Lactose is the most important carbohydrate in the milk of most species. Its biosynthesis takes place in the mammary gland. The molecular structures of α- and β -lactose differ in the orientation of a hydrogen- and a hydroxyl group on carbon atom no.1 in the glucose moiety. Both forms change into one another continuously. At room temperature, the equilibrium results in a ratio of about 40% α-lactose and 60% β-lactose. The fact that two forms of lactose exist which differ in molecular structure has profound effects on various properties of lactose such as crystallization behavior, crystal morphology, solid-state properties, and solubility. The intestine does not actively absorb lactose unless it is split into its two-monosaccharide components, i.e. glucose and galactose. This hydrolysis of lactose is affected by the enzyme lactase, which is produced by the epithelium cells in the brush-border of the small intestine. Thus, the capacity of mammals to digest lactose is dependent on the lactase activity in the intestine. The maximum activity of the enzyme occurs shortly after birth and declines during the weaning period, after which it remains at a relatively constant level. Genetically determined factors governing residual lactase activity also exist. Individuals having low lactase activity are called lactose malabsorbers. Lactose intolerance is a condition in which people have symptoms due to the decreased ability to digest lactose. The principal symptom of lactose intolerance is an adverse reaction to products containing lactose (primarily milk), including abdominal bloating and cramps, flatulence, diarrhea, nausea, borborygmi, and vomiting (particularly in adolescents). These appear one-half to two hours after consumption.