Lactose (=milk sugar) is contained in milk and milk products as well as in foods and dishes made from it. Normally, the disaccharide lactose is broken down in the small intestine by lactase, an enzyme produced in the intestinal villi, and absorbed into the bloodstream. If there is too little lactase or no lactase at all, the lactose reaches deeper parts of the intestine undigested, where it is broken down by bacteria into gases (especially carbon dioxide and hydrogen) and short-chain fatty acids. These fermentation products in turn cause an increased influx of water into the large intestine and can cause symptoms such as flatulence, soft, greasy stools or diarrhoea, abdominal pain (sometimes colicky), but also fullness, nausea, belching and in rare cases constipation or vomiting. In addition, undigested lactose binds tryptophan to itself and prevents its absorption. Since tryptophan is the raw material for the messenger serotonin and the sleep hormone melatonin, lactose intolerance can increase depression, pain and sleep disorders.