All milk contains sugar. Milk from most mammals contains lactose, a sugar composed of glucose and galactose molecules. This sugar is not as sweet as other disaccharides and is generally found in low quantities in cow's milk (about 5 percent).
Lactose is an essential component of milk and the main source of energy for an animal's young. Some humans' digestive systems cannot process lactose. This condition, which is called lactose intolerance, occurs when the intestines cannot produce enough lactase, an enzyme that breaks down lactose before it can be used by the body. Without this enzyme, fermentation can occur in the digestive tract, causing the person pain and discomfort.