Jung Choi, OpenStax from Biology 2e (2018) is licensed under Creative Commons Attribution 4.0 International license.

In the small intestine, lactose is broken down into its monosaccharide components by the enzyme lactase, which is produced by the enterocytes (the epithelial cells lining the intestinal wall). However, in many individuals, lactase production decreases after weaning (the transition from exclusively breastfeeding or formula-feeding to consuming other foods), leading to a condition known as lactose intolerance (Figure 4.7). Some people, however, do continue to produce lactase into adulthood, a trait known as lactase persistence.

Figure 4.7 Lactose intolerant individuals do not have the enzyme lactase available to break down the disaccharide lactose in the small intestine. This results in the breakdown of lactose in the large intestine by bacteria, typically leading to gastrointestinal distress symptoms. Image by Author.

People with lactose intolerance, however, are unable to fully digest lactose in the small intestine, resulting in the lactose reaching the large intestine undigested. Bacteria in the large intestine break down the lactose, leading to gastrointestinal symptoms such as bloating, gas, abdominal pain, and diarrhea. The severity of symptoms varies depending on the amount of lactose consumed and the individual's remaining lactase activity. Management typically involves reducing or eliminating lactose-containing foods, using lactose-free alternatives, or taking lactase enzyme supplements.

Sucrose and maltose are also disaccharides. Sucrose, commonly known as table sugar, is composed of one molecule of glucose and one molecule of fructose linked by an α(1→2) glycosidic bond. It is naturally found in many fruits and vegetables and is commercially extracted from sugar cane and sugar beets, processed to make molasses, and further treated to produce crystallized sucrose (table sugar) for widespread use as a sweetener. Sucrose is easily digested in the small intestine by the enzyme sucrase, which breaks it down into glucose and fructose for absorption. Sucrose is a significant source of added sugars in the modern diet.

Maltose, on the other hand, is composed of two glucose molecules joined by an α(1→4) glycosidic bond. It is not found in many foods, but is produced during the enzymatic breakdown of starch, particularly during digestion or the malting process used in brewing beer. For example, enzymes convert starches from barley to maltose, which is then fermented by bacteria to produce alcohol. During digestion in the small intestine, the enzyme maltase hydrolyzes maltose into two glucose molecules, which can then be used for energy.

Naturally occurring sugars are found in whole, unprocessed foods such as fruits, vegetables, dairy products, and some grains. These sugars—primarily fructose, glucose, and lactose—are naturally integrated into the food’s matrix, which also includes fiber, vitamins, minerals, and other beneficial compounds. For example, fruits have fructose but are also rich in vitamins, minerals, and fiber. In contrast, added sugars, though chemically identical to naturally occurring sugars, are those incorporated during food processing or preparation, such as sucrose or high-fructose corn syrup added to sodas, baked goods, and condiments. Baked goods with added sugars, for example, don’t often have nutritional value beyond the calories they contain.

Excessive intake of added sugars has been associated with an increased risk of obesity, type 2 diabetes, cardiovascular disease, and dental caries. The Dietary Guidelines for Americans recommends that no more than 10% of one’s daily caloric intake should come from added sugars. For example, for a 2,000 calorie diet, no more than 200 calories should come from added sugars. This equates to approximately 12 teaspoons of added sugar from both food and beverages. Unfortunately, the average American consumes upwards of 20 teaspoons of added sugars each day.

In response to these recommendations and statistics, and amid growing health concerns, artificial sweeteners were developed (Figure 4.8). These synthetic or naturally derived compounds, such as aspartame, sucralose, saccharin, and stevia, provide intense sweetness with little to no caloric content. They are widely used in "diet" or "sugar-free" products and are regulated for safety by agencies like the FDA. While artificial sweeteners can help reduce added sugar intake, their long-term health effects continue to be a topic of research and debate, especially regarding their impact on appetite regulation, gut microbiota, and metabolic health.

Figure 4.8 Artificial sweeteners. Image from Flickr by abbyladybug licensed by CC BY-NC 2.0.

Complex Carbohydrates

Oligosaccharides

Oligosaccharides consist of between three and ten monosaccharides joined by glycosidic linkage. Raffinose and stachyose are the two most common oligosaccharides, found as components of glycoproteins and glycolipids in the cell membrane and are important for cell-tocell recognition and interactions (Figure 4.9). Raffinose and stachyose are found primarily in sources such as dried beans, soybeans, peas, and lentils. Humans lack the enzymes necessary to break these oligosaccharides down, therefore, they pass undigested into the large intestine where they are broken down by bacteria. As a result, individuals often experience gastrointestinal symptoms such as bloating, cramps, or gas production when consuming common sources of these oligosaccharides.

Figure 4.9 Oligosaccharides are found in the cell membrane as part of glycoproteins and/or glycolipids. “Fluid Mosaic Model” by Connectivid-D is licensed under the Creative Commons Attribution Share-Alike 4.0 International license.

Oligosaccharides consist of between three and ten monosaccharides joined by glycosidic linkage. Raffinose and stachyose are the two most common oligosaccharides, found as components of glycoproteins and glycolipids in the cell membrane and are important for cell-tocell recognition and interactions (Figure 4.9). Raffinose and stachyose are found primarily in sources such as dried beans, soybeans, peas, and lentils. Humans lack the enzymes necessary to break these oligosaccharides down, therefore, they pass undigested into the large intestine where they are broken down by bacteria. As a result, individuals often experience gastrointestinal symptoms such as bloating, cramps, or gas production when consuming common sources of these oligosaccharides.

Polysaccharides

Though polysaccharides are classified as complex carbohydrates with more than 10 monosaccharides linked together, most polysaccharides are made of hundreds of monosaccharides bonded together in various arrangements. Ultimately, the types and arrangements of sugar molecules determine the shape and form of each polysaccharide. The three most common polysaccharides include starch, glycogen, and dietary fiber.

Starch is a complex carbohydrate most commonly found in plant-based foods such as grains like corn and wheat, pasta, bread, legumes, and potatoes. It is composed of two types of polysaccharides: amylose and amylopectin, both entirely made of glucose monomers. Amylose consists of long, unbranched chains of glucose molecules linked primarily by α-1,4 glycosidic bonds, which makes it more resistant to digestion. In contrast, amylopectin is highly branched, containing both α-1,4 and α-1,6 glycosidic bonds (α-1,6 bonds at the branch points of the structure). This extensive branching allows for quicker digestion and a faster rise in blood glucose levels when the structure is broken down. The ratio of amylose to amylopectin in a food affects its digestibility, glycemic response (how quickly and how much a food raises blood glucose levels after consumption), and texture.

Glycogen is another polysaccharide made entirely of glucose molecules, acting as the primary form of storage of glucose in animals and humans and serving as a readily available energy reserve. It is a highly branched polysaccharide composed of glucose units linked mainly by α-1,4 glycosidic bonds, with branching occurring through α-1,6 bonds approximately every 8 to 12 glucose units (Figure 4.10). This extensive branching allows for rapid mobilization of glucose when energy is needed, especially during periods of physical activity or between meals. Glycogen is primarily stored in the liver and skeletal muscles; liver glycogen helps maintain blood glucose levels, while muscle glycogen is used locally to fuel muscular activity. Because of its structure and function, glycogen plays a critical role in energy metabolism and blood sugar regulation.

Figure 4.10 Glycogen is a highly branched arrangement of glucose molecules consisting of both α-1,4 glycosidic bonds and α-1,6 glycosidic bonds at branch points (shown in red). “Glycogen structure" by Mikael Häggström is made available under the Creative Commons CC0 1.0 Universal Public Domain Dedication.

Dietary Fiber

Dietary fiber is a type of complex carbohydrate found in plant-based foods that the body cannot digest. Fiber is made entirely of glucose molecules, though the bonds between the glucose molecules are different from what is seen in starch or glycogen. Fiber contains upward facing β-1,4 glycosidic bonds that are resistant to digestive enzymes. As a result, fiber passes from the small intestine to the large intestine relatively intact and is eventually broken down by the gut bacteria. Fiber promotes the growth of beneficial intestinal bacteria, playing a vital role in maintaining digestive health and preventing chronic diseases.

Fiber is not a required dietary component, however, many health benefits have been shown to be associated with a diet including fiber-rich foods. There are two main types of dietary fiber: soluble and insoluble. Soluble fiber absorbs water to form a gel-like substance in the intestines that protects the GI tract. This has been shown to slow digestion and help relieve constipation by softening fecal matter. It is found in foods such as oats, beans, lentils, apples, and citrus fruits. Insoluble fiber does not dissolve in water and adds bulk to the stool, promoting regular bowel movements and preventing constipation. Sources of insoluble fiber include whole grains, nuts, seeds, and the skins of fruits and vegetables (Table 4.1).

Table 4.1 Common high-fiber foods

Whole Grains	Barley, oats, quinoa, whole wheat pasta, brown rice
Legumes	Lentils, split peas, black beans, garbanzo beans, kidney beans, edamame
Vegetables	Artichoke, green peas, broccoli, Brussels sprouts, sweet potato (with skin)
Fruits	Raspberries, blackberries, pear (with skin), apple (with skin), avocado
Nuts & Seeds	Chia, flaxseed, almonds, pistachios

In addition to supporting digestive health, dietary fiber offers a range of health benefits. It helps regulate blood sugar levels by slowing digestion and allowing the body to properly absorb nutrients in food and prevent glucose spikes; this makes it especially beneficial for individuals with or at risk for type 2 diabetes. Fiber also contributes to heart health by lowering cholesterol levels by binding fats in the intestine and helping to eliminate them with stools. Furthermore, a high-fiber diet is associated with a reduced risk of developing colorectal cancer and can aid in achieving and maintaining a healthy weight by promoting a feeling of fullness and reducing overall calorie intake. Both types of fiber are essential for optimal health and should be included as part of a balanced diet.

Total fiber intake includes dietary fiber that exists naturally in plants plus any functional fiber added to a food. Functional fiber is typically derived from natural fibrous plant sources and is added to food as an ingredient during manufacturing, though has also been shown to have beneficial physiological effects.