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Medicine LibreTexts

4.1: Introduction to Carbohydrates

  • Page ID
    21111
  • Learning Objectives

    • Identify food groups that are sources of carbohydrates.
    • Classify carbohydrates as simple or complex.
    • Describe various simple and complex carbohydrates.
    • Identify the body's preferred fuel source.

    Carbohydrate Sources

    Food groups that contain carbohydrates include:

    • fruits
    • vegetables
    • dairy
    • protein (only in beans and some processed meats)
    • grains

    Many people immediately think of grains when they think about carbohydrates. You likely eat grains every day—cereal, a sandwich, pasta, or your favorite rice dish. When eating grains, it's important to know that overwhelming scientific evidence now shows that diets containing high amounts of whole grains rather than refined grains decrease weight gain and the risk for many chronic diseases, including certain types of cancer and diabetes. Whole grains contain the bran, endosperm, and germ of the grain (Figure \(\PageIndex{1}\)). Refined grains (e.g., white rice) only contain the endosperm. The Food and Drug Administration (FDA) has provided the food industry with specifics on how to label whole-grain foods—to label it as made from 100% percent whole grains. The best way to ensure the product is made from 100% whole grains is to check the ingredient list and look for "whole (grain)" (e.g., whole wheat) as the first ingredient.

    Figure \(\PageIndex{1}\): Wheat kernel anatomy and composition

    Carbohydrate Classifications

    Carbohydrates are the perfect nutrient to meet your body’s nutritional needs. They provide energy to all of your cells, including your brain and nervous system. Digestible carbohydrates provide energy, vitamins, and minerals, while indigestible carbohydrates provide a good amount of fiber with a host of other health benefits.

    Carbohydrates are broadly classified into two subgroups, “simple” and “complex". Simple carbohydrates are grouped into monosaccharides and disaccharides. The simplest unit of a carbohydrate is a monosaccharide. If you break down the word 'monosaccharide' it means 'one sugar' (mono=one, saccharide=sugar) which is referring to the fact that each monosaccharide is one molecule of sugar. Complex carbohydrates are long chains of monosaccharides. (Figure \(\PageIndex{2}\)).

    ed929503f6e47180eb307d29ab220c86.jpg
    Figure \(\PageIndex{2}\): Carbohydrate Classification Scheme. Carbohydrates are broken down into “simple” and “complex” carbohydrates. These subgroups are further categorized into mono-, di-, and polysaccharides.

    Simple Carbohydrates

    Simple carbohydrates are grouped as either monosaccharides or disaccharides. Monosaccharides include glucose, fructose, and galactose, and the disaccharides include lactose, maltose, and sucrose.

    Monosaccharides

    For all organisms, from bacteria to plants to animals (including humans), glucose is the preferred fuel source. The brain is completely dependent on glucose as its energy source (except during extreme starvation conditions). The monosaccharide galactose differs from glucose only in that a hydroxyl (−OH) group faces in a different direction on the number four carbon (Figure \(\PageIndex{3}\)). This small structural alteration causes galactose to be less stable than glucose. As a result, the liver rapidly converts galactose to glucose. Most absorbed galactose is utilized for energy production in cells after its conversion to glucose. 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 \(\PageIndex{3}\)). 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.

    Figure \(\PageIndex{3}\): Structures of the three most common monosaccharides: glucose, galactose, and fructose. Circled areas indicate the structural differences between the three molecules.

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

    Lastly, there are the sugar alcohols, which are industrially synthesized derivatives of monosaccharides. Some examples of sugar alcohols are sorbitol, xylitol, and glycerol. Sugar alcohols are often used in place of table sugar to sweeten foods as they are incompletely digested and absorbed, and therefore contain fewer calories. The bacteria in your mouth opposes them, hence sugar alcohols do not cause tooth decay. Interestingly, the sensation of “coolness” that occurs when chewing gum that contains sugar alcohols comes from them dissolving in the mouth, a chemical reaction that requires heat from the inside of the mouth.

    Disaccharides

    Disaccharides are composed of pairs of two monosaccharides linked together. Disaccharides include sucrose, lactose, and maltose. All of the disaccharides contain at least one glucose molecule.

    Sucrose, which contains both glucose and fructose molecules, is also 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. Lactose, which is commonly known as milk sugar, is composed of one glucose molecule and one galactose molecule. 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.

    Complex Carbohydrates

    Complex carbohydrates are polysaccharides, long chains of monosaccharides that may be branched or not branched. There are three main groups of polysaccharides: starch, glycogen, and fiber.

    Starch

    Starch molecules 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 (Figure \(\PageIndex{4}\)). Amylopectin, another plant starch, is a branched chain containing thousands of glucose units. These large starch molecules (amylose and amylopectin) are the storage form of glucose in plants.

    Glycogen

    Humans and animals store glucose energy in the form of the very large molecule, glycogen (Figure \(\PageIndex{4}\)). 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.

    83d76e165d784a9bc705cc259e416ed9.jpg
    Figure \(\PageIndex{4}\): Structures of the plant starches (amylose and amylopectin) and glycogen

    Fiber

    Fiber is a polysaccharide that is highly branched and cross-linked. Our bodies do not produce the enzymes that can break down fiber which means that most fiber passes through our intestines without being digested and absorbed.

    Fiber can be classified in a few ways. One way is by using the categories dietary fiber, functional fiber, and total fiber.

    • Dietary fiber: nondigestible parts of plants that form the supportive structure of the plant's seeds, leaves, and stems
    • Functional fiber: nondigestible forms of carbohydrates that are extracted from plants or are synthetically made (and have been shown to provide health benefits to humans); one example of a functional fiber is psyllium
    • Total fiber: dietary fiber + functional fiber

    Fiber can also be classified according to its chemical and physical properties (soluble vs. insoluble):

    • Soluble fiber: dissolves in water and is easily digested by bacteria in the large intestine (colon); citrus fruits, oats, and beans are foods that contain soluble fiber; research indicates that diets high in soluble fiber reduce the risk of cardiovascular diseases and Type 2 diabetes by lowering blood cholesterol and blood glucose levels
    • Insoluble fiber: does not dissolve in water and is not easily digested so it helps promote regular bowel movements; whole grains and vegetables are examples of food that contain insoluble fiber

    Key Takeaways

    • Carbohydrates are broadly classified into two subgroups, simple and complex carbohydrates.
    • Simple carbohydrates include monosaccharides (glucose, galactose, and fructose) and disaccharides (sucrose, lactose, and maltose).
    • Complex carbohydrates include the following polysaccharides: starch (amylose and amylopectin), glycogen, and fiber.
    • Fiber-rich foods help reduce the risk of heart disease and diabetes and promote regular bowel movements.
    • Glucose is the most important monosaccharide in human nutrition; it is the body's preferred fuel source.