5.2: Types of Carbohydrates

Simple and Complex Carbohydrates

Carbohydrates are the perfect nutrient to meet your body’s nutritional needs. They nourish your brain and nervous system, provide energy to all of your cells, and help keep your body fit and lean. Specifically, digestible carbohydrates provide bulk in foods, vitamins, and minerals, while indigestible carbohydrates provide a good amount of fiber with a host of other health benefits. In this section, you will learn about the different types of carbohydrates based on the way they release energy in your body.

The word "carbohydrate" means carbon (carbo-) and water (hydrate). Carbohydrates are a group of organic compounds containing a ratio of one carbon atom (C) to two hydrogen atoms to one oxygen atom. Basically, they are hydrated carbons. All carbohydrates have this same formula, (CH₂0)_n, but each carbohydrate can vary in the number of carbons and how the carbon atoms are connected. Carbohydrates are classified based on the structure of the carbon atoms (Figure \(\PageIndex{1}\)). The simplest carbohydrates are called monosaccharides. This term originates from the Greek word "mono," which means one, and "saccharide," which means sugar. Simple sugars in our diet usually have 5 or 6 carbons. Monosaccharides are the building blocks for disaccharides, which contain two connected monosaccharides, and polysaccharides, which contain many monosaccharides connected in long chains. Monosaccharides and disaccharides are also called simple sugars because they have simple structures and are more easily broken down for energy. Polysaccharides are also called complex sugars because they have complicated structures and are slowly broken down to release energy. Some complex carbohydrates, like fibers, are not broken down to release energy but are still important for digestion and overall health. Because carbohydrates are so important for energy, we will describe each type of carbohydrate below based on the way energy is released when we eat them.

Types of Carbohydrates

Figure \(\PageIndex{1}\): Classification of carbohydrates. Carbohydrates are classified based on the structure of the carbon atoms. Source: Erin Shanle licensed under CC BY 4.0.

Fast-Releasing Carbohydrates

In nutrition, carbohydrates can be broadly classified into two subgroups based on the way energy is released in our bodies. Monosaccharides and disaccharides are fast-releasing carbohydrates and are known more simply as sugars. As discussed later in this chapter, fast-releasing carbohydrates are quickly absorbed and released into the bloodstream. This can give a fast boost of energy because our cells can easily use these sugars, but that energy boost does not last long.

Monosaccharides

In our diets, the most common monosaccharides include glucose, fructose, and galactose (Figure \(\PageIndex{2}\)). These simple sugars are the building blocks for disaccharides and polysaccharides.

Glucose is the preferred energy source for all organisms, from bacteria to plants to animals. The brain is completely dependent on glucose as its energy source (except during extreme starvation conditions). In fact, it is the most abundant carbohydrate in the human body. Glucose has 6 carbons, so its chemical formula is C₆H₁₂0₆. Plants synthesize glucose through the process of photosynthesis using carbon dioxide, water, and the sun’s energy. When we eat plants, we harvest the energy stored in glucose to support life’s processes. So, the ultimate energy source for all of us is the sun!

The monosaccharide galactose differs from glucose only in that a hydroxyl (−OH) group faces in a different direction on the number four carbon (Figure \(\PageIndex{2}\)). This small structural alteration causes galactose to be less stable than glucose. As a result, the liver rapidly converts it to glucose. Most absorbed galactose is utilized for energy production in cells after its conversion to glucose. (Galactose is one of two simple sugars that are bound together to make up the sugar found in milk. It is later freed during the digestion process.)

Fructose also has the same chemical formula as glucose but differs in its chemical structure, as the ring structure contains only five carbons and not six (Figure \(\PageIndex{2}\)). Fructose, in contrast to glucose, is not an energy source for other cells in the body. Mostly found in fruits, honey, and sugarcane, fructose is one of the most common monosaccharides in nature. It is also found in soft drinks, cereals, and other products sweetened with high fructose corn syrup.

Less common monosaccharides are the pentoses, which have only 5 carbons and not 6. The pentoses are abundant in the nucleic acids RNA and DNA, and also as components of fiber.

Finally, the sugar alcohols are industrially synthesized derivatives of monosaccharides. Some examples of sugar alcohols are sorbitol, xylitol, and glycerol. Xylitol is similar in sweetness to table sugar, so it is often used in place of table sugar to sweeten foods. Sugar alcohols are incompletely digested and absorbed and are therefore less caloric. The bacteria in your mouth oppose them; thus, sugar alcohols do not cause tooth decay. However, consuming too much sugar alcohol can lead to digestive issues, such as diarrhea. Interestingly, the sensation of “coolness” that occurs when chewing gum that contains sugar alcohols comes from their dissolving in the mouth, a chemical reaction that requires heat from the inside of the mouth.

Common Monosaccharides

Figure \(\PageIndex{2}\): Structures of the three most common monosaccharides: glucose, galactose, and fructose. Red circles indicate the major structural differences between glucose and the other two monosaccharides. Source: Adapted from "File:217 Five Important Monosaccharides-01.jpg" by OpenStax College is licensed under CC BY 3.0.

Disaccharides

Disaccharides are composed of pairs of two monosaccharides linked together. Disaccharides include sucrose, lactose, and maltose (Figure \(\PageIndex{3}\)). Each of these disaccharides contains at least one glucose molecule.

Sucrose, which contains both glucose and fructose molecules, is otherwise known as table sugar. Sucrose is also found in many fruits and vegetables and at high concentrations in sugar beets and sugar cane, which are used to make table sugar. You may have also heard of high fructose corn syrup, which is a common sweetener used in many processed foods. High-fructose corn syrup is produced by processing corn starch to change some of the glucose into fructose. This creates a sweetener that is cheaper and easier to handle than table sugar. However, there are some health concerns about eating too much high-fructose corn syrup.

Lactose, commonly known as milk sugar, comprises one glucose unit and one galactose unit. Lactose is prevalent in dairy products such as milk, yogurt, and cheese.

Maltose consists of two glucose molecules bonded together. It is a common breakdown product of plant starches and is rarely found in foods as a disaccharide.

Common Disaccharides

Figure \(\PageIndex{3}\): Structures of the three most common disaccharides: sucrose, lactose, and maltose. Source: "218 Three Important Disaccharides-01" by OpenStax Anatomy and Physiology is licensed under CC BY 4.0.

Slow-Releasing Carbohydrates

Slow-releasing carbohydrates are complex carbohydrates or polysaccharides that contain long chains of monosaccharides. These chains may be branched or not branched, which means there can be a lot of variety in their structures. Slow-releasing carbohydrates take longer to digest and break down, so energy is released gradually. There are two main groups of slow-releasing carbohydrates: starches and fibers (Figure \(\PageIndex{4}\)).

Starches

Starches are found in abundance in grains, legumes, and root vegetables, such as potatoes. Amylose, a plant starch, is a linear chain containing hundreds of glucose units. Amylopectin, another plant starch, is a branched chain containing thousands of glucose units. These large starch molecules form crystals and are the energy-storing molecules of plants. These two starch molecules (amylose and amylopectine) are contained together in foods, but the smaller one, amylose, is more abundant. Eating raw foods containing starches provides very little energy as the digestive system has a hard time breaking them down. Cooking breaks down the crystal structure of starches, making them much easier to break down in the human body. The starches that remain intact throughout digestion are called resistant starches. Bacteria in the gut can break some of these down and may benefit gastrointestinal health. Isolated and modified starches are used widely in the food industry and during cooking as food thickeners.

Humans and animals store glucose energy from starches in the form of the very large molecule glycogen. It has many branches that allow it to break down quickly when energy is needed by cells in the body. It is predominantly found in liver and muscle tissue in animals.

Common Polysaccharides

Figure \(\PageIndex{4}\): Structures of polysaccharides. Polysaccharides can be branched or unbranched chains of monosaccharides (shown as hexagons). Source: "File:219 Three Important Polysaccharides-01.jpg" by OpenStax College is licensed under CC BY 3.0.

Dietary Fibers

Dietary fibers are polysaccharides that are highly branched and cross-linked. Some dietary fibers are pectin, gums, cellulose, and lignin. Humans do not produce the enzymes that can break down dietary fiber into simple sugars; however, bacteria in the large intestine (colon) can break down some fibers. Dietary fibers are very beneficial to our health. Diets with adequate fiber reduce the risk for obesity and diabetes, which are primary risk factors for cardiovascular disease. However, more than 90%t of women and 97% of men in America do not meet recommended intakes for dietary fiber, so it is considered a dietary component of public health concern.¹

Dietary fiber is categorized as either water-soluble or insoluble. Some examples of soluble fibers are inulin, pectin, and guar gum and are found in peas, beans, oats, barley, and rye. Soluble fibers can help regulate blood sugar levels, reduce blood cholesterol levels, and modulate the gut microbiome.² Insoluble fibers help materials pass through the digestive system and can prevent constipation. Cellulose and lignin are insoluble fibers, and a few dietary sources of them are whole-grain foods, flax, cauliflower, and avocados. Cellulose is the most abundant fiber in plants, making up the cell walls and providing structure. Soluble fibers are more easily accessible to bacterial enzymes in the large intestine, so they can be broken down to a greater extent than insoluble fibers, but even some breakdown of cellulose and other insoluble fibers occurs.

Some fibers can be classified as functional fibers. Functional fibers may be extracted from plants or synthetically made, and they are added to foods to provide health benefits to humans. An example of a functional fiber is psyllium-seed husk. Scientific studies show that consuming psyllium-seed husk reduces blood-cholesterol levels, and this health claim has been approved by the FDA.³ Total dietary fiber intake is the sum of dietary fiber and functional fiber consumed.

Attributions

• Zimmerman, "An Introduction to Nutrition (Zimmerman)," CC BY-NC-SA 3.0. Figures were updated and text was slightly changed.

References

1. US Department of Agriculture and US Department of Health and Human Services. Dietary Guidelines for Americans 2020-2025. 9th ed. dietaryguidelines.gov. Published December 2020. Accessed July 22, 2023. https://www.dietaryguidelines.gov/sites/default/files/2021-03/Dietary_Guidelines_for_Americans-2020-2025.pdf.
2. Tosh SM, Bordenave N. Emerging science on benefits of whole grain oat and barley and their soluble dietary fibers for heart health, glycemic response, and gut microbiota. Nutr. Rev. 2020;78(Supplement_1):13-20. doi:10.1093/nutrit/nuz085.
3. Office of Nutrition and Food Labeling, Center for Food Safety and Applied Nutrition, Food and Drug Administration, US Department of Health and Human Services. Review of the Scientific Evidence on the Physiological Effects of Certain Non-Digestable Carbohydrates. fda.gov. Published June 2018. Accessed July 30, 2023. https://www.fda.gov/files/food/published/Review-of-the-Scientific-Evidence-on-the-Physiological-Effects-of-Certain-Non-Digestible-Carbohydrates-PDF.pdf.