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.    

    第一課  營 養

    人們的營養情況在很大程度上取決于人們吃什么和吃多少。而這兩方面又受體內、體外多種因素的影響。

    凡是渴望找到“應該吃什么才會有良好的營養”這一問題答案的人也許很容易會墜入復雜的知識迷宮，很容易被科學家所提供的豐富技術信息所迷惑，也很容易被銷售商的簡單化答案所誤導。事實上，這中間有些信息是有一定道理的常識，我們可以利用這些信息來指導（我們去）尋求正確的營養知識。

    首先，我們要懂得一些常用營養學術語的定義，并找出一些準則，以便幫助我們去衡量飲食的質量和制定有益健康的飲食結構。

    營養與飲食的定義

    由于營養和食物的關系密切，所以營養和食物這兩個詞常常成對出現。他們互相依賴，但不能互相替換。

    食物可定義為食后起滋養作用的任何可食用的物質。食物由多種天然成分（全都是化學物質）組成。這些成分有各種各樣的功效：如提供色、香、味和營養等等。那些具有營養功能的成分稱為營養素。

    這些營養素可分為脂肪、蛋白質、碳水化合物（糖和淀粉）、礦物質、維生素和水。我們稱這些營養素為必須營養素，因為沒有它們我們就不能生存。我們需要這些營養素來提供能量，構成和修補機體組織，調節機體生理過程。這是食物在人體內的三種基本功能。

    營養可定義為人們賴以獲得的必須營養素，并利用這些營養素來制造人體所需的其他物質的過程。這一過程理應包括攝入和消化食物以及吸收和利用（即同化）其中所含的營養素。

    那么能夠從食物中獲得全部必須營養素。盡管如此，我們也可以不通過食物的攝入和消化而得到豐富的營養。例如：靜脈進食的情形就是直接將營養素注射到人體的靜脈。

    由此可見，唯有營養素才是真正必需的，而正常提供營養素的則是食物。食物是維持生命所必不可少的，因此我們必須懂得各種食物的營養素含量，必須懂得哪些食物是各種各樣營養素最豐富的來源，懂得怎樣把它們配成有益健康的膳食。營養良好這一術語是指我們處在從食物中獲取為保持機體正常活動和維持最佳健康狀況所需的全部營養素。營養良好最簡明的說法是：“適量食用恰當的食物”。

    營養學家的任務之一是解決營養素方面的問題，即營養素在人體內的功能、每天營養素的人體需要量以及營養素攝入過多或過少會產生什么樣的后果等等；二是解決有關食物和膳食方面的問題，即我們應該吃什么，應該吃多少。

    然而，從廣義來看，營養科學還涉及更多得方面：如食物的色、香、味、質構等感官因素對食用性能的影響；攝食的心理、文化教養、情緒、社交活動等的狀態；甚至還包括獲得食物的經濟性以及消費者購買食物的態度。

    營 養 素 

    到目前為止，營養學家已確認40～45種物質為必須營養素。然而，隨著不斷的鑒定出新的營養素，這個數目會進一步增多。營養科學的歷史記載著多種食物怎樣被確認為必須營養素的有趣的故事。在有些情況下，醫學研究者在尋找某種特殊疾病的病因時發現問題就由于缺乏某一種物質上，并且發現當把這種物質加入到病人的飲食中之后，病癥便消失了。許多維生素就是這樣被發現的。

    根據人體需要量的不同，營養素可分為量大類：需要量大的稱為常量營養素（碳水化合物、脂肪、蛋白質和水）；需要量少的稱為微量營養素（礦物質和維生素）。除了礦物質和水之外，所有其他營養素都屬于有機化合物，因為它們都含有碳元素。礦物質和水都是無機化合物，因為它們不含碳元素。

    維生素按其可溶于水或脂肪分為兩大類：脂溶性維生素，有維生素A、D、E、K；水溶性維生素，包括維生素C（抗壞血酸）、煙酸、硫胺素、核黃素、葉酸、泛酸、吡哆醇、維生素B12和生物素。

    礦物質按人體需要量分兩類。人體需要量較大的稱為常量元素，如鈉、鉀、鈣、磷；人體需要量較小的稱為微量元素，如鐵、碘、錳、鋅、氟。
推薦膳食標準 

    當一種營養素被確認后，營養學家的主要任務之一就是人們在不同年齡和不同的階段需要多少這種營養素。初步研究一般通過動物實驗進行，但這些研究所得的數據資料不能直接應用于人類，因為人的營養需要量常常不同于動物的營養需要量。另一方面，人體營養需要量好事廠、費用大，而切難于進行，特別是因為控制可變因素和可能對有關人員的危害問題上存在著困難。由于收集正確數據不便，目前我們對營養素的需要量的知識并不完備，因而許多營養素的需要量還未確定下來。

    盡管這樣，營養學家還是運用了目前可利用的有關人類和動物營養素需要量的數據來確定能滿足大多數健康人每天必須營養素需要量的概約值，在美國，使用最為普遍的營養素標準是“推薦膳食標準（RDA）”，它是由美國科學院全國科學研究委員會食品營養研究會頒布的。

    RDA供不同年齡-體重-性別組，如嬰兒、兒童、青少年、孕婦、產婦、成年人和老年人作為膳食營養的標準。但這些標準只是為了滿足全人口各組平均健康水平者適當攝入必須營養素的推薦量，而不是起平均需要量。制定RDA并不考慮某些個別人由于遺傳體質、代謝紊亂、慢性傳染病以及其他會引起不同營養水平需要的人體異常所造成的特殊需要量。

    考慮個別差異的安全余量 

    考慮到個別的種種差異，RDA中通常規定了充裕的安全余量。因此一般認為RDA能滿足各年齡-性別組內95～97%的認得營養需要。換句話說，RDA標準超過大部分個體的需要量，以便保證幾乎所有得人的需要量都能得到滿足。因此，若某人飲食中提供的一種或多種必須營養素低于RDA，他的膳食也未必就是缺乏營養的膳食。不過可以肯定，必須營養素低于RDA越多，則缺乏營養的可能性就越大。另一方面，如某人攝如的所有必須營養素都正好等于或高于RDA按其年齡規定的標準，那么他的膳食就很大可能是營養充足的膳食。

    例外的情況是RDA對能量（熱量）的要求并不設計成個體熱量需要的標準。在個體熱量需要量方面，要涉及到諸如體形、體力活動等其他可變因素，這些因素是RDA中所沒有規定的。

    考慮到營養素應用上的差異 

    制定RDA時所考慮的另一個主要方面是營養素的利用律以及影響它在人體內如何被有效的利用的各種因素。對于某些營養素如鐵，它在人體內的吸收和利用可能是不完全的，因此RDA為考慮到這種情況而將這種營養素的需要量訂得很高。而對另一些營養素，由于它們可以從食物中某些稱為前體的物質在人體內轉化而成，故RDA對此酌情減低標準。例如存在于胡蘿卜和其它果蔬中的橙色的胡蘿卜素，它可在人體內轉變成維A。

    另一方面，攝取某些營養素過多（大大超過RDA），也有可能與這些營養素攝取不足一樣有害。某些維生素（如VA、VD）和礦物質若大計量的食用超過一段時間會有很大的毒性。因此，可把RDA作為從最高、最低兩限量來看都是最佳的營養素攝入量標準來使有。