Lesson 5

Reading Material Vitamin

Vitamins are organic compounds present in small amounts in foods，which must be provided to the body，to ensure normal growth and maintenance of the body. Now we know that vitamins are one of the six classes of nutrients supplied by food. Vitamins are important for the body regulatory and protective functions. Unlike most other nutrients，they are required in very small amounts. With few exceptions，humans cannot synthesize most vitamins and therefore need to obtain them from food and supplements. Insufficient levels of vitamins result in deficiency diseases（e. g.，scurvy and pellagra，which are due to the lack of ascorbic acid（vitamin C）and niacin，respectively）.

The main exceptions are vitamin D，a hormone-like vitamin that can be synthesized in the skin upon exposure to sunlight and other forms of ultraviolet radiation，and therefore is not absolutely required in the diet，and vitamin K，which can be synthesized by intestinal microflora. Some vitamins have multiple chemical forms or isomers that vary in biological activity. For example，four forms of tocopherol occur in food，withα-tocopherol having the greatest vitamin E activity. Numerous carotenoids occur in nature and in food，but only a few have vitamin A activity. Some carotenoids，such as lycopene，do not possess vitamin A activity but，nonetheless，have important health benefts.

Classification

Vitamins are conveniently classified into two groups on the basis of their solubility（in fat or in water）into fat-soluble and water-soluble vitamins. Fat-soluble vitamins include A，D，E and K，and water-soluble vitamins include the B-group [e. g. vitamins B₁（thiamin），B₂（riboflavin），B₃（niacin），B₅（pantothenic acid），B₆（pyridoxine），B₁₂（cyanocobalamin），biotin，and folic acid] and vitamin C. Foods differ greatly in the amount and kinds of the vitamins they supply.

Fat- soluble vitamins are capable of being stored in the body and can accumulate. They are not easily destroyed by heat during cooking or processing or through exposure to air. In contrast，water - soluble vitamins usually do not accumulate in the body because they are stored only in small quantities. However，this property allows deficiencies of water-soluble vitamins to occur faster than deficiencies of fat-soluble vitamins. Excessive dosages of water-soluble vitamins generally are excreted in the urine but can lead to toxicity if the dosages are high enough.

Function

The vitamins comprise a diverse group of organic compounds that are nutritionally essential micronutrients. Vitamins function in vivo in several ways，including（a）as coenzymes or precursors（niacin，thiamin，riboflavin，biotin，pantothenic acid，vitamin B₆，vitamin B₁₂，and folate）；（b）as components of the antioxidative defense system（ascorbic acid，certain carotenoids，and vitamin E）；（c）as factors involved in genetic regulation（vitamins A，D，and potentially several others）；and（d）with specialized functions such as vitamin A in vision，ascorbate in various hydroxylation reactions，and vitamin K in specific carboxylation reactions.

Most of the vitamins exist as groups of structurally related compounds exhibiting similar nutritional function. Various forms of each vitamin can exhibit vastly different stability（e. g.，pH of optimum stability and susceptibility to oxidation）and reactivity. For example，tetrahydrofolic acid and folic acid are two folates that exhibit nearly identical nutritional properties.

Toxicity

In addition to the nutritional role of vitamins，it is important to recognize their potential toxicity. Vitamins A，D，and B₆are of particular concern in this respect. Episodes of vitamin toxicity are nearly always associated with overzealous consumption of nutritional supplements. Potential toxicity also exists from inadvertent excessive fortification，as having occurred in an incident with vitamin D-fortified milk. Instances of intoxication from vitamins occur endogenously in food are exceedingly rare.

Sources of Vitamins

Although vitamins are consumed in the form of supplements by a growing fraction of the population，the food supply generally represents the major and most critically important source of vitamin intake. Foods，in their widely disparate forms，provide vitamins that occur naturally in plant，animal，and microbial sources as well as those added in fortifications. In addition，certain dietetic and medical foods，enteric formulas，and intravenous solutions are formulated so that the entire vitamin requirements of the individual are supplied from these sources.

The popular literature has emphasized that natural vitamins are superior to synthetic ones，even though the active ingredient is often chemically identical in both forms. However，added folic acid in dietary supplements or fortified foods is better absorbed than naturally occurring folates in foods. Pregnant women administered synthetic folic acid（pteroylglutamic acid）had higher serum folate levels than women given the natural conjugated form of the vitamin.

Regardless of whether the vitamins are naturally occurring or added，the potential exist for losses by chemical or physical（leaching or other separations）means. Losses of vitamins are，to some degree，inevitable in the manufacturing，distribution，marketing，home storage，and preparation of processed foods，and losses of vitamins，can also occur during the post-harvest handling and distribution of fruits and vegetables and during the post-slaughter handling and distribution of meat products. Since the modern food supply is increasingly dependent on processed and industrially formulated foods，the nutritional adequacy of the food supply depends，in large measure，on our understanding of how vitamins are lost and on our ability to control these losses.

Vocabulary

ascorbic acid 抗坏血酸（维生素C）

fat-soluble 脂溶性

carotenoids 类胡萝卜素

intestinal 肠道的

microflora 微生物群落

tocopherol 生育酚

niacin 烟酸

thiamin 硫胺素（维生素B₁）

regulatory 调节的

vitamin 维生素

supplement 补充剂

参考译文 维生素

维生素是食品中含量较低的一类有机化合物，它们是机体维持正常生理功能所必需的。我们知道维生素是食物中的六大营养物质之一，它们对机体有重要的调节和保护功能。与其他大多数营养物质不同，维生素的需求量非常少。除个别维生素外，人类不能合成大多数维生素，需要从食物和补充剂中获得。维生素缺乏会导致一些疾病[如坏血病和糙皮病分别是由于抗坏血酸（维生素C）和烟酸的缺乏]。

维生素D是个例外，它是一种类似激素的维生素，皮肤在阳光和其他形式的紫外线照射下能合成维生素D，因此饮食中并不是绝对需要维生素D，维生素K则可以通过肠道微生物合成。有些维生素有多种化学形式或异构体，生物活性各异。例如，生育酚在食品中有四种形式，其中α-生育酚具有最大的维生素E活性。许多类胡萝卜素存在于自然界和食物中，但只有少数有维生素A活性。一些类胡萝卜素如番茄红素等，虽不具有维生素A的活性，但对健康大有益处。

分类

根据维生素溶解性（溶于脂肪或水）可将其简单的分成脂溶性和水溶性维生素两类。脂溶性维生素包括维生素A、维生素D、维生素E、维生素K，水溶性维生素包括B族维生素和维生素C，其中B族维生素如维生素B₁（硫胺素）、维生素B₂（核黄素），维生素B₃（烟酸）、维生素B₅（泛酸）、维生素B₆（吡哆醇）、维生素B₁₂（氰钴胺素）、生物素和叶酸。不同食物所含维生素的含量和种类不同。

脂溶性维生素能够储存在生物体内，因此可以积累。它们在烹饪、加工或暴露于空气时不易被热量破坏。相比之下，水溶性维生素通常不会积聚在体内，因为它们只能少量储存。然而，这种性质使水溶性维生素的缺乏症比脂溶性维生素的缺乏症的发生率更大。过量的水溶性维生素通常在尿中排出，但如果剂量足够高，则会引起毒性。

功能

维生素是由各种各样的有机化合物构成的，它们是营养必需的微量营养物质。维生素在体内的功能有：（a）作为辅酶或前体物质（如烟酸、硫胺素、核黄素、生物素、泛酸、维生素B₆，维生素B₁₂，叶酸）；（b）作为抗氧化防御系统的组成部分（抗坏血酸、类胡萝卜素、维生素E）；（c）作为基因调控因子（维生素A、维生素D，以及潜在的其他维生素）；（d）一些特殊功能，如维生素A与视觉，抗坏血酸的各种羟基化反应，维生素K的特殊羧基化反应。

大多数维生素以一组结构相似的化合物形式存在，它们具有相似的营养功能。每种维生素的不同形式则表现出不同的稳定性（如最稳定pH，氧化敏感性）和反应活性。例如，四氢叶酸和叶酸具有几乎相同的营养特性。

毒性

维生素除了具有营养功能外，认识到它们的潜在毒性也是非常重要的。在这方面，维生素A、维生素D和维生素B₆需要特别注意。维生素中毒几乎总是与过量摄取营养补充剂有关。维生素体现潜在毒性也存在于无意间的过度强化，如维生素D强化牛乳的中毒事件就发生过。在食物中因内源性维生素中毒的案例极少发生。

维生素来源

随着人口增长，维生素常以补充剂形式来补充摄入，但通过食物供应是主要的和最重要的维生素来源。食物中维生素的来源形式广泛且多样，包括从植物、动物和微生物中获取，以及那些添加了维生素的强化剂。此外，也出现了一些饮食和医疗食品、肠溶配方和静脉注射液的维生素补充源，来满足个体对全部维生素的需求。

比较流行的说法是天然维生素优于合成的维生素，即使它们的活性成分的化学性质相同。然而，叶酸的膳食补充剂或强化食品比食品中的天然叶酸更易吸收。孕妇服用合成叶酸（叶酸）后，血清中的叶酸水平高于服用天然共轭形式的叶酸维生素。

无论维生素是自然存在还是添加的，化学或物理（浸出或其他分离方法）方式都有可能造成维生素损失。一定程度上，在食品的生产、分销、营销、家庭储藏和加工食品的准备过程中维生素的损失都是不可避免，并且维生素的损失也可能发生在果蔬的采后处理、分装及肉制品屠宰后的处理和流通过程中。由于现在的食品供应越来越依赖加工和工业配制的食品，这些食品的营养充足程度在很大程度上取决于维生素的损失程度以及我们控制这些损失的能力。

Exercise

1.Answer questions

（1）What is Vitamin? What are the common features of vitamins?

（2）What are the functions of vitamin?

（3）List three important dietary sources of the following vitamins：

（a）Vitamin A

（b）Thiamin

（c）Riboflavin

（d）Niacin

（e）Ascorbic acid

2.Translation

（1）Vitamins are organic compounds present in small amounts in foods，which must be provided to the body，to ensure normal growth and maintenance of the body.

（2）Vitamins are conveniently classified into two groups on the basis of their solubility（in fat or in water）into fat-soluble and water-soluble vitamins. Fat-soluble vitamins include A，D，E and K，and water-soluble vitamins include the B-group [e. g. vitamins B₁（thiamin），B₂（riboflavin），B₃（niacin），B₅（pantothenic acid），B₆（pyridoxine），B₁₂（cyanocobalamin），biotin，and folic acid] and vitamin C.