A carbohydrate is an important organic nutrient that has numerous roles in living things. It stores energy, acts as a structural component, at least in plants, and forms the backbone of genetic molecules such as DNA and RNA. Carbohydrates are made out of smaller building blocks that form a class of molecules known as sugars; when they are digested, it is more accurate to say that carbohydrates are "reduced" to sugars rather than "turned into" sugars.
In common parlance sugar usually refers to sucrose, which is common table sugar, but scientifically it means a type of simple crystalline carbohydrate that has a sweet flavor. Carbohydrates are composed of one or more units known as saccharides. Saccharide literally means "sugar" in Greek, and is made out of a carbon ring structure with a precise ratio of two hydrogen atoms for every one oxygen atom. Sugar refers to both the basic unit of a carbohydrate molecule — the saccharide — and other simple carbohydrate molecules made from the union of two saccharides.
Types of Carbohydrates
If a carbohydrate is composed of just one saccharide, then it's known as a monosaccharide. Examples include the galactose found in milk, the fructose found in honey and fruit, and the ubiquitous glucose. When these individual units bond together in sets of two, they form disaccharides. For example, sucrose is a combination of glucose and fructose. Any carbohydrate that combines more than two monosaccharides is called a polysaccharide. One such example is the starch found in plants. Polysaccharides can be made up of tens, hundreds or thousands of linear or branched saccharide units chained together. Monosaccharides and disaccharides fall into the category of simple sugars or simple carbohydrates. Polysaccharides are known as complex carbohydrates.
Carbohydrate units are bonded together by a single oxygen atom. The digestive system breaks down these bonds by enlisting the use of acids and enzymes. Through this process complex carbohydrates such as starch revert first into the disaccharide maltose and then the simple sugar glucose, although some of the components, such as fiber, are undigestible. Disaccharides, on the other hand, require only a single step; they break down quickly into the two monosaccharides. Eventually most carbohydrates end up as glucose, which the cells use primarily as a source of energy. The amount of glucose in the blood, known as blood sugar, is a critical gauge of cellular energy and must be kept within a certain range by your body.
The rate at which the digestive system breaks down and absorbs carbohydrates is partly influenced by the complexity of the molecule. However, this is only a general rule, and there are many factors that affect digestive speed, including the type of food in question and the way in which it's prepared. Milk, for example, only has a low to moderate impact on blood sugar. Fructose is digested in such a way that it may not influence blood sugar much at all. Sharp rises in the total amount of blood sugar are known as hyperglycemia. Over long periods of time hyperglycemic states can eventually cause diabetes, heart disease and nerve damage. Generally, it's a good idea to consume carbohydrates that are broken down into sugars much more slowly.
- "Essentials of Glycobiology"; Saccharide Structure and Nomenclature; A. Varki, et al.; 1999
- Kimball's Biology Pages; Carbohydrates; John W. Kimball; March 2011
- National Digestive Diseases Information Clearinghouse; Your Digestive System and How It Works; April 2008
- Oregon State University Linus Pauling Institute; Glycemic Index and Glycemic Load; Jane Higdon; December 2005