The job of the digestive system is to take the foods you eat and break them down into molecules your body can absorb and use as nutrients. In the case of carbohydrates, the foods that contain them are digested through stages into smaller sugar molecules that the body can use as fuel. Glucose, sucrose and lactose fall along the different stages needed to digest carbohydrates.
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As the body's main source of energy, carbohydrates are found in many of the foods we eat including grains, beans, fruits, vegetables and dairy products. Carbohydrates are made up of sugars, starches and fiber, and can be either complex or simple. Simple sugars are called monosaccharides or disaccharides. They are more easily digested and found in fruits and dairy. Grains, beans and vegetables contain complex carbohydrates which are made up of chains of polysaccharides that go on to form starches. Fiber is the only form of carbohydrate that cannot be digested by the body.
Many disaccharides exist, but the most important for humans are sucrose, maltose and lactose. Sucrose, or table sugar, is a disaccharide formed when the monosaccharides glucose and fructose are linked together. It is found naturally in fruits and vegetables, but is most commonly obtained from sugar cane or sugar beets. Sucrose is used commercially as a sweetener for many different types of foods. When the body digests sucrose, it is broken back down into glucose and fructose before being absorbed.
Lactose is a sugar found in milk and other dairy products. It is a disaccharide made up of glucose and galactose. Much attention is given to lactose since it is the component of dairy to which many people are intolerant. The body produces an enzyme called lactase which is supposed to break down any lactose consumed. However, those who are lactose intolerant do not have enough lactase to properly digest this disaccharide and consequently experience symptoms such as gas, bloating and diarrhea.
Glucose is the most abundant monosaccharide in nature. It is the sugar molecule the body uses to create energy, and it is the fuel used by brain cells. Most carbohydrates end up as glucose after being digested. Glucose is known as the "blood sugar" because it is found in high concentrations in the bloodstream. Many people know of glucose because of the glucose test, which measures the amount of glucose in the blood -- one of the markers for diabetes.