Glucose and lactose are sugars commonly ingested through food. Glucose is also the basic molecule by which carbohydrate is utilized as energy. Lactose is the sugar found in milk. All foods people eat are broken down into glucose, although it may ultimately be stored as carbohydrate or fat.
Glucose
Glucose is the simplest sugar humans ingest, and is the only fuel used by the brain. Carbohydrates, including pasta, potatoes, grains, and bread, are easily digestible, and glucose is easily separated from the remaining elements of the food so that glucose may be utilized as fuel. Muscles and organs use glucose as fuel, but muscles and organs can also fat and protein as fuel. Glucose is stored in muscle and the liver as glycogen. Glucose in excess of caloric requirements is stored as fat in adipose tissue.
Glucose and Fructose
Glucose combined with fructose creates sucrose, what is often thought of as "table sugar." Fructose is the sugar most abundant in fruit. Glucose and fructose are the simplest forms of sugar. Sport drinks usually contain glucose and fructose to maximize the speed of gastric emptying during exercise. The faster the gastric emptying, the faster the glucose can be used for muscular contraction.
Lactose
Lactose is the sugar contained in milk, made of glucose and galactose. Some people do not have the enzyme lactase required to metabolize lactose. Lactose intolerance is most commonly a genetically transmitted disorder. People who do not produce lactase or enough lactase may simply wish to avoid consuming products with lactose, namely milk and dairy products.
Ends in -ose
If a substance ends in the suffix -ose it is likely a sugar, and it has a corresponding enzyme, ending in -ase, that aids its in digestion. Although not often considered, milk has a sweet flavor, and that flavor comes from the sugar, lactose. There is lactose-free milk, which could be more accurately called, "glucose, galactose, and lactase" milk, because it contains lactase that has already broken the lactose into the simpler sugars.
Glucose Tolerance
You may have had a laboratory test known as the "oral glucose tolerance test," abbreviated OGTT. Blood may be collected one hour after you have ingested 50 mg of glucose. Or you may have taken the three-hour OGTT, during which your blood is collected after a 10- to 12-hour fast, then at one hour intervals for three hours after you have ingested 75 to 100 mg of glucose. Your glucose tolerance is indicative of your body's ability to produce insulin and clear ingested glucose from your bloodstream. "Failing" the OGTT could indicate conditions ranging from glucose intolerance to type II diabetes.
