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(yǎng)

    人們的營養(yǎng)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(nèi)、體外多種因素的影響。

    凡是渴望找到“應(yīng)該吃什么才會有良好的營養(yǎng)”這一問題答案的人也許很容易會墜入復(fù)雜的知識迷宮，很容易被科學(xué)家所提供的豐富技術(shù)信息所迷惑，也很容易被銷售商的簡單化答案所誤導(dǎo)。事實(shí)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(dǎo)（我們?nèi)ィ�尋求正確的營養(yǎng)知識。

首先，我們要懂得一些常用營養(yǎng)學(xué)術(shù)語的定義，并找出一些準(zhǔn)則，以便幫助我們?nèi)ズ饬匡嬍车馁|(zhì)量和制定有益健康的飲食結(jié)構(gòu)。

營養(yǎng)與飲食的定義

    由于營養(yǎng)和食物的關(guān)系密切，所以營養(yǎng)和食物這兩個(gè)詞常常成對出現(xiàn)。他們互相依賴，但不能互相替換。

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

    這些營養(yǎng)素可分為脂肪、蛋白質(zhì)、碳水化合物（糖和淀粉）、礦物質(zhì)、維生素和水。我們稱這些營養(yǎng)素為必須營養(yǎng)素，因?yàn)闆]有它們我們就不能生存。我們需要這些營養(yǎng)素來提供能量，構(gòu)成和修補(bǔ)機(jī)體組織，調(diào)節(jié)機(jī)體生理過程。這是食物在人體內(nèi)的三種基本功能。

    營養(yǎng)可定義為人們賴以獲得的必須營養(yǎng)素，并利用這些營養(yǎng)素來制造人體所需的其他物質(zhì)的過程。這一過程理應(yīng)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(yǎng)素。

    那么能夠從食物中獲得全部必須營養(yǎng)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(yǎng)。例如：靜脈進(jìn)食的情形就是直接將營養(yǎng)素注射到人體的靜脈。

    由此可見，唯有營養(yǎng)素才是真正必需的，而正常提供營養(yǎng)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(yǎng)素含量，必須懂得哪些食物是各種各樣營養(yǎng)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(yǎng)良好這一術(shù)語是指我們處在從食物中獲取為保持機(jī)體正常活動(dòng)和維持最佳健康狀況所需的全部營養(yǎng)素。營養(yǎng)良好最簡明的說法是：“適量食用恰當(dāng)?shù)氖澄铩薄?/FONT>