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  3. Human Nutrition and Diet The scope of human nutrition extends far beyond the classical study of the physiological and bioche

Human Nutrition and Diet The scope of human nutrition extends far beyond the classical study of the physiological and bioche

admin2010-07-14  34
问题                                           Human Nutrition and Diet
    The scope of human nutrition extends far beyond the classical study of the physiological and biochemical processes involved in nourishment; i.e. , how substances in food are converted into energy and body tissues. Human nutrition has come to involve all the effects on humans of any component found in food; these include most chronic degenerative diseases (dental decay, coronary heart disease, some cancers, etc. ), which are now major targets of research activity. The scope of nutrition extends to the effects of food on human function; e.g. , mental function, athletic performance, resistance to infection, and fetal health and development. There is a growing interaction between nutritional science and genetics because of the diversity of human chemical make-up and because food components of which most people are unaware can have marked effects on some individuals. Last, nutrition also considers why people choose to eat the foods they do, even after they have been advised that doing so may be unhealthy. The study of food habits and people’s attitudes, beliefs, likes, and dislikes overlaps with the social sciences of physiology, anthropology, sociology, and economics. Dietetics is the application of nutrition in the health sciences.
    The principal evidence that an organic compound is essential is that it consistently cures a specific deficiency disease. An inorganic clement is essential if it regularly occurs in the body and is demonstrated to have a function (such as being an integral part of an enzyme). Humans require oxygen, water, food energy, protein, 14 other organic compounds (vitamins and essential fatty acids), and some 18 inorganic elements, as well as carbon, hydrogen, and nitrogen.
    Growing children need more protein per kilogram of body weight than do adults. Protein requirement at all ages is increased by infections not only because there is an increased utilization of protein but also because illness usually impairs the appetite and thus reduces dietary intake of all substances, including protein. In many countries children are weaned on a diet of cereal paps with little or no supplement of milk or other protein containing foods. Such a diet at the least retards growth and development. If a child on such a diet suffers from an acute infection, notably measles or gastroenteritis (胃肠炎), a severe illness may ensue. The death rate from protein-calorie malnutrition in many poverty stricken communities is high. Primary protein deficiency is not common among adults, for whom cereals in general satisfy the protein requirement.
    Iron is required for the synthesis of hemoglobin (血红蛋白), the oxygen-binding pigment in red blood ceils. Normally the iron liberated from old ceils is retained and can be reutilized. When, however, there is chronic bleeding from wounds or there is severe and prolonged menstruation, the normal amount of dietary iron may be insufficient to replenish the body’s supply. Losses of iron in the menses, the needs of a fetus, and the inevitable loss at labour and in the milk of a lactating (哺乳期的) woman increase the iron requirements of women during their reproductive life.
    Calcium is the most obvious and persistent of the mineral nutrients, yet it is more difficult to measure the adequacy of its intake than for other nutrients. More than 99 percent of the body’s calcium is in the skeleton, where it not only provides structural support but also is a large reservoir for maintaining a constant calcium concentration in plasma. Several hormones are all involved in this regulation. The amount of calcium in the bones is nearly 30 grams at birth and builds up to about 1,200 grams in an adult. Hence, an average of 180 milligrams of calcium must be retained in the body throughout childhood, and the individual daily amounts should reach 400 milligrams during the adolescent growth spurt. Absorption of calcium, like that of many other metallic elements, is inefficient. The diet must therefore supply more calcium than the amount theoretically retained for skeletal growth. Three major questions about calcium are as yet not fully answered by nutritional science: (1) whether generous intakes of calcium during childhood and adolescence will lead to taller adults or heavier bones, (2) whether a generous intake of calcium from about 45 years of age will delay the progress of osteoporosis (骨质疏松), which occurs in older people and is more likely to cause symptoms in women after menopause (更年期), and (3) what mechanisms enable people in Africa, for example, who grow up on cereal diets low in calcium, to retain enough calcium to achieve a skeleton of similar height to those in northern countries where milk and cheese (both rich in calcium) are staples in the diet.
    Sodium is present only in small quantities in most natural foods, but salt is added, often in large amounts, in food processing and by cooks to enhance flavour. Sodium is the predominant ion in extracellular fluid; an excess can cause edema (水肿), especially in conditions such as congestive heart failure. A low sodium intake leads to a lowering of the blood pressure and brings about diuresis (利尿), ridding the body of the excess extracellular fluid. There is now much evidence that excess dietary salt may contribute to high blood pressure in some individuals, but other factors are also responsible.
    For normal nutrition and metabolism the body requires certain organic substances, called vitamins, which it cannot make for itself, at least in sufficient quantities. Many of them function as components of enzyme systems. Because they are required in such small quantities, vitamins do not contribute significantly to the energy needs of the body. In their absence, however, the carbohydrates, fats, and proteins required for energy production and tissue maintenance cannot be properly metabolized. Vitamins were originally identified by letters as each new one was discovered, but this method is being replaced as the vitamins become known by their chemical names.
Nutritional science ______ three major questions about calcium.
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答案has not fully answered
解析 文章第五自然段最后一句说明此点。营养科学尚未能完全解答有关人体钙含量的三个问题。
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