Lactose Intolerance: The condition resulting from the absence or deficiency of LACTASE in the MUCOSA cells of the GASTROINTESTINAL TRACT, and the inability to break down LACTOSE in milk for ABSORPTION. Bacterial fermentation of the unabsorbed lactose leads to symptoms that range from a mild indigestion (DYSPEPSIA) to severe DIARRHEA. Lactose intolerance may be an inborn error or acquired.Lactose Tolerance Test: A measure of a patient's ability to break down lactose.Lactase: An enzyme which catalyzes the hydrolysis of LACTOSE to D-GALACTOSE and D-GLUCOSE. Defects in the enzyme cause LACTOSE INTOLERANCE.Flatulence: Production or presence of gas in the gastrointestinal tract which may be expelled through the anus.Lactose: A disaccharide of GLUCOSE and GALACTOSE in human and cow milk. It is used in pharmacy for tablets, in medicine as a nutrient, and in industry.Dairy Products: Raw and processed or manufactured milk and milk-derived products. These are usually from cows (bovine) but are also from goats, sheep, reindeer, and water buffalo.Breath Tests: Any tests done on exhaled air.Lactase-Phlorizin Hydrolase: The multifunctional protein that contains two enzyme domains. The first domain (EC 22.214.171.124) hydrolyzes glycosyl-N-acylsphingosine to a sugar and N-acylsphingosine. The second domain (EC 126.96.36.199) hydrolyzes LACTOSE and is found in the intestinal brush border membrane. Loss of activity for this enzyme in humans results in LACTOSE INTOLERANCE.Hydrogen: The first chemical element in the periodic table. It has the atomic symbol H, atomic number 1, and atomic weight [1.00784; 1.00811]. It exists, under normal conditions, as a colorless, odorless, tasteless, diatomic gas. Hydrogen ions are PROTONS. Besides the common H1 isotope, hydrogen exists as the stable isotope DEUTERIUM and the unstable, radioactive isotope TRITIUM.Milk: The white liquid secreted by the mammary glands. It contains proteins, sugar, lipids, vitamins, and minerals.Kwashiorkor: A syndrome produced by severe protein deficiency, characterized by retarded growth, changes in skin and hair pigment, edema, and pathologic changes in the liver, including fatty infiltration, necrosis, and fibrosis. The word is a local name in Gold Coast, Africa, meaning "displaced child". Although first reported from Africa, kwashiorkor is now known throughout the world, but mainly in the tropics and subtropics. It is considered to be related to marasmus. (From Dorland, 27th ed)Abdominal Pain: Sensation of discomfort, distress, or agony in the abdominal region.Malabsorption Syndromes: General term for a group of MALNUTRITION syndromes caused by failure of normal INTESTINAL ABSORPTION of nutrients.Calcium, Dietary: Calcium compounds used as food supplements or in food to supply the body with calcium. Dietary calcium is needed during growth for bone development and for maintenance of skeletal integrity later in life to prevent osteoporosis.National Institute of Mental Health (U.S.): A component of the NATIONAL INSTITUTES OF HEALTH concerned with research, overall planning, promoting, and administering mental health programs and research. It was established in 1949.Glucose Intolerance: A pathological state in which BLOOD GLUCOSE level is less than approximately 140 mg/100 ml of PLASMA at fasting, and above approximately 200 mg/100 ml plasma at 30-, 60-, or 90-minute during a GLUCOSE TOLERANCE TEST. This condition is seen frequently in DIABETES MELLITUS, but also occurs with other diseases and MALNUTRITION.Diarrhea: An increased liquidity or decreased consistency of FECES, such as running stool. Fecal consistency is related to the ratio of water-holding capacity of insoluble solids to total water, rather than the amount of water present. Diarrhea is not hyperdefecation or increased fecal weight.Irritable Bowel Syndrome: A disorder with chronic or recurrent colonic symptoms without a clearcut etiology. This condition is characterized by chronic or recurrent ABDOMINAL PAIN, bloating, MUCUS in FECES, and an erratic disturbance of DEFECATION.Intestine, Small: The portion of the GASTROINTESTINAL TRACT between the PYLORUS of the STOMACH and the ILEOCECAL VALVE of the LARGE INTESTINE. It is divisible into three portions: the DUODENUM, the JEJUNUM, and the ILEUM.Fructose Intolerance: An autosomal recessive fructose metabolism disorder due to deficient fructose-1-phosphate aldolase (EC 188.8.131.52) activity, resulting in accumulation of fructose-1-phosphate. The accumulated fructose-1-phosphate inhibits glycogenolysis and gluconeogenesis, causing severe hypoglycemia following ingestion of fructose. Prolonged fructose ingestion in infants leads ultimately to hepatic failure and death. Patients develop a strong distaste for sweet food, and avoid a chronic course of the disease by remaining on a fructose- and sucrose-free diet.beta-Galactosidase: A group of enzymes that catalyzes the hydrolysis of terminal, non-reducing beta-D-galactose residues in beta-galactosides. Deficiency of beta-Galactosidase A1 may cause GANGLIOSIDOSIS, GM1.Orthostatic Intolerance: Symptoms of cerebral hypoperfusion or autonomic overaction which develop while the subject is standing, but are relieved on recumbency. Types of this include NEUROCARDIOGENIC SYNCOPE; POSTURAL ORTHOSTATIC TACHYCARDIA SYNDROME; and neurogenic ORTHOSTATIC HYPOTENSION. (From Noseworthy, JH., Neurological Therapeutics Principles and Practice, 2007, p2575-2576)Lactose Synthase: An enzyme complex that catalyzes the transfer of GALACTOSE from UDP GALACTOSE to GLUCOSE, forming LACTOSE. The enzyme complex is composed of a B subunit, ALPHA-LACTALBUMIN, which changes the substrate specificity of the A subunit, N-ACETYLLACTOSAMINE SYNTHASE, from N-ACETYLGLUCOSAMINE to glucose making lactose synthesis the preferred reaction.