The food scientist has a many-sided interest in carbohydrates. He is concerned with their amounts in various foods, availability (nutritional and economic), methods of extraction and analysis, commercial forms and purity, nutritional valve, physiological effects, and functional properties in foods. Understanding their functional properties in processed foods requires not only knowledge of the physical and chemical properties of isolated carbohydrates, but also knowledge of the reactions and interactions that occur in situs between carbohydrates and other food constituents and the effects of these changes upon food quality and acceptance. This is a tall order for knowledge. Because processing affects both nutritional and esthetic values of food, knowledge of the changes that carbohydrates undergo during milling, cooking, dehydration, freezing, and storage is especially important.

    Students are advised to study the fundamental chemistry underlying useful carbohydrates properties Of service will be an understanding of the association of polar molecules through hydrogen bonding, ionic effects, substituent effects, chelation with inorganic ions, complexing with lipids and proteins, and decomposition reaction. This background will provide an understanding of properties that affect the texture and acceptance of processed foods (e.g., solubility, hygroscopicity, diffusion, osmosis, viscosity, plastity, and flavor production), properties that enable the formation or high quality pastries, gels, coatings, confections, and reconstitutable dehydrated and frozen foods.

    Ability to predict what changes in functional properties are likely to ensue from incorporating various types of carbohydrates into processed foods is a practical goal of the food scientist.Such forecasting requires either a wealth of experience with trial-and-error methods or a deep knowledge of carbohydrate properties as related to structure—perhaps both. However, scientific knowledge of cause and effect is highly respected when it shortens industrial development time 　
 
    Source, Types, and Terminology 
 
    The layman’s conception of carbohydrates generally involves only the sugars and starches of foods—those that generate calories and fat. The food chemist knows many other types that are ingested.

    Because most people enjoy the sweetness of sugars and the mouth feel of cooked starches, they become familiar by association with table sugar (sucrose), invert sugar’s hydrolyzed sucrose, corn syrup sugars (D-glucose and maltose), milk sugar (lactose), and the more starchy foods. These carbohydrates are nutritionally available; i .e., they are digested (hydrolyzed to component monosaccharides) and utilized by the human body。Carbohydrates of dietary fiber (cellulose, hemicelluse,  pentosans, and pectic substances), in contrast, tend to be overlooked because they are largely unavailable. Digestive enzymes do not hydrolyze them significantly; nevertheless, they may be quite important for human health.

    The carbohydrates of natural and processed foods are divided into available and unavailable types. The available carbohydrates vary in degrees of absorption and utilization depending upon quantities ingested, accompanying food types, and human differences in complements of defective enzymes and intestinal transport mechanisms. Malabsorption difficulties and adverse physiological effects are known for all the available carbohydrates but gelatinized starches give little or no trouble.

    It is important to realize that in ruminants the unavailable and most abundant polysaccharide cellulose is partially hydrolyzed to the same highly available sugar that starch provides upon digestion; i.e. D-glucose. Grazing animals do it through the celluloses generated by the microorganisms of their rumen. Cellulose is, therefore, a contributing source of voluble animal protein. Food chemists probably can improve upon the efficiency and economics of the ruminant’s conversion of cellulose to nutrients. Development of celluloses that are stable outside the cells of microorganisms enables the culturing of fungi and with yeasts on cellulose hydrolyzates. Fungi (e.g., mushrooms) can produce protein with the biological value of animal protein. The conversion of cellulose wastes to animal feed and human food is an intriguing prospect for limiting environmental pollution and for feeding the world’ expending population.

    Carbohydrates were first named according to their natural sources; e.g., beet sugar, cane sugar, grape sugar, malt sugar, milk sugar, cornstarch, liver glycogen, and sweet corn glycogen. Trivial names were then formed, in English terminology, often from a prefix related to the source followed by the suffix “-ose” to denote carbohydrate. Names arising in this way, for example, are fructose, maltose, lactose, xylose, and cellulose. These short, well-established names are still commonly used. They furnish no information on the chemical structures however, so a definitive carbohydrate nomenclature has been developed. From the definitive names, structural formulas can be written. Some of the terms involved in the definitive nomenclature are explained in the following paragraphs.

    The simple sugars (monosaccharides0 are basically aliphatic polyhydroxy aldehydes and ketones: HOCH2- (CHOH) n-CHO and HOCH2- (CHOOH) n-1-C-O-Ch2OH, called “aldoses” and “ketoses,” respectively. However, it must be understood that cyclic hemiacetals of those open-chain forms prevail I solids and at equilibrium in solutions. In the definitive nomenclature, the suffix “ose” is appended to prefixes denoting the number of carbon atoms in the nomosaccaride; e.g. trioses (n=1), tetroses (n=2), pentoses (n=3), hexoses (n=4) to distinguish aldoses from ketoses, ketoses are designated as”-uloses.” Thus, the simplest ketose, HOCH2-C:O-CH2OH, is a triulose; the most common ketose, D-fructose (levulose), is a hexlose. To designate the configurations of hydroxyl groups on the asymmetric carbon atoms of monosaccharides, the prefixes D and L are used together with prefixes derived from the trivial sugar names (e.g., D-glycero-, L-arabino-, D-xylo-) followed by pentose, hexose hexulose, etc.

    As open-chain hydroxy aldehydes and hydroxyl ketenes, the monosaccharides are very reactive. They readly enolize in alkaline soluions to reduce ions such as Cu2+ and Fe(CN)63-. Therefore, they are called “reducing sugars”. Plants protect the reactive monosaccharides for transport and storage by condensing them with loss of water, into less reactive sugars, e.g., D-glucose and D-fructose, are condensing in such a way that their reactive functions are lost to form the disaccharide no reducing sugar, sucrose. The less reactive sucrose is then transported to all parts of the plant for enzymin syntheses of oligo-and polysaccharides. From thousands or more D-glucose moieties of sucrose the glucans, starch and cellulose, are built. From the D-fructose moiety of sucrose, fructans such as inulin are assembled. Other polysaccharides are formed from other sugar, which rose by enzymic transformations of phosphorylated hexoes and sugar nucleotides.

    The prefix “glyc,” is used in a generic sense to designate sugars and their derivatives; e.g., glycoses, glycosides, glycosans, glyconic glyceric, and glycuronic acids. The generic name for polysaccharides is “glycan”homoglycansare composed of single monosaccharide; for example, the D-glucans, cellulose and starch, release only D-glucose by hydrolysis. Other homoglycans (e.g., the hexcsans, D-galactan and D-manan, and the pentosans, L-arabinan and D-xy-lan) are uncommon in nature. Heteroglycans, composed of two or more different monosaccharides, are widely distributed than the homoglycans that are not glucans. Galactomnnans, glucomammans, arabinogalactans, and arabinoxylans are common diheteroglycans(composed of two sugars).the glycant vail over free glycoses in natural foods.

    The reducing sugars are readily oxidized. mild oxidation of aldoses yields aldonic acids, HOCH2-(CHOH)n-COOH; e.g., gluconic acid(n=4).oxidation of both ends of the aldose molecule yields aldaric acids, HOOC-(CHOH)n-COOH; e.g., tartaric acid(n=2). Oxidation of the terminal CH2OH group of hexoses without oxidation of the reducing function (protected) produces hexuronic acids, HOOC-(CHOH)-CHO. The hexuronic acids are common monosaccharide constituents of many heteroglycans .for example, they are found in acidic hemicelluloses, pectic substances, alginpl and exudate gumes, and the mucopolysaccharides of mammalian tissues. Penturonic acids have not been found in nature.

    Reduction of aldoses or ketoses yield sugar alcohols ,properly called ‘alditols,” HOCH2-(CHOH)n-CH2OH.the suffix “-itol “ is applied to the trivial prefixes to denote different alditols; e.g., D-glucitol, D-manniitol, xylitol. The triitol, gllyceritol (by common usage, glycerol, n=1), is the alditol moiety of fats.Glycerol and D-glucitol(sorbitol) are acceptable and useful food addiaffinity for water. Pentitols(n=3) and hexutols(n=4) are found in small amount in many fruits, vegetables and hexitol, perseitol (n=5), and an octitol have been isolated from avocados. Some aditols are nutritionally available; others are not.

    Other types of carbohydrates found in food are the condensed N-acetylated amino sugars of mucopolysaccharides, glycoproteins, and chitin; the condense deoxy sugars of gum, mucilages, and nucleotides; glcosides (sugars condensed with nonsugars); glucosinolates (toxic thioglycosides); cyclitols (myoinositol, phytic acid); and reductone, L-ascorbic acid.

    Complex carbohydrates, such as cellulose and hemicellulose, are largely indigestible, as are a number of origins
 
    Carbohydrate Composition of Foods
 
    Detains need more exact information on the carbohydrate compassion of foods. Food pressers also make practical use of carbohydrate composition data. For example, the reducing sugar content of fruits and vegetables that are to be dehydrated or processed with heat is frequently an indicator of the extent of nonenzymic browing that can expected during processing and storage. The possible hydrolysis of sucrose to reducing sugars during processing also is to be considered .the natural changes in carbohydrate composition that occur during maturation and post harvest ripening of plant foods is therefore of particular interest to food chemists.

    Citrus fruits, which normally ripen on the tree and contain no starch, undergo little change in carbohydrate composition following harvest. However, in fruit that are picked before complete ripening (e.g., apples, bananas, pears), much of the stored starch is converted to sugars as ripening process. The reducing sugar content of potatoes also increase during the sun drying of grapes and dates, sucrose is converted to D-glucose and D-fructose; accordingly, the color of the dried products is deepened by nonenzymic browning reactions.

    Green peas, green beans, and sweet corn are picked before maturity to obtain succulent textures and sweetness. Later the sugars would be converted to polysaccharides, water would be lost, and tough textures would develop. In soybean, which is allowed to mature completely before harvest, the starch reserve is depleted as sucrose and galactosy lsucroses (raffinose, stachyose, verbascose, etc.) are form in the malting of cereal grains, rapid conversions of reserve carbohydrate to sugars occur as enzymes are strongly activated.

    In foods of animal origin, postmortem activity of enzymes must be considered when carbohydrate composition data is obtained. The glycogen of animal tissues, especially liver is rapidly depolymerized to D-glucose after slaughter, and immediate deep freezing is required to preserve the glycogen. Mammalian internal organs, such as liver, kidney, and brains also eggs and shellfish, provide small amount of D-glucose in the diet .Red fresh meats contain only traces of available carbohydrate (D-glucose, D-fructose, and D-ribose) and these are extracted into bouillons and broths. Dairy products provide the main source of mammalian carbohydrate. Whole cow’s milk contains about 4.9% carbohydrates and dried skim milk contains over 50% lactose.

    Examination of food composition tables shows that in general, cereals are highest in starch content and lowest in sugars. Fruit are highest in free sugars and lowest in starch .on a dry basis, the edible portions of fruit usually contain 80-90% carbohydrate. Legumes occupy intermediate portion with regard to starch and are high in unavailable carbohydrate.

    Glycosides of many types are widely distributed in plants. Certain biologically active thioglucosides, properly called “glucosinolates”, are found in significant amount in crucifers. Mustard oils, nitriles, and goitrins are released by enzymic hydrolysis. Their suspected goitrogenic in humans have been investigated, but the amount of glucosnolates normally consumed in food such as fresh cabbage (300-1000ppm), cauliflower, Brussels sprouts, turning, rutabagas, and radishes are not now believed to cause adverse physiological effects. Cyan genetic glycosides, which release hydrogen cyanide by enzymic hydrolysis under certain condition of vegetable maceration, are known to be sources of acute toxicity in certain animal feeds; however they are not active in the customary foods of developed countries. Certain foreign varieties of lima beans and manic root (cassava) may yield up to 0.3% hydrogen cyanide by weight, but lima beans distributed in the United States yield less than 0.02%. Saponins, the surface-active glycosides of steroids and triterpenoids, are found in low concentrations in tealeaves, spinach, asparagus, beets sugar beet (0.3%), yams, soybeans (0.5%), alfalfa (2-3%), and peanuts and other legumes.

    食品科學(xué)家對碳水化合物有著多方面的興趣。他們關(guān)心碳水化合物在各種食品中的含量，關(guān)心它的可利用性（營養(yǎng)上和經(jīng)濟上），它的提取方法和分析方法，它的商品形式和純度，它的營養(yǎng)價值、生理效用以及在食品中的功能特性。要了解碳水化合物在加工食品中的功能特性，不僅要有單離態(tài)碳水化合物的物理、化學(xué)性質(zhì)的知識，還要有碳水化合物與其它食物成分之間在加工食品中就地所發(fā)生的反應(yīng)和相互作用的知識，以及這些反映變化對食品質(zhì)量和食品可接受性的影響的知識。顯然要了解這些知識是不容易的。由于食品加工過程既影響食品的營養(yǎng)價值，也影響它的美學(xué)價值。所以了解碳水化合物在研磨、熱處理、脫水、冷凍和貯藏過程中所經(jīng)受的變化就顯得特別重要。

    建議學(xué)員們學(xué)習(xí)基礎(chǔ)化學(xué)，這時認(rèn)識碳水化合物有用性質(zhì)的基礎(chǔ)。了解和認(rèn)識極性分子通過氫鍵、離子效應(yīng)、取代基效應(yīng)、與無機離子螯合、與脂類和蛋白質(zhì)絡(luò)和以及分解反應(yīng)等的締和作用將具有重要的意義。這些基礎(chǔ)知識將有助于我們了解影響加工食品的質(zhì)構(gòu)和可接受性的性質(zhì)（例如溶解度、吸濕性、擴散性、滲透性、粘度、可塑性、風(fēng)味形成），了解使優(yōu)質(zhì)糕點、凝膠食品、糖衣、糖果和可復(fù)原脫水食品、冷凍食品得以形成的性質(zhì)。

    食品科學(xué)家實際工作目標(biāo)之一是能夠預(yù)測在把各種各樣碳水化合物摻到加工食品中之后，可能會發(fā)生什么樣的功能性質(zhì)變化。作這樣的預(yù)測要有豐富的用試探法摸索的經(jīng)驗，或者要具備有關(guān)結(jié)構(gòu)的碳水化合物的性質(zhì)的深奧知識，也可能上述兩者都要具備。不過，科學(xué)的因果關(guān)系知識只有在它縮短了工業(yè)產(chǎn)品試制周期之時才受到高度的重視。

    碳水化合物的來源、種類和技術(shù)名稱 

    外行人的碳水化合物概念一般僅包括食物中能產(chǎn)生熱量和脂肪的糖和淀粉，而食品科學(xué)家還知道許多攝入的他種碳水化合物。

    由于大多數(shù)人喜歡糖的甜味和熟淀粉的口感，所以它們由于經(jīng)常打交道而對食糖、（蔗糖）、轉(zhuǎn)化糖（蔗糖水解產(chǎn)物）、淀粉糖漿（D-葡萄糖和麥芽糖）、乳糖和含淀粉多的食品十分熟悉。這些碳水化合物都有很好的營養(yǎng)價值，即它們可分為人體所消化（水解成單糖成分）和利用。相反，食用纖維類碳水化合物（纖維素、半纖維素、戊聚糖、果膠物質(zhì)）因其大部分不能為人體所利用而往往被忽視。食用纖維類碳水化合物不能有效的被消化酶水解；盡管這樣，他對人體的健康可能相當(dāng)重要。

    天然食物和加工食物中的碳水化合物被分成人體可利用和不可利用的兩類。可利用的碳水化合物在吸收、利用的程度上也有差別，要看他的攝入量、伴隨食物的種類以及人體在消化酶互補情況上和腸道輸送機制上的差異而定。大家都知道，除了糊化淀粉大致上沒有問題以外，所有可利用碳水化合物都有吸收不良的問題和有害的生理影響。

    重要的是要意識到，在反芻動物方面，數(shù)量最豐富的人類不可利用的多糖類纖維素受到了部分水解，變成了淀粉消化時所形成的高度可利用糖，即D-葡萄糖。食草動物利用它們瘤胃中微生物所產(chǎn)生的纖維素酶來水解纖維素。因此，纖維素對有價值的動物蛋白而言是有一定貢獻(xiàn)的資源。反芻動物將纖維素轉(zhuǎn)化為營養(yǎng)素的效率和經(jīng)濟性有可能經(jīng)過食品化學(xué)家來改進。開發(fā)在微生物細(xì)胞外部能穩(wěn)定存在的纖維素酶使真菌和酵母在纖維素水解產(chǎn)物上培養(yǎng)有了可能。真菌（如蘑菇）能產(chǎn)出具有動物性蛋白質(zhì)生理效價的蛋白質(zhì)。對于控制環(huán)境污染和供應(yīng)增長著的世界人口的食品來說，將纖維素廢料轉(zhuǎn)化為動物飼料和人類食物有著誘人的前景。

    碳水化合物最初是按照它們的天然來源來命名的，例如甜菜糖、甘蔗糖、葡萄糖、麥芽糖、乳糖、玉米淀粉、肝糖原、甜玉米糖原。以后的英語名稱中就形成了唱以前綴表示來源，加后綴“-ose”表示碳水化合物的俗名。由此法產(chǎn)生的名稱，有如：fructose（果糖）、maltose（麥芽糖）、lactose（乳糖）、 xylose （木糖）、cellulose（纖維素）等。這些簡短而明確的名稱現(xiàn)在仍通用。可是，這些名稱不反映其化學(xué)結(jié)構(gòu)。于是就產(chǎn)生了碳水化合物的定形命名法。國際定形名稱，可以寫出其結(jié)構(gòu)式。下面幾段將對某些涉及定形命名法的專門術(shù)語作出解釋。

    單糖本質(zhì)上是脂族的多羥基醛和酮，即HOCH2-(CHOH)2-CHO和HOCH2-(CHOH)n-1-C:O-CH2OH,分別稱為醛糖和酮糖。不過要知道，固體中和平衡時溶液中的單糖是以開鏈型環(huán)狀半縮醛占多數(shù)。在定形命名法中，單糖名由表示單糖碳原子數(shù)的前綴加后綴“-ose”構(gòu)成。例如丙糖（trioses，n=1）、丁糖（tetroses，n=2）、戊糖（pentoses，n=3），己糖（hexose，n=4），為區(qū)別醛糖和酮糖，就把酮糖后綴加成“-ulose”。這樣，最簡單的酮糖HOCH2-C：O-CH2OH即為triulose（丙酮糖）；最常見得酮糖D-果糖（左旋糖）即為hexulose（己酮糖）。為表明單糖不對稱碳原子上羥基的構(gòu)型，可將前綴D和L與取自俗名的前綴一起使用（例如D-甘油-、L-阿拉伯-、D-木-），后面在接上戊糖、己糖、己酮糖等等。

    以開鏈的羥基醛和羥基酮形式 存在的單糖非�；顫�。它們在堿性溶液中易于烯醇化而使Cu2+、Fe（CN）63+之類的離子還原，因此稱它們?yōu)?ldquo;還原糖”。植物依靠脫水縮和方式將活潑的單糖變成不大活潑的糖類來保存單糖的還原活性，以便輸送和貯存。例如，D-葡萄糖和D-果糖縮合成無還原性雙糖——蔗糖時，變失去了活潑的功能。不太活潑的蔗糖然后被輸送到植物的各個部位，由酶合成低聚糖和多聚糖。由數(shù)千或更多的半個蔗糖分子D-葡萄糖構(gòu)成葡聚糖、淀粉和纖維素；而由另半個蔗糖分子D-果糖聚合成為果聚糖，如菊粉。其它多聚糖則是由磷酸化己糖和核苷酸糖通過酶作用生成的其它糖聚合形成的。

    前綴“glyc”（甘、糖）在一般意義上用來表示糖及其衍生物。例如：glycoses（單糖）、glycosides（糖苷）、glycosans（聚糖）、glyconic acids (糖酸) 、glyconic acids（甘油酸）、glycuronic acids（糖醛酸）。多聚糖的俗名是“聚糖”。均聚糖由一種單糖構(gòu)成。例如D-葡聚糖、纖維素和淀粉，它們水解時只產(chǎn)生D-葡萄糖。其它均聚糖（例如己聚糖D-半乳聚糖和D-甘露聚糖；戊聚糖有L-阿拉伯聚糖D-木聚糖）在自然界中很少見到。雜聚糖由兩種或多種單糖構(gòu)成。它們在自然界的分布比均聚糖（除葡聚糖外）廣泛 。半乳甘露聚糖、葡甘露聚糖、阿拉伯半乳聚糖、阿拉伯木聚糖時常見的雙雜聚糖（由兩種糖組成）。天然食物中的聚糖大大多余游離單糖。

    還原糖易于氧化。醛糖經(jīng)輕度氧化可產(chǎn)生醛糖酸，HOCH2-（CHOH）n-COOH，如葡萄糖酸（n=4）。醛糖分子兩端氧化形成糖二酸，HOOC-（CHOH）n-COOH，例如酒產(chǎn)生的糖醛酸，HOOC-（CHOH）4-CHO。己糖醛酸是組成多種雜聚糖的常見單糖成分，例如在酸性半纖維素、果膠物質(zhì)、藻酸、植物滲出膠和哺乳動物組織中的粘多糖中就有己糖醛酸。戊糖醛酸還未在自然界中發(fā)現(xiàn)。

    醛糖或酮糖被還原后變產(chǎn)生糖醇，確實的叫法為“多羥糖醇”（alditols），HOCH2-（CHOH）n-CH2OH。其后綴“-itol”（糖醇）接在俗名前綴之后表示不同的多羥糖醇，例如D-葡糖醇、D-甘露糖醇、木糖醇。丙糖醇，即甘油醇（常稱為甘油，n=1），是脂肪的多羥基糖醇部分。甘油和葡糖醇（山梨醇）成為受歡迎的有用的食品添加劑，因為它們能生成葡萄糖，并依靠其強烈的親水作用保持食品是濕潤。許多水果、蔬菜和菇類中存在有少量的戊糖醇（n=3）和己糖醇（n=4）。庚糖醇、甘露庚糖醇（n=5）和某種辛糖醇已從鱷梨中分離出來。有些多羥糖醇是有營養(yǎng)的，有些則無營養(yǎng)。

    在食物中發(fā)現(xiàn)它種碳水化合物還有：粘多糖、糖蛋白、殼多糖之類的縮聚N-乙酰氨基糖；植物膠、粘漿、核苷酸之類的縮合脫氧糖；糖苷（糖與非糖縮合）；芥子油苷（毒性的硫代糖苷）；環(huán)多糖（肌醇、植酸）；L-抗壞血酸（一種還原酮）。

    復(fù)雜碳水化合物如纖維素、半纖維素基本上是不能消化的，就象植物性食物中所找到的許多低聚糖、某些其它碳水化合物、樹膠和纖維質(zhì)材料一樣。

    食物中碳水化合物組成

    營養(yǎng)學(xué)家需要有較為嚴(yán)格的有關(guān)食物碳水化合物組成的知識。食品加工者也把碳水化合物組成的知識用于實際工作。例如，欲待脫水或熱處理的水果和蔬菜，其還原糖含量常常時一項預(yù)示加工和貯藏過程中非酶褐變程度的指標(biāo)。當(dāng)然也要考慮加工過程中蔗糖有可能水解變成還原糖。因此，食品化學(xué)家對植物性食物在成熟和后熟期間所發(fā)生的碳水化合物組成的自然變化特別感興趣。

    在果樹上正常成熟且不含淀粉的柑桔類水果，其碳水化合物組成在摘下后很少變化�？墒牵�在許多完全成熟前摘下的水果（如蘋果、香蕉、梨）中，貯存的大部分淀粉會在繼續(xù)成熟時轉(zhuǎn)化為糖。馬鈴薯在冷藏間還原糖含量也會增加。葡萄和海棗在曬干期間，隨其內(nèi)源蔗糖酶活性的不同，會不同程度地使蔗糖轉(zhuǎn)化D-葡萄糖和D-果糖。因此，其干制品的顏色也會因非酶褐變而加深。

    豌豆、青刀豆和甜玉米在成熟前摘下是為了獲得多汁鮮嫩的質(zhì)構(gòu)和甜味。遲了，所含的糖就會轉(zhuǎn)成多聚糖，水分會失掉，就會產(chǎn)生韌性的質(zhì)構(gòu)。熟透后在摘收的大豆，其淀粉儲量隨蔗糖和類半乳蔗糖（棉籽糖、水蘇糖、毛蕊花糖等）的形成而耗盡。用谷物加工麥芽糖使，由于酶被極度活化而使得貯存的碳水化合物迅速轉(zhuǎn)化成糖。

    在動物性食物方面，當(dāng)獲得碳水化合物組成數(shù)據(jù)時，必須仔細(xì)估量動物宰后酶的活性。動物組織（尤其是肝臟）中的糖原在宰后會迅速解聚成D-葡萄糖，所以，要保存糖原，就得立即進行深度冷凍。哺乳動物內(nèi)部器官（如肝、腎、腦和蛋類、貝類所提供的D-葡萄糖只占飲食中D-葡萄糖的一小部分。紅色的新鮮肉類僅含極少可利用的碳水化合物（D-葡萄糖、D-果糖和D-核糖），而這些碳水化合物被溶出進入肉汁和肉湯。乳制品提供了哺乳動物碳水化合物的主要來源，牛全乳含碳水化合物約4.9%，脫脂乳粉含乳糖量超過50%。

    查看一些食物成分表便可知道，通常谷類的淀粉含量最高而糖含量最低。水果類游離糖最多，淀粉最少。以干基計算，水果的可食部分通常含80～90%的碳水化合物。豆類含淀粉不多不少，但含有大量的不可利用的碳水化合物。

    在植物界，廣泛分布著多種糖苷。在十字花科植物中大量存在著某些具有生理活性的硫葡糖苷（確切稱之為“芥子油柑”）。芥子油柑經(jīng)酶水解產(chǎn)生芥子油、腈和甲狀腺腫素。人們已經(jīng)研究了它在人體中可疑的甲狀腫病源性質(zhì)。但目前還不認(rèn)為食用新鮮卷心菜（其中芥子油柑量為300～1000ppm）、花椰菜、抱子甘藍(lán)、蕪菁、蕪菁甘藍(lán)、小蘿卜之類食物中的芥子油苷數(shù)量會引起有害的生理效應(yīng)。生氰苷在一定蔬菜浸漬條件下，通過酶水解能釋放出氰化氫。人們都知道生氰苷是某些動物飼料劇毒性的主要來源�？墒窃诎l(fā)達(dá)國家的平常食物中，生氰苷沒有活性。某些外國品種的菜豆和木薯可產(chǎn)生高達(dá)0.3%的氰化氫（按重量計），但遍布在美國的菜豆產(chǎn)氰化氫低于0.02%。皂角苷類（類固醇和三萜系化合物的表面活性糖苷）會以低濃度出現(xiàn)在茶葉、菠菜、蘆筍、甜菜、糖用甜菜（0.3%）、甘薯、大豆（0.5%）、苜蓿（2～3%）、花生及其它豆類。