Lactose intolerance is the inability to digest significant amounts of lactose, the predominant sugar found in milk. This inability results from a shortage of the enzyme lactase, which is normally produced by the cells that line the small intestine. Lactase breaks down milk sugar into simpler forms that can then be absorbed into the bloodstream. When there is not enough lactase to digest the amount of lactose consumed, the results, although not usually dangerous, may be very distressing. While not all people deficient in lactase have symptoms, those who do are said to be lactose intolerant. Common symptoms include nausea, cramps, bloating, wind, and diarrhoea, which begin about 30 minutes to two hours after eating or drinking foods containing lactose. The most common test for measuring absorption of lactose in the digestive system is the hydrogen breath test.