Cecil Textbook of Medicine 1-2. [antikvár]

I. kötet: PREFACE The 19th edition of the Cecil Textbook of Medicine appears on the one-hundredth anniversary of the publication of William Osler's influential The Principles and Practice of Medicine, a monumental single-authored volume notable for its comprehensive clinical cover-age, authoritative pathologic descriptions, and literary qualities. Microbiology was then the newest medical science. A tone of therapeutic nihilism was the book's most salutary contribu-tion. At least two generations of physicians would fali under its influence. The textbook ushered in a period of increasingly exact diagnosis, especially in infectious diseases, and an ever more critieal evaluation of drugs, remedies, and nostrums in the practice of medicine. It alsó led to the establishment of the Rockefeller Institute, founded to address the pervasive ignorance of the pathophysiology of disease so abundantly displayed in Osler's textbook. avant garde though it was for its day. Thirty-five years later, in 1927, Russell Cecil introduced "A Text-book of Medicine by American authors." Single-authored textbooks had largely given way to books jointly authored by a small number of writers, but the idea of an edited textbook compiled by multiple authors, each writing on topics of personal interest and experience, was new. Basic biologic sciences were making increasingly important contributions to clinical medicine, and these were to be accorded substantial attention. The maturing sciences were physiology, pharmacology, and biochemistry. With succeeding editions, Cecil's philosophy became more explicit. Cecil believed that ". . . in terms of biological processes, fragmentation of the discussion of disease is artificial" (Preface, lOth edition, 1959). Each chapter was a treatise in which clinical description, pathologic information, pathophysiologic knowledge, diagnostic criteria, and therapeutic measures were well integrated, so that students and physicians consulting the text could secure the most authoritative information available and find it in one place (Beesőn and McDermott, Preface, llth edition, 1963). Cecil's inaugural philosophy continues into the 19th edition of the Cecil Textbook, 65 years later, appropriately adapted to ever-changing circumstances. By 1992, several generations of physicians have learned medicine with the help of Cecil. The series spans a period of remarkable progress in biomedical and behavioral sciences, and each new edition has incor-porated new insights on disease causation, prevention, and treatment. The pace has quickened as we approach the twenty-fírst century. New technologies have revolutionized molecular genetics, neurobiology, immunology, cell biology, and structural biology; the application of these disciplines to all branches of the traditional biomedical sciences proceeds apace. The structure of DNA was elucidated less than 40 years ago, and recombinant DNA technology was discovered less than 20 years ago. Today, the leitmotif of biologic science, regardless of its disciplinary name, is increasingly cell and molecular biology. This theme is now permeating medicine and prefiguring the developments of the next few decades. Beyond these contributions from the biologic sciences, applications of the physical and mathematical sciences, especially in diagnostic imaging (CT, MRI, PET, and sonography) and in the information sciences, continue to altér medical practice. In such a climate of change, medical competence itself is fragile. It must be constantly renewed or else it will erode. To reflect the best in medical practice, a major textbook of medicine must alsó be constantly renewed. In that spirit, this edition of the Cecil Textbook of Medicine has been thoroughly revised. As before, approximately one third of the book is "new" in that different authors have been selected, in this way assuring that their chapters have been completely recast. All other chapters have been revised and updated by their current authors, carefully chosen authorities in their respective subjects. The editors are deeply grateful to all retiring authors for the high standards of their contributions. We have retained the two-color presentation of figures and charts, so well received in the 18th edition, and have expanded the color plates from 8 to 16 pages. The most extensive change in the 19th edition is the further expansion of space clevoted to the acquired immunodeficiency syndrome (AIDS), a still unfolding epidemic. This condition II. kötet: PART XV NUTRITIONAL DISEASE S 199 Nutrient Requirements Róbert M. Russell Recommended Dietary Allowances Recommended Dietary Allowances (RDA's) have been estab-lished for most essential nutrients by the Food and Nutrition Board of the National Academy of Sciences. A nutrient is defined as essential if its absence from the diet results in a deficiency disease. For certain nutrients, notably somé trace elements, essentiality has not been established. The United States RDA's are but one set of many recommendations put out by various countries and organizations (e.g., World Health Organization, Food and Agriculture Organization). In the United States the RDA's are used as a standard upon which several food assistance programs are based. For example, the school lunch program must meet 33 per cent of the RDA's for 12-year-old children in its meal planning. It is necessary in meeting such standards, however, that planners choose foods that will be eaten and enjoyed. The RDA's (Table 199-1) do not represent nutrient requirements for individuals; they are designed as guidelines for the daily intake of nutrients sufficient to ensure that almost all members of the population are not at risk of developing nutrient defícits. Thus, the RDA's exceed the nutrient requirements for most healthy individuals. Recommendations for energy intakes are an exception in that they represent values derived by multi-plying resting energy expenditure (REE) by an activity factor for particular age and sex groups. RDA's have been determined by balance studies, measurement of the amount of a nutrient needed to result in tissue saturation, examination of the food supplies of healthy populations, exami-nation of minimai nutrient intakes requircd to prevent or correct either a naturally occurring or an experimentally produced deficit, epidemiologic observations, and animal studies. Precise RDA's have not been established for somé nutrients (e.g., biotin, manganese) because of limited experimentál data. However, ranges of safe intakes of these nutrients have been determined by the National Academy of Sciences and are provided in Table 199-2. Continued consumption of trace minerals above the upper limit of the recommended ranges can lead to toxic effects, as is the case with most individual nutrients. The RDA's should be met by a variety of foods for two major reasons. First, certain dietary components (e.g., carotene, fiber, and possibly others as yet undefined) that are not considered "required" may nevertheless have a beneficial effect on body functioning. For example, if an individual is limited to a diet containing only preformed vitamin A, he or she could be deprived of the alleged beneficial effects of carotene (a vitamin A precur-sor). Second, a monotonous diet over a prolonged period may not supply a beneficial ratio of individual nutrients (e.g., a diet of very high carhohydrate content may increase the body's need for thiamine). Although other nutrient interactions have been defined (vitamin B12 is necessary for the demethylation of 5-methyl tetrahydrofolate; zinc is needed íbr the oxidation of retinol to photochemically active retinaldehyde), many such interactions are not fully known at present. Body growth, body size, pregnancy, and lactation altér the RDA's. Pregnancy increases nutrient needs for the expansion of hlood volume and for the growth and development of the fetus, placenta, uterus, and breasts. Similarly, lactation increases nutrient needs in proportion to the quantity of milk produced. Pregnancy and lactation RDA adjustments are provided in Table J99-1. Other factors that result in an alteration of dietary needs include environmental temperature, fever, menstruation (an in-creased requirement for iron), disease, and medication. Disease and/or drugs may change nutrient requirements by altering nutrient absorption or bioavailability, storage capacity, or excre-tion or by changing a nutrient s metabolism. For example, kidney disease may result in a decreased ability to change 25-hydroxy-vitamin D to its active 1,25-dihydroxylated form (Ch. 233); drugs that stimulate microsomal cytochrome P-450-mediated enzyme activities (e.g., alcohol) cause an increased hepatic metabolism of vitamin A. The phvsician should remember that the RDA's were not designed for sick or traumatized patients or for individuals with metabolic disorders such as hvperthyroidism. Despite all of these caveats, the RDA's do sérve as useful guidelines for the practitioner to judge the adequacy of an individual's diet. Table 199-3 provides a guide to the possible effects of medication on nutrient requirements and the mechanisms by which these interactions occur. Nutrient requirements and dietary recommendations for adults are defined in broad age classes in Table 199-1, namely 19 to 24 years, 25 to 50 years, and 51 years and older. In the absence of adequate information, the present recommendations for the el-derly are the same as for the young aduit population. However, old people eat fewer calories than young people, and accompa-nying this diminished calorie intake is a concomitant reduction in intake of almost all other nutrients. For nutrients whose requirements are fixed (rather than relatíve to calories), such across-the-board reductions may result in intakes that are insuf-ficient to meet metabolic demands; for example, the amount of dietary protein needed for nitrogén equilibrium is not reduced with age. Age-related changes affect the absorption, metabolism, and excretion of many nutrients, so that age-specific standards for the elderly are needed. In addition, chronic disability, illness, and the increased use of medications in the elderly introduce other variables. Suffice it to say that the elderly person's diet should be of high quality in terms of nutrient density (i.e., quantity of nutrients/calorie). Since parenteral administration of somé nutrients bypasses any problems due to limited absorption, nutrient requirements are generallv less when delivered by the parenteral route than by the enteral route. However, the underlying cause (e.g., disease, trauma) that necessitates parenteral delivery in a patient often dictates overall higher nutrient requirements (see Ch. 207). WATER Owing to a high ratio of surface area to volume, infants are more prone to dehydration than adults. The average aduit needs a minimum of 700 to 1000 ml of water per day in order to survive, but 2000 ml per day (1 ml per Kcal intake) provides a safe and adequate hasal maintenance amount. Approximatelv 100 ml per day of water is lost in feces, 500 to 1000 ml in evaporation and exhalation (insensible loss), and the remainder in urine. Diets providing a high renal solute load (e.g., diets high in protein, sodium, potassium, chloride) will result in higher urinary water losses. In a sick patient additional water must be supplied if body temperature is elevated (each 1°C elevation over normál results in an additional obllgatory water loss of 200 ml per day), if diarrhea is present, or if polyuria is present (e.g., from uncon-trolled diabetes mellitus or kidney disease). In such cases, measured losses may be added to the maintenance requirements. Elevated environmental temperature and exercise increase insensible losses (for each 2°C rise in temperature above 32°C, 500 ml of extra water should be provided). Acute alterations in water balance can be estimated by rapid changes in body weight.