Lactose

A carbohydrate found predominantly in milk.  Lactose is a disaccharide consisting of a glucose + galactose simple sugar bound together.

Lactose intolerance is the inability to digest significant amounts of lactose.  This lack of digestion results from a shortage of the enzyme lactase, which is normally produced by the cells that line the small intestine.  Lactase breaks down the lactose milk sugar into a simpler form which can then be absorbed into the bloodstream.  When there is not enough lactase to digest the amount of lactose consumed, a large proportion of the lactose can pass whole into the digestive tract.

The most common symptoms of this include nausea, cramps, bloating, gas, and diarrhoea, occurring approximately 30 minutes to 2 hours after eating or drinking foods containing lactose. The severity of symptoms varies depending on the amount of lactose each individual can tolerate.  These symptoms are not dangerous, but they can be quite distressing and make you feel extremely uncomfortable.