NUTRITION  
 
    What we eat as well as how much we eat determine our nutrition status to an important extent, and influenced by a diversity of external and internal factors.

    The person who wants to fine the answer to the question" what should I eat for good nutrition? ”,might easily become lost in the maze of informational corridors, confused by the wealth of technical information provided by scientists or mislead by simplistic answers provided by those with products to sell.Somewhere in between is some reasonable, commonsense information that we can use to guide us our quest for sound nutrition knowledge  

    To begin, we need to learn some definitions of commonly used nutrition terms and find out what sorts of guidelines are available to help us measure the quality of our diets and to develop healthful eating patterns.
                        
    NUTRITION AND FOOD: DEFINITIONS  
 
    The word nutrition is often paired with the word food because the two go together. They are interdependent, but not interchangeable. 

    Food might be defined as any edible substance that provides nourishment when consumed. It is made up of many natural ingredients all chemicals that have different functions such as providing odor, flavor, color, and nourishment. The ingredients that give us nourishment are called nutrients. 

    These nutrients are categorized as fats, proteins carbohydrates (sugars and starches), minerals, vitamins, and water. They are called essential nutrients because we cannot get along without them. We need them for energy, for building and maintaining body tissue; and for regulating body processes the three essential functions of foods in the body. 

    Nutrition might be defined as the process whereby we obtain the essential nutrients and use them to make many other substances our bodies need, this process would include eating and digesting food and absorbing and using, or metabolizing, the nutrients it contains. 

    We can obtain all of the essential nutrients from food. However, it is possible to obtain nourishment without eating and digesting food-if, for example, the nutrients are injected directly to our veins as in intravenous feeding. 

    Thus, it is the nutrients that are essential and the food that normally provides them. Since food is vital, we need to know the nutritive content of foods, which ones are the best sources of the various nutrients and how to combine them into a healthful diet. 

    The term good nutrition implies that we are obtaining from our food all of the essential nutrients in the amounts needed to keep our bodies functioning and to maintain optimum health. A very simplified definition of good nutrition might be" eating the right foods in the right amounts.”

    The work of nutrition scientists involves finding in the body, the amount of each that we need, what happenings when we receive too much or too little and about food and died-what foods we should eat and in what amount. 

    Yet nutrition science in its broadest sense has many more facets: the influence of sensory factors of flavor, color, and texture of food on eating behavior; the psychological, cultural, emotional, and social aspects of food intake; and even the economics of food availability and consumer behavior in the purchase of food
                                  
    THE NUTRIENTS 
 
    To date, nutrition scientists have identified some 40 to 45 substances as essential nutrients. But the list is growing as new nutrients continue to be identified; the history of nutrition science contains fascinating stories about the ways food substances have been identified as essential nutrients. In some instances, medical researchers seeking the cause of a particular disease found that problem was due to a single substance, and that when this substance was added to the diet, the symptoms of the disease disappeared. A number of vitamins were discovered in this way. 

    Nutrients might be divided into two genial categories based on the amount that we need. These are the macronutrients (carbohydrates, fats, proteins, and water), which we need in relatively large amounts and the micronutrients) mineral elements and vitamins), which we need in relatively small amounts. All of the nutrients except for mineral elements and water are classified as organic chemicals because they contain the element carbon. Mineral elements and water are inorganic chemicals because they do not contain carbon. 

    The vitamins are divided into two general categorizes based on their solubility in either water or fat .the fat-soluble vitamins are vitamins A, D, E, and K; the water-soluble vitamins include vitamins C (ascorbic acid), niacin, thiamin, riboflavin, flacon (also called folic acid) antithetic acid, pyridoxine, vitamin B12 and biotin. 

    The mineral elements are divided into two categories based on the quantity of them that we need. Microelements are those needed in relatively large amounts, while microelements are those needed in very small amounts, some example of microelements are sodium, calcium, and phosphorus. Some example of microelements is iron, iodine, manganese, zinc, and fluorine. 
 
    RECOMMENDED DIETARY ALLOWANCES 
 
    Once a nutrient is identified, one of the principal research efforts of nutrition scientist is to determine how much of it is needed by people at various ages and stages of life. Initial studies usually are conducted with laboratory animals, but the information developed in these studies cannot be applied directly to humans since people’s needs often are quite different from animals’ needs. Human nutrition studies on the other hand, are time-consuming, costly, and difficult to conduct, especially because of the problems of controlling variables and possibly causing harm to the individuals involved. Because of the obstacles to collecting, accurate data, our present knowledge of nutrient needs is incomplete, and the requirements of humans for many nutrients have not been established. 

    However, the data on human and animal needs currently available are used by nutrition scientists to establish estimates of the amounts of essential nutrients per day that will meet the needs of most health persons. In the United States, the most widely used nutrient guidelines are the recommended dietary allowance (RDA), which are issued by the national academy of sciences, national research council, and food and nutrition board.   

    The RDA serves as dietary of nutritional standards for a wide range of age-weight-sex groups such as infants, children, adolescents, pregnant and lactating women, and younger and older adults. They are recommendations, not average requirements, for satisfactory levels of intake of essential nutrients of population groups of average, healthy people. They do not take account of special needs certain individuals may have due to genetic make up, metabolic disorders, chronic infections, and other abnormalities, which may result in their needing different levels of nutrients.                  
 
    MARGIN  of  SAFETY  ALLOWS  for  INDIVIDUAL DIFFERENCES 
 
    To allow for individual difference, the usually are set with a generous margin of safety. Thus, they are thought to meet the needs of 95 to 97 percent of the people within each age-sex group. In other words, the RDA exceed the requirements of most individuals to ensure that the needs of nearly all are meet. For this reason, a person who consumes a diet that provides less than the RDA for one or more essential nutrients is not necessarily getting a diet that is nutritionally inadequate. What can be concluded, however, is that the farther the intake of an essential nutrient falls below the RDA, the greater the probability of nutritional inadequacy, on the other hand, if an individual is getting all the essential nutrients at or above the RDA level of his or her age, chances are good that diet is nationally adequate. 

    An exception is the RDA for energy or calories, which are not designed, as guides for individual caloric needs. Other variables not included in the RDA, such as body size and physical activity, are involved in an individual’s caloric requirements
 
    DIFFERENCES  IN  NUTRIENT  UTILIZATION  CONSIDERED  
 
    Another factor considered when the RDA are established is the availability of the nutrient and factors that affect how efficiently it is used in the body for some nutrients, such as iron, absorption or use in the body may be incomplete; so the RDA needs to be set high enough to allow for this. And because in the case of certain other nutrients, substance found in carrots and other vegetables and fruits, which our bodies convert to vitamin A. 

    On the other side of the coin, receiving too much of certain nutrients, amounts significantly above the RDA, can be just harmful as not obtaining enough certain vitamins (such as A and D) and minerals can be highly toxic if high for optimal nutrient intake from the standpoint of both maximum and minimum levels.    

    第一课  营 养

    人们的营养情况在很大程度上取决于人们吃什么和吃多少。而这两方面又受体内、体外多种因素的影响。

    凡是渴望找到“应该吃什么才会有良好的营养”这一问题答案的人也许很容易会坠入复杂的知识迷宫，很容易被科学家所提供的丰富技术信息所迷惑，也很容易被销售商的简单化答案所误导。事实上，这中间有些信息是有一定道理的常识，我们可以利用这些信息来指导（我们去）寻求正确的营养知识。

    首先，我们要懂得一些常用营养学术语的定义，并找出一些准则，以便帮助我们去衡量饮食的质量和制定有益健康的饮食结构。

    营养与饮食的定义

    由于营养和食物的关系密切，所以营养和食物这两个词常常成对出现。他们互相依赖，但不能互相替换。

    食物可定义为食后起滋养作用的任何可食用的物质。食物由多种天然成分（全都是化学物质）组成。这些成分有各种各样的功效：如提供色、香、味和营养等等。那些具有营养功能的成分称为营养素。

    这些营养素可分为脂肪、蛋白质、碳水化合物（糖和淀粉）、矿物质、维生素和水。我们称这些营养素为必须营养素，因为没有它们我们就不能生存。我们需要这些营养素来提供能量，构成和修补机体组织，调节机体生理过程。这是食物在人体内的三种基本功能。

    营养可定义为人们赖以获得的必须营养素，并利用这些营养素来制造人体所需的其他物质的过程。这一过程理应包括摄入和消化食物以及吸收和利用（即同化）其中所含的营养素。

    那么能够从食物中获得全部必须营养素。尽管如此，我们也可以不通过食物的摄入和消化而得到丰富的营养。例如：静脉进食的情形就是直接将营养素注射到人体的静脉。

    由此可见，唯有营养素才是真正必需的，而正常提供营养素的则是食物。食物是维持生命所必不可少的，因此我们必须懂得各种食物的营养素含量，必须懂得哪些食物是各种各样营养素最丰富的来源，懂得怎样把它们配成有益健康的膳食。营养良好这一术语是指我们处在从食物中获取为保持机体正常活动和维持最佳健康状况所需的全部营养素。营养良好最简明的说法是：“适量食用恰当的食物”。

    营养学家的任务之一是解决营养素方面的问题，即营养素在人体内的功能、每天营养素的人体需要量以及营养素摄入过多或过少会产生什么样的后果等等；二是解决有关食物和膳食方面的问题，即我们应该吃什么，应该吃多少。

    然而，从广义来看，营养科学还涉及更多得方面：如食物的色、香、味、质构等感官因素对食用性能的影响；摄食的心理、文化教养、情绪、社交活动等的状态；甚至还包括获得食物的经济性以及消费者购买食物的态度。

    营 养 素 

    到目前为止，营养学家已确认40～45种物质为必须营养素。然而，随着不断的鉴定出新的营养素，这个数目会进一步增多。营养科学的历史记载着多种食物怎样被确认为必须营养素的有趣的故事。在有些情况下，医学研究者在寻找某种特殊疾病的病因时发现问题就由于缺乏某一种物质上，并且发现当把这种物质加入到病人的饮食中之后，病症便消失了。许多维生素就是这样被发现的。

    根据人体需要量的不同，营养素可分为量大类：需要量大的称为常量营养素（碳水化合物、脂肪、蛋白质和水）；需要量少的称为微量营养素（矿物质和维生素）。除了矿物质和水之外，所有其他营养素都属于有机化合物，因为它们都含有碳元素。矿物质和水都是无机化合物，因为它们不含碳元素。

    维生素按其可溶于水或脂肪分为两大类：脂溶性维生素，有维生素A、D、E、K；水溶性维生素，包括维生素C（抗坏血酸）、烟酸、硫胺素、核黄素、叶酸、泛酸、吡哆醇、维生素B12和生物素。

    矿物质按人体需要量分两类。人体需要量较大的称为常量元素，如钠、钾、钙、磷；人体需要量较小的称为微量元素，如铁、碘、锰、锌、氟。
推荐膳食标准 

    当一种营养素被确认后，营养学家的主要任务之一就是人们在不同年龄和不同的阶段需要多少这种营养素。初步研究一般通过动物实验进行，但这些研究所得的数据资料不能直接应用于人类，因为人的营养需要量常常不同于动物的营养需要量。另一方面，人体营养需要量好事厂、费用大，而切难于进行，特别是因为控制可变因素和可能对有关人员的危害问题上存在着困难。由于收集正确数据不便，目前我们对营养素的需要量的知识并不完备，因而许多营养素的需要量还未确定下来。

    尽管这样，营养学家还是运用了目前可利用的有关人类和动物营养素需要量的数据来确定能满足大多数健康人每天必须营养素需要量的概约值，在美国，使用最为普遍的营养素标准是“推荐膳食标准（RDA）”，它是由美国科学院全国科学研究委员会食品营养研究会颁布的。

    RDA供不同年龄-体重-性别组，如婴儿、儿童、青少年、孕妇、产妇、成年人和老年人作为膳食营养的标准。但这些标准只是为了满足全人口各组平均健康水平者适当摄入必须营养素的推荐量，而不是起平均需要量。制定RDA并不考虑某些个别人由于遗传体质、代谢紊乱、慢性传染病以及其他会引起不同营养水平需要的人体异常所造成的特殊需要量。

    考虑个别差异的安全余量 

    考虑到个别的种种差异，RDA中通常规定了充裕的安全余量。因此一般认为RDA能满足各年龄-性别组内95～97%的认得营养需要。换句话说，RDA标准超过大部分个体的需要量，以便保证几乎所有得人的需要量都能得到满足。因此，若某人饮食中提供的一种或多种必须营养素低于RDA，他的膳食也未必就是缺乏营养的膳食。不过可以肯定，必须营养素低于RDA越多，则缺乏营养的可能性就越大。另一方面，如某人摄如的所有必须营养素都正好等于或高于RDA按其年龄规定的标准，那么他的膳食就很大可能是营养充足的膳食。

    例外的情况是RDA对能量（热量）的要求并不设计成个体热量需要的标准。在个体热量需要量方面，要涉及到诸如体形、体力活动等其他可变因素，这些因素是RDA中所没有规定的。

    考虑到营养素应用上的差异 

    制定RDA时所考虑的另一个主要方面是营养素的利用律以及影响它在人体内如何被有效的利用的各种因素。对于某些营养素如铁，它在人体内的吸收和利用可能是不完全的，因此RDA为考虑到这种情况而将这种营养素的需要量订得很高。而对另一些营养素，由于它们可以从食物中某些称为前体的物质在人体内转化而成，故RDA对此酌情减低标准。例如存在于胡萝卜和其它果蔬中的橙色的胡萝卜素，它可在人体内转变成维A。

    另一方面，摄取某些营养素过多（大大超过RDA），也有可能与这些营养素摄取不足一样有害。某些维生素（如VA、VD）和矿物质若大计量的食用超过一段时间会有很大的毒性。因此，可把RDA作为从最高、最低两限量来看都是最佳的营养素摄入量标准来使有。