Sucrose, glucose and fructose are important carbohydrates, commonly referred to as simple sugars. Sugar is found naturally in whole foods and is often added to processed foods to sweeten them and increase flavor. Your tongue can’t quite distinguish between these sugars, but your body can tell the difference. They all provide the same amount of energy per gram, but are processed and used differently throughout the body.
Simple carbohydrates are classified as either monosaccharides or disaccharides. Monosaccharides are the simplest, most basic units of carbohydrates and are made up of only one sugar unit. Glucose and fructose are monosaccharides and are the building blocks of sucrose, a disaccharide. Thus, disaccharides are just a pair of linked sugar molecules. They are formed when two monosaccharides are joined together and a molecule of water is removed — a dehydration reaction.
The most important monosaccharide is glucose, the body’s preferred energy source. Glucose is also called blood sugar, as it circulates in the blood, and relies on the enzymes glucokinase or hexokinase to initiate metabolism. Your body processes most carbohydrates you eat into glucose, either to be used immediately for energy or to be stored in muscle cells or the liver as glycogen for later use. Unlike fructose, insulin is secreted primarily in response to elevated blood concentrations of glucose, and insulin facilitates the entry of glucose into cells.
Fructose is a sugar found naturally in many fruits and vegetables, and added to various beverages such as soda and fruit-flavored drinks. However, it is very different from other sugars because it has a different metabolic pathway and is not the preferred energy source for muscles or the brain. Fructose is only metabolized in the liver and relies on fructokinase to initiate metabolism. It is also more lipogenic, or fat-producing, than glucose. Unlike glucose, too, it does not cause insulin to be released or stimulate production of leptin, a key hormone for regulating energy intake and expenditure. These factors raise concerns about chronically high intakes of dietary fructose, because it appears to behave more like fat in the body than like other carbohydrates.
Sucrose is commonly known as table sugar, and is obtained from sugar cane or sugar beets. Fruits and vegetables also naturally contain sucrose. When sucrose is consumed, the enzyme beta-fructosidase separates sucrose into its individual sugar units of glucose and fructose. Both sugars are then taken up by their specific transport mechanisms. The body responds to the glucose content of the meal in its usual manner; however, fructose uptake occurs at the same time. The body will use glucose as its main energy source and the excess energy from fructose, if not needed, will be poured into fat synthesis, which is stimulated by the insulin released in response to glucose.