Glucose is the metabolized form of “sugar” in the body. It is found in some fruits, such as grapes. It can also be hydrolyzed from starch, cane sugar (sucrose), milk sugar (lactose), and malt syrup (maltose). Glucose is carried in the blood and is the principal sugar used by the tissues and cells for energy. Glucose can be measured by the “blood sugar” test, which reads the current concentration of glucose in the serum or plasma. A high glucose level can signal a diabetic condition; low blood sugar is called hypoglycemia. Both of these abnormalities can become chronic and even life-threatening. The adrenal and pancreatic hormones, adrenaline and insulin, are very important in sugar metabolism. High-sugar and refined-carbohydrate diets, stress, and lack of exercise can generate elevated glucose levels in both blood and tissue. Pregnancy is also a stress on carbohydrate metabolism because of its high metabolic demands.
Fructose is found in most fruits and fruit juices, as well as in honey and some vegetables. Fructose is sweeter than cane sugar and is absorbed directly into the blood. Cane sugar (sucrose) is metabolized into fructose and glucose. Fructose can be changed to glucose in the liver or in the small intestine for a quick source of energy.
Galactose comes from the metabolism of the milk sugar lactose, which breaks down into galactose and glucose. Galactose is converted to glucose in the liver and is synthesized in the mammary glands to make the lactose of mother’s milk.
These sugars can be hydrolyzed into two monosaccharides with the addition of a water molecule. They are all water soluble and will crystallize when dehydrated.
Lactose (milk sugar) is the only sugar of animal origin, the sugar of mother’s milk. It is composed of one molecule each of glucose and galactose. Lactose is broken down by the enzyme lactase, which may be deficient or absent in some races of people, leading to problems with milk digestion.
Sucrose (“white sugar”) is found in sugar cane and sugar beets, maple syrup, molasses, sorghum, and pineapple. Sucrose is composed of one molecule each of fructose and glucose. Sucrose is very sweet and can be metabolized in the body. Its crystalline form, table sugar, is used excessively in our society, not only as a sweetener on food and in beverages but also in cooking and “hidden” in preparation of many other common foods and condiments, such as catsup, mayonnaise, salad dressings, and baby foods. Addiction to sucrose begins early and is supported by millions of dollars of advertising. The average American consumes well more than 100 pounds of sucrose a year, and this particular disaccharide is responsible for a wide variety of problems. It has been implicated in obesity, tooth decay, diabetes, and many psychological and emotional problems, including premenstrual syndrome and stress/burnout syndromes. Using less sugar would be a boost to anyone’s health.
Maltose (malt sugar) is a short chain of two glucose molecules. It is produced during the breakdown of starches in many cereal grains. Maltose is present in beers, malted snacks, and some breakfast cereals and is the sweetener of many crackers (read labels!). It is easily broken down into glucose molecules for quick utilization by the body.