The oesophagus extends from the pharynx at the level of the sixth cervical vertebra to the stomach at the level of T10. It lies in close relation to the aorta from T3 downwards and is behind the heart in the anterior/posterior plane from T5 to T10. It is lined by squamous epithelium.

The Gastrointestinal Tract

The principal conclusion reached in Part I of this article (1) was that the group differences found in primary adult lactose intolerance among the world's peoples are largely genetic in origin. In this part, after a review of recent research that adds to Part I in important ways, we turn to a consideration of the conditions of genetic selection that may have led some groups of men to have persistently high levels of intestinal lactase throughout life, and others not. Low incidence of intolerance, it is held, would develop over time in a group that has an abundant milk supply, that has alternate foodstuffs inadequate in amount and quality, and that consumes milk in lactose-rich forms. Since such selection cannot have occurred among groups that did not use milk, areas of nonmilking in the modern world are first delimited. The origins and diffusion of dairying are then sketched to determine the length of time that milk was consumed in various regions. With the background thus gained, the present-day occurrence of various Old World groups with high and low incidences of intolerance is explained. All groups studied so far within the traditional areas of nonmilking are found to have high incidences of intolerance. Overseas groups, such as American Negroes, whose ancestors came from nonmilking regions also have high incidences of intolerance. Within the areas of milking in Africa and Europe, moreover, the known group differences in tolerance are found to be in accord with the hypothesis. Then, that the hypothesis may further be confirmed, specific additional research efforts are suggested.

Primary adult lactose intolerance and the milking habit : a problem in biologic and cultural interrelations II. A culture historical hypothesis

Studies of small intestine during development. I. Distribution and activity of beta-galactosidase.

The ability to assimilate lactose, a primary constituent of milk, is dependent upon the presence of lactase, a genetically controlled enzyme in the small intestine. In most mammals, levels of lactase are high during infancy, but fall rapidly following weaning; adults are lactase-deficient. In man, some populations, including Orientals, Australian Aborigines, and Negroes, are deficient as adults, while others, including Nilotic Negroes and Europeans, maintain consistently high levels of lactase throughout life regardless of individual lactose intake. Ingestion of even small amounts of lactose by lactase-deficient individuals usually gives rise to a variety of symptoms, including cramps, bloating, and diarrhea. It is hypothesized that prior to the domestication of animals and the development of dairying, the normal condition for all men was adult lactase deficiency, but that with the introduction of lactose into the adult diet in certain cultures, new selective pressures were created that favored the genoty...

Lactase Deficiency: An Example of Dietary Evolution

The administration of a carbohydrate-containing diet for 24 hours to rats previously fasted for 3 days led to a twofold increase in total intestinal sucrase and sucrase specific activity The specific activity of maltase was similarly increased, but lactase activity was unaffected The sucrose-containing diet led to a greater increase in sucrase than maltase activity, whereas the converse was true of the maltose-containing diet A carbohydrate-free isocaloric diet led to a slight increase in the total intestinal sucrase, but sucrase specific activity was unchanged Assay of sucrase activity of mixed homogenates from casein-fed and sucrose-fed rats or fasted and sucrose-fed animals yielded activities that were additive The Michaelis constant (Km) of the enzyme hydrolyzing sucrose was similar in the fasted, casein-fed, and sucrose-fed rats The maximal velocity (Vmax) was twice greater in sucrose-fed as compared to casein-fed or fasted rats, suggesting an increased quantity of enzyme subsequent to sucrose feeding Adrenalectomized rats maintained on 10% salt intake had sucrase and maltase levels comparable to those of controls Steroid administration did not significantly increase their activities The response to sucrose feeding was similar in both control and adrenalectomized rats, indicative of the absence of steroidal control on sucrase and maltase activity in the adult animal Studies using intestinal ring preparations indicated that sucrose hydrolysis by the intact cells proceeded more rapidly when animals were fed sucrose Additional corroboration of the physiologic significance of the increased enzyme levels in homogenates was afforded by intestinal perfusion studies Sucrose hydrolysis increased twofold and fructose absorption fourfold in animals fed sucrose when compared to either fasted or casein-fed rats

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V. Schwarz

Papers

Defective lactose absorption causing malnutrition in infancy.