The carbohydrates you eat are either complex starches or simple sugars. No matter what the source, your body breaks down your dietary carbohydrates into their simplest component, usually glucose. The primary function of sugar in your body metabolism is to provide energy to power your activities. Additionally, sugar can convert to a stored form of energy in your body, and it plays a role in conserving your lean muscle mass.
The starches you consume consist of large molecules of glucose linked together. Digestive enzymes secreted from your salivary glands and pancreas clip the starch into individual glucose units as your food passes from your mouth through your stomach and into your small intestine. Glucose, along with other simple sugars you eat, pass through the cells of your small intestine, enter your bloodstream and, with the help of insulin, move into the cells of your liver and other tissues. From here, sugar molecules can burn as fuel or convert to different types of physiological molecules.
Your body derives energy from the sugar in your cells. In a process called glycolysis, your cells oxidize glucose to produce either pyruvate or lactate, which then is further metabolized to ultimately produce adenosine triphosphate, or ATP. ATP is a high-energy molecule that supplies your cells with fuel for activities such as muscle contraction. Your cells can also derive energy from sugars other than glucose, including fructose -- from fruit or table sugar -- and galactose -- from milk. Your dietary sugars supply four calories of energy for each gram of carbohydrate you consume.
When you eat more dietary carbohydrate than your body needs as fuel, some of the excess sugar may convert to glycogen. Glycogen is a storage form of starch your muscles and liver stockpile in modest amounts. While your muscle glycogen is available strictly for use by your muscle tissue, your liver glycogen can travel to other tissues as needed. This matters because some organs, such as your brain, require glucose as an energy source. Your liver glycogen thus serves as a means to fuel your brain when your blood sugar levels are low.
Sparing of Protein
Although your body prefers to burn dietary carbohydrates for energy over proteins or fats, when sugars are scarce, your body may turn to other sources to fuel itself. For example, if your sugar intake is low, you may burn the amino acids from your dietary protein to supply energy to your tissues. When you do this, however, the protein you consume is unavailable for other uses, such as building muscle or maintaining your current muscle mass. Sufficient dietary carbohydrates help you conserve your muscle tissue.
In addition to glycogen formation, excess dietary sugars may convert to amino acids or fatty acids. This process depends not only on the amount of sugars you consume but also on your other physiological needs. For example, if your needs for sugar metabolism are satisfied, your cells may use part of an excess glucose molecule to synthesize a needed amino acid. Alternatively, you may convert excess carbohydrates to fatty acids for long-term storage in your fat tissue.
- University of Illinois McKinley Health Center; Macronutrients: the Importance of Carbohydrate, Protein, and Fat; March 2008
- The Medical Biochemistry Page; Digestion of Dietary Carbohydrates; July 2011
- The Medical Biochemistry Page; Fructose Metabolism; February 2011
- The Medical Biochemistry Page; Glycogen; February 2011