The carbohydrates you consume contain starch and sugars. Starch is a large molecule comprising glucose units arranged in a branched configuration. Food sugars are commonly monosaccharides, or single sugar molecules, and disaccharides, which are molecules of two sugars joined together. For example, glucose and fructose are monosaccharides, while sucrose is a disaccharide composed of one glucose and one fructose. Your body processes sugars through digestion, absorption and metabolism.
If the food you eat contains starch, its digestion begins in your mouth, where salivary amylase cleaves glucose molecules from the starch molecule. The remainder of starch digestion occurs in your small intestine with the action of another amylase secreted by your pancreas. In addition, digestive enzymes known as disaccharidases split disaccharides into monosaccharides. For instance, sucrase clips sucrose into glucose and fructose, and lactase cleaves lactose into glucose and galactose. When your body processes all your digestible carbohydrates into single-sugar monosaccharides, they are ready for absorption by your small intestine.
Lining your small intestine are multitudes of folds called villi, and lining your villi are structures known as microvilli. This architectural feature of your small intestine greatly increases the absorptive surface area of your digestive system. On the gut side of the villi and microvilli intestinal cells are special proteins called hexose transporters. These sugar transporters bind the free monosaccharides in your gut and ferry them, one by one, across the cell membrane to the interior of the intestinal cell. Once inside the cell, the sugars move to the other side of the cell, where different hexose transporters deposit them into the capillaries abutting the intestinal cell. From there, the sugars pass into your blood for further processing.
When glucose, fructose and galactose molecules reach your blood, they travel to your liver and to other cells. Your body can then process these sugars to synthesize glycogen, fatty acids or certain amino acids. Alternatively, your cells can oxidize monosaccharides to derive the energy cells need for various physiological functions. The oxidation of sugars within your cells results in the formation of adenosine triphosphate, a high-energy molecule that provides energy to drive the biological reactions in your cells; carbon dioxide, a waste product of sugar metabolism; and water.
Although proteins and fats can provide energy to your body, sugars are the predominant energy source your cells use as fuel. Carbohydrates provide 4 calories of energy per g you consume, whether you eat them as starch or as simple sugars. Consuming more carbohydrates than your body needs, however, may cause you to convert the excess calories to fat for long-term storage.