Simple sugars are either monosaccharides, composed of one sugar molecule, or disaccharides, composed of two sugar molecules. The principal monosaccharides that occur in foods are glucose and fructose. The major disaccharides are sucrose, also known as white sugar and composed of one molecule of glucose and one molecule of fructose; maltose, which is composed of two molecules of glucose; and lactose, which is composed of one molecule of glucose and one molecule of galactose. Glucose is not particularly sweet-tasting as compared to fructose and sucrose. It is found abundantly in fruits, honey, sweet corn, and most root vegetables.

Glucose is also the primary repeating sugar unit of most complex carbohydrates (starches). Fructose, or fruit sugar, is the primary carbohydrate in many fruits, maple syrup, and honey. Fructose, which is very sweet, is roughly 1.5 times sweeter than sucrose (white sugar). Although fructose has the same chemical formula as glucose, its structure is quite different. In order to be utilized by the body, fructose must first be converted to glucose within the liver. Sucrose, which is common table sugar, is found in a large number of processed foods and some fruits. Maltose is found in malted grain products and syrups derived from grains such as barley and sorghum. Lactose is the sugar found in milk. Humans have an enzyme called lactase to digest lactose as infants but later start to lose this enzyme. By age four, most of our lactase production is absent. In some people, particularly those of African-American descent, lactase production is completely absent, causing lactose intolerance with symptoms of gas, bloating, and diarrhea when dairy products are consumed. The lactase enzyme can be supplemented for people who are lactose-intolerant to enable the occasional consumption of dairy foods. Sucrose, maltose, and lactose are broken down into their constituent sugars in the small intestine. Glucose is the primary form of sugar that enters the bloodstream. In fact, most of the content of other sugars is converted to glucose at the surface of the intestine or in the liver.