الفهرس 
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Abstract
The ancient Greek physician Hippocrates (460-370 B.C.) first noted gastrointestinal upset and skin problems in some who consumed milk patients experiencing the former symptom may likely have been suffering from lactose intolerance. However, it was only in the 20th century that the syndrome was more widely described by modern medical science. The condition was first recognized in the 1950s and 1960s when various organizations like the United Nations began to engage in systematic famine-relief efforts in countries outside Europe for the first time. Holzel et al. (1959) and Durand (1959) produced two of the earliest studies of lactose intolerance. As anecdotes of embarrassing dairyinduced discomfort increased, the First World donor countries could no longer ascribe the reports to spoilage in transit or inappropriate food preparation by the Third World recipients. Lactose was discovered in milk in 1619 by Fabriccio Bartoletti, and identified as a sugar in 1780 by Carl Wilhelm Scheele Lactose is actually two sugar molecules joined together: it is known as a disaccharide (di = two saccharide = sugar). The digestive enzyme called lactase splits lactose into its two constituent sugars: glucose and galactose. These are single sugars, called monosaccharides (mono = one; saccharide = sugar). Monosaccharides are small enough to be transported across the cells that line the digestive tract and into circulation, where they are an important source of energy for many body functions. Lactose, being a disaccharide, cannot pass across the lining of the digestive tract. Human milk has more lactose but less protein than cow or goat milk. The composition of milk varies from differs various breeds. However, commercial milks are a blend of many animals of many breeds and have the average compositions. Human milk has the highest lactose percentage at around7%. Unprocessed cow milk has 4.7% lactose. Unprocessed milk from other bovids contains similar lactose percentages (goat milk 4.7%, buffalo 4.86%, and sheep milk 4.6%). Lactose intolerance has been recognized for many years as common problem in many children and most adults throughout the world. Lactose intolerance is a clinical syndrome of one or more of the following: abdominal pain, diarrhea, nausea, flatulence, and/or bloating after the ingestion of lactose or lactose-containing food substances. Lactose, a disaccharide that comprises the monosaccharides glucose and galactose, is the primary carbohydrate found exclusively in mammalian milk. Absorption of lactose requires lactase activity in the small intestinal brush border to split the bond linking the 2 monosaccharides. A β-galactosidase termed “lactase-phlorizin hydrolase” (lactase) accounts for most of the lactase activity in the intestinal mucosa. For effective utilization, lactose requires hydrolysis by the enzyme lactase and, during infancy, provides an excellent source of energy at a time of rapid growth and development. An enhanced understanding of lactase and its deficiency and why there is a special carbohydrate in milk is important for improved management of lactose intolerance. A deficiency of intestinal lactase prevents hydrolysis of ingested lactose. The osmotic load of the unabsorbed lactose causes secretion of fluid and electrolytes until osmotic equilibrium is reached. Dilation of the intestine caused by the osmosis induces an acceleration of small intestinal transit, which increases the degree of maldigestion. Within the large intestine, free lactose is fermented by colonic bacteria to yield short-chain fatty acids and hydrogen gas. The combined increase in fecal water, intestinal transit, and generated hydrogen gas accounts for the wide range of gastrointestinal symptoms. Lactose intolerance is not an allergy, We are dealing with intolerance, and not an allergy, though lactose intolerance can exacerbate allergic symptoms. An allergy involves an immune response to a foreign protein, resulting in a reaction with antibodies IgE or IgG. These antibodies bind the allergen, and then the antigen-antibody complexes activate cells in the immune system- lymphocytes to generate more antibodies, phagocytes to release oxygen metabolites and proteases, and importantly, mast cells to release histamine. These substances then cause contraction of smooth muscle and inflammation, with resulting breathing difficulties, skin itching and rashes. In its severest form a patient may suffer anaphylactic shock that can be lethal. In contrast, an intolerance is a biochemical defect that prevents the normal metabolism of a specific substance. Most commonly, such biochemical intolerances are to a carbohydrate, amino acid or other small organic molecule. Lactose intolerance is caused by an impaired capacity to digest lactose properly, and thus a reduced capacity to absorb into the body its two constitutive sugars, galactose and glucose. Primary disease is the most common form .Approximately 70% of the world’s population has primary lactase deficiency. The percentage varies according to ethnicity and is related to the use of dairy products in the diet, resulting in genetic selection of individuals with the ability to digest lactose. In populations with a predominance of dairy foods in the diet, particularly northern European people, as few as 2% of the population has primary lactase deficiency. . In the US, it is far more common in black people, Native Americans, Hispanic people, and Asian people than in white people. North Americans, white Northern Europeans, and Australians have the lowest rates, ranging from 2% to 15%. In contrast, the prevalence of lactase deficiency is 50% to 80% in Hispanics; around 60% to 80% in black people and Ashkenazi Jewish people; and almost 100% in Americans of Indian ancestry and people from some Asian countries. The lactase gene is approximately 50 kb in size and located on chromosome 2.Lactose persistence and non-persistence reflects inheritence of different alleles of the lactase gene. Congenital lactase deficiency (CLD) has an incidence of 1:60,000 and is one of the 36 rare monogenic disorders enriched in the Finnish population. Congenital lactase deficiency is an extremely rare autosomal recessive disorder associated with a complete absence of lactase expression. teleologically, infants with congenital lactase deficiency would not be expected to survive before the 20th century, when no readily accessible and nutritionally adequate lactose-free human milk substitute was available. Developmental lactase deficiency is now defined as the relative lactase deficiency observed among preterm infants of less than 34 weeks’.