NUTRITION.  Every person, like every other living being, needs certain substances called nutrients, to build and maintain the body’s structure and to supply the energy that powers all vital activities. The science that studies nutrients—determining what they are, in what quantities they are found in different foods, how they are utilized by the body, and in what quantities they are needed---is called nutrition. The present article discusses human nutrition, but much of what is said applies to other animals. For a discussion of plant nutrients, see FERTILIZER.

营养。每个人，就像所有其它的生命个体一样，都需要某种称为营养物的物质，来建立和维持身体的结构，并为所有生命活动提供动力能量。该研究营养物的学科---确定它们是什么，发现它们在不同食物中的数量，它们是如何被身体利用的，以及它们需的数量，被称为营养学。该文讨论的是人类的营养，但所涉及的许多部分也适用于其它动物。有关植物营养的讨论，可参阅肥料词条。

There are two main classes of nutrients: the macronutrients, which are needed in relatively large amounts and make up the bulk of all diets, and the micronutrients, which are needed in very much smaller amounts. The macronutrients, including proteins, carbohydrates, and fats, supply calories (energy) as well as structural materials. The micronutrients, including vitamins and minerals, do not serve as calorie sources but play essential roles in the body’s chemistry. In addition to these generally recognized nutrients, certain other components of foodstuffs affect the nutritional value of diets. Fiber, for example, is not a nutrient but does add desirable bulk and roughage to the diet.

营养物主要有两类：常量营养物质，对它们的需要量相对较大，并且构成了所有饮食的主体，以及微量营养物质，对它们的需要量要少得多。常量营养物质，包括蛋白质、碳水化合物以及提供热量（能量）和结构材料的脂肪。微量营养物质，包括维生素和矿物质，不作为热量的来源，但在身体的化学过程中发挥着重要作用。除了这些一般公认的营养物质外，食物的一些其它成分也会影响饮食的营养价值。例如，纤维并不是一种营养物质，但它确实增加了值得拥有的饮食纤维和粗粮。

What Is an Adequate Diet?  An adequate diet is one that provides enough of all nutrients to keep a person healthy. In the United States the most authoritative dietary recommendations are those of the Food and Nutrition Board of the National Academy of Science, a nongovernmental organization. The recommendations of this board, published under the title Recommended Daily Dietary Allowances, are revised every five years.

什么是适当的饮食？适当的饮食就是一种提供足够的营养以保持人的健康。在美国，最权威的饮食建议是国家科学院食品与营养委员会，一个非政府组织的那些建议。这个委员会的建议发表在题为《每日饮食摄入量建议》上，每5年修订一次。

RDA’s are not minimum or marginal requirements; they represent the amounts of essential nutrients that the Food and Nutrition Board judged “to be adequate to meet the known nutritional need of practically all healthy persons.” In some cases these amounts have been estimated fairly precisely, but in other cases they are more of an informed guess. If too little information is available to supply a reasonably accurate allowance for a particular nutrient, the Food and Nutrition Board does not establish an RDA for it. As a rule, the RDA’s include a generous margin of safety. For this reason a diet that falls somewhat short in some of the RDA’s is not necessarily deficient.

每日建议饮食量不是最低或边际的要求；它们代表着必须的营养物数量，即食品与营养委员会判断的“足以满足差不多所有健康人已知营养的需求”。在某些情况下，这些数量的估计相当精确，但在另一些情况下，它们像是有根据的猜测。如果可用的信息太少，对于特殊的营养物就无法提供合理准确的用量，食品与营养委员会就无法为其建立每日建议饮食量。通常，每日建议饮食量包括充足的安全边际。由于这个原因，低于每日建议饮食量的饮食并不一定有缺陷。

RDA’s are precise amounts of specific nutrients, such as 54 grams of protein, 15 milligrams of thiamine, or 400 international units of vitamin D, and not amounts of the foods that contain these nutrients, such as a pound of steak or a head of lettuce. How then can one plan a diet that will provide the RDA for each nutrient?

每日建议饮食量是特殊营养物的精确数量，例如54克蛋白质，15毫克硫胺素，或多或400国际单位的维生素D，而不是含这些营养物的数量，比如，一磅肉排或一颗生菜。那么一个人该如何计划每日建议饮食量为每种营养物提供的饮食呢？

Most people can easily achieve a balanced diet by eating foods from each of the following four groups each day:

大多数人可以很容易地通过每天食用以下4组中的每种食物，实现均衡饮食：

1.      Whole milk and skim milk products, especially milk, cheese, and yogurt.

1、全脂牛奶和脱脂奶制品，尤其是牛奶、奶酪和酸奶。

2.      Meat, fish, poultry, eggs, and legumes and nuts.

2、肉、鱼、家禽、蛋，以及豆类和坚果。

3.      Vegetables and fruits.

3、蔬菜和水果。

4.      Breads and cereals.

4、各种面包和谷物。

One should vary the selection within each major food group from day to day, and within the vegetable group a citrus fruit and a leafy vegetable should be eaten each day.

人们应该在每天的主要食物组中进行不同的选择，每天应该食用蔬菜组中的柑橘类水果和叶菜类蔬菜。

The first group is an excellent source of protein and is our main source of calcium. Fortified milk also provides vitamins A and D. The section group provides much protein and also iron and many B vitamins, and is a major source of calories in many diets. Within the THIRD group, leafy green and yellow vegetables are rich sources of vitamin A, and citrus fruits provide the richest source of vitamin C. Most vegetable and fruits provide important vitamins, minerals, and dietary bulk. Grains and cereals provide an inexpensive source of calories along with vitamins, minerals, and protein.

第一组是极好的蛋白质来源，也是我们钙的主要来源。强化牛奶还提供了维生素A和D。该组提供了许多蛋白质，还提供了铁和许多B族维生素，并且是许多饮食中主要的热量来源。在第三组中，绿叶和黄色蔬菜富含维生素A，而柑橘水果提供了最丰富的维生素C。大多数蔬菜和水果都提供了重要的维生素、矿物质和膳食纤维。粮食和谷物都提供了廉价的热量来源，连同多种维生素、矿物质和蛋白质。

Generally, foods that supply mainly calories, such as sugar and soft drinks, should be avoided. They contribute to taste, but not to nutrition except as sources of calories, which for most people in the United States and other developed countries are not in short supply.

通常，应该避免主要提供热量的食物，诸如糖和软饮料。它们有助于口感，但除了热量来源外对营养没有贡献，它对美国和其它发达国家的大多数人而言，并不短缺。

Detailed information on the nutritional value of particular foods is given in several of the books listed in the Bibliography at the end of this article. See also DIET

有关特殊食物的营养价值的详细信息在该文末尾的文献目录所列出的一些书中有所呈现。可参阅饮食词条。

ENERGY REQUIREMENTS

Although green plants and certain bacteria can produce energy-containing food directly from sunlight, animals typically are unable to do so. Thus man and other animals depend on the energy produced by plants and stored in the tissues of both plants and plant-eating animals. These energy-storing materials are called food.

能量需求

尽管绿色植物和某些细菌可以从阳光中直接产生含有能量的食物，但动物通常无法这样做。因此，人类和其它动物都要依赖植物产生的能量，并将能量储存在植物和食草动物的组织中。这些储存能量的材料被称为食物。

The energy content of food is measured in calories. The calorie is defined as the heat energy needed to raise the temperature of 1 kilogram (2.2 pounds) of water from 14.5゜ C to 15.5 ゜C. The calorie used in nutrition is sometimes spelled with a capital “C” to distinguish it from the much smaller energy calorie used in physics and chemistry, but it is more properly called the kilogram-calorie, or kilocalorie, because it is precisely 1,000 times the smaller unit, or gram-calorie. The energy content of food is stored in the chemical bonds that link its atoms and molecules. Part of this energy is captured by biochemical reactions within our bodies and used to drive the life processes of our cells.

食物的能量含量是以热量测量的。热量被定义为将1公斤（2.2磅）的水从14.5 摄氏度提升到15.5摄氏度的温度所需要的热能。营养物中使用的热量有时被写成大写字母“C”，以区别物理和化学中使用的小得多的能量卡，但它更适当的称谓是千克卡，或叫千卡，因为它恰好是那些更小的单位，或者克卡的1千倍。食物能量的含量贮存在连接其元子和分子的化学键中。在我们的身体内，该能量的一部分由生物化学反应来捕获，用来驱动我们细胞的生命过程。

Most human food energy is provided by carbohydrates, fats, and protein, with a small contribution from alcohol. Protein and carbohydrates each yield 4 calories per gram (113 per ounce). Alcohol (ethanol) yields 7 calories per gram (198 per ounce), and fat is the richest in calories, yielding 9 per gram (255 per ounce).

大多数的人类食物能量是由碳水化合物、脂肪和蛋白质提供的，酒精也起了一些作用。蛋白质和碳水化合物会产生4个热卡/每克（每盎司113个热卡）。酒精（乙醇）会产生7个热卡/每克（每盎司198个热卡），而脂肪的热量最丰富，会产生9个热卡/每克（每盎司255个热卡）。

The calorie content of a food also depends on its content of indigestible and non-caloric substances, such as cellulose and water. An apple, for example, is 85% water and 1% indigestible fiber, while the rest is mostly 4-calorie-per-gram carbohydrate. One hundred grams (3.5 ounces) of apple provides only about 60 calories. In contrast, butter has only 16% water, negligible fiber, and a predominance of 9-calorie-per-gram fat. Thus 100 grams of butter provide 716 calories. In general, the cheapest sources of calories are foods rich in carbohydrates, namely starches and sugars, and the most costly sources are protein-rich foods, such as meat and eggs.

食物的热量含量还取决于它不消化的和无热量物质的含量，纤维素和水。例如，一只苹果85%是水分和1%难消化的纤维，而其它部主要是每克4个热卡的碳水化合物。100克（3.5 盎司）的苹果仅提供了大约60个热卡。相比之下，奶油只有16%的水分，纤维可忽略不计，而优势是每克脂肪含有9个热卡。因此，100克奶油可提供716个热卡。一般来说，最便宜的热量来源是那些富含碳水化合物的食物，也就是淀粉和糖类，而最昂贵的资源是富含蛋白质的食物，比如肉和蛋类。

The minimal calorie requirement varies with a person’s weight, basal metabolic rate, and physical activity. In addition, energy needs vary in response to environmental temperature and to special requirements such as those of growing children and pregnant or lactating women.

最低的热量需求会随着人的体重、基础代谢率以及身体的运动而变化。另外，能量需求会对环境温度和特殊要求的变化做出响应，诸如那些成长的孩子和怀孕或哺乳的妇女。

Food energy in excess of that used or excreted is stored in the body mostly in the form of fat. A chronic shortage of calories results in loss of weight and, if extreme, in deficiency diseases, starvation, or death.

超过消耗和排泄的食物能量主要以脂肪的形式贮存在身体内。长期缺乏热量会导致体重下降，如果是极端情况会导致营养缺乏症、饥饿或死亡。

PROTEIN

The several different types of proteins play very different roles in the body. Enzymes are proteins that catalyze most of the body’s chemical reactions. Other proteins are key parts of certain hormones, such as insulin, that regulate communication among our organs and cells. But most of the body’s protein occurs in the form of structural units that make up muscles and connective tissues, and it forms the major part of such organs as the heart and the skeletal muscles.

蛋白质

几种不同类型的蛋白质在身体内起着非常不同的作用。酶是催化身体大部分化学反应的蛋白质。其它的蛋白质是某些激素的关键部分，比如，调节我们器官和细胞间交流的胰岛素。但身体的大部分蛋白质都是以构成肌肉和结缔组织的结构单位形式存在的，而且它形成了像心脏和骨骼肌这类器官的主要部分。

The structures of protein molecules are as diverse as the functions they perform in the body. Essentially a protein consists of a large number of amino-acid units linked together into a chain that is then twisted and folded into the usually highly complex form characteristic of each particular kind of protein. The amino acids in proteins are joined together by peptide bonds, in which the nitrogen of one amino acid is attached to a specific carbon atom of another amino acid. Short chains of amino acids linked together by peptide bonds are called peptides, and longer chains are called polypeptides. See also PROTEIN.

蛋白质分子的结构就像它们在身体内承担的功能一样是多样的。本质上，蛋白质是由大量氨基酸单位连成的一个链，然后被扭曲并折叠成各自特定种类蛋白质通常所具有的高度复杂的形式特点。蛋白质中的氨基酸通过肽键连接在一起，其中，一种氨基酸的氮连接着另一种氨基酸的特定碳原子。由肽键连接在一起的氨基酸短链被称为肽，而长链被称为多肽。可参阅蛋白质词条。

Protein is not used by the body in the form in which it is eaten. Instead, food protein is digested to nitrogen-containing amino acids, which in turn are used by the body to make the particular kinds of protein that it needs. Protein digestion is performed by enzymes that disrupt the peptide bonds to release amino acids. These enzymes, called proteases, are secreted by the stomach, pancreas, and intestine. The amino acids resulting from digestion are absorbed through the intestine into the blood, which carries them to the body tissues. Foods that are poorly digested lose much of their protein in the feces.

蛋白质并不是以它被食用的形式被身体所利用。相反，食物蛋白质是被消化为含氮的氨基酸，转而被身体利用来制造它所需要的特定种类的蛋白质。蛋白质的消化是由改变肽键释放氨基酸的酶来完成的。这些被称为蛋白酶的酶是由胃、胰腺和小肠分泌的。由消化产生的氨基酸通过小肠进入血液，将它们输送到身体的组织中。消化不良的食物会在排泄物中丢失许多它们的蛋白质。

Proteins are continuously manufactured and destroyed in the body’s tissues. Ordinarily, the processes of protein building and degradation are in equilibrium, and the same amino acid units may be reused many times to make different proteins for different tissues.

蛋白质在身体组织中会不断地被制造和破坏。通常，蛋白质的构建和降解过程是处于平衡状态，而且相同的氨基酸单位可被重复利用许多次，为不同的组织制造不同的蛋白质。

Of the 21 amino acids that are found in human proteins, the body can synthesize only 12. These are called the nonessential amino acids, because they can be omitted if enough nitrogen and other nutrients are supplied. The nine other amino acids cannot be made by the body, but must be taken directly from the food we eat. These are the essential amino acids: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. The border between essential and nonessential amino acids is not sharp, since some of the essential amino acids can be synthesized by the body in small amounts or in special situations. But for most nutritional planning it remains useful to distinguish between the essential amino acids, which must be included in the diet, and the nonessential ones, which need not be included in the diet.

在人类蛋白质中发现的21种氨基酸，身体能合成的仅为12种。这些被称为非必需氨基酸，因为，如果提供足够的氮和其它的营养物，它们是可以忽略的。其它的9种氨基酸身体是无法制造的，而必须从我们吃的食物中直接获取。这些是必需氨基酸：组氨酸、异亮氨酸、亮氨酸、赖氨酸、蛋氨酸、苯基丙氨酸、苏氨酸、色氨酸和缬氨酸。在必需氨基酸与非必需氨基酸之间的边界并不明显，因为一些必需氨基酸可在少量或特殊情况下由身体合成。但对大多数营养的计划而言，区别必需氨基酸与无需包含在饮食中的非必需氨基酸依然是很有用的。

To utilize food protein efficiently, the body requires all the essential amino acids to be present at the same time and in specific proportions. If even one essential amino acid is not provided, or is present in too small an amount, then all or part of the other amino acids will be useless for making protein, because they cannot be stored for future use. Instead they are broken down into energy-yielding compounds, and their nitrogen is excreted. As a result, the body’s ability to make protein is severely impaired. Similarly, if more protein is eaten than is needed to provide amino acids for making the body’s own proteins, the excess is broken down into energy-yielding compounds. The energy that is obtained in this way though valuable, can usually be obtained much more cheaply from non-protein foods.

为了有效地利用食物蛋白质，身体需要所有必需氨基酸，同时以特定的比例存在。即使缺少一种必需氨基酸，或含量过小，那么其它的氨基酸的全部或部分将无法制造蛋白质，因为它们无法贮存，以备将来使用。相反，它们会被分解成产生能量的化合物，而它们的氮则会排泄掉。因此，身体制造蛋白质的能力会严重受损。同样，如果摄入了更多的蛋白质超过了提供制造人体自身蛋白质所需要的氨基酸，过量部分会被分解成生产能量的化合物。用这种方式获得的能量虽然有价值，但通常可以从非蛋白质的食物中更便宜地获得。

The usefulness of a particular food protein in human nutrition is called its quality. Protein quality is determined by the protein’s digestibility and by the amount and proportion of the essential amino acids that are present. The quality of a protein can be measured by its net protein utilization (NPU). The NPU is the percent of ingested nitrogen—nearly all of which comes from protein—that is digested and retained in the body. The highest quality protein is that in the white of eggs (NPU 94%). Other foods that contain large amounts of high-quality protein include milk, cheese, fish, poultry, soybeans, and meats. It is a common, but incorrect, IMPRESSION that the protein quality of the muscle meat of cattle and pigs is superior to that of other foods, but all of the above-mentioned foods are actually comparable or superior in both the quality and quantity of their protein content.

在人类营养中，一种特定食物蛋白质的有效性被称为品质。蛋白质的品质是由该蛋白质的可消化性、数量和必需氨基酸所呈现的比例来决定的。一种蛋白质的品质可用其净蛋白质利用率（NPU）来衡量。NPU是吸收氮的百分比---几乎所有这些都来自蛋白质---换言之，就是吸收和保留在身体中的蛋白质。最优质的蛋白质是蛋清（NPU 94%）中的蛋白质。其它包含大量优质蛋白质的食物包括牛奶、奶酪、鱼、家禽、大豆和肉类。这是一种常见，但不正确的印象，即牛和猪的肌肉部位的肉的蛋白质品质比其它食物的品质都要更好，但实际上，上面提到的所有食物在它们的蛋白质含量的品质和数量上都是相当的或更好的。

Foods that are deficient in different essential amino acids can complement each other’s deficits if they are eaten together. For example, corn-meal is deficient in lysine but adequate in methionine. Certain beans are deficient in methionine but rich in lysine. Together, their protein quality is 50% higher than when they are eaten separately.

如果一起食用缺乏不同必需氨基酸的那些食物，就能弥补彼此的不足。例如，玉米粉缺乏赖氨酸，但是蛋氨酸充足。某些豆类缺乏蛋氨酸，但赖氨酸丰富。一起食用，它们的蛋白质质量要比单独食用它们时高出50%。

The minimum daily protein requirements for optimal growth and good health can be estimated only approximately. The current RDA for protein is 56 grams (2 ounces) of protein a day for a standard 70-kg (154-pound) man and 46 grams (1.6 ounces) for a standard 58-kg (132-pound) woman. These amounts of protein would be provided, for example, by 265 grams (9.33 ounces) of lean round beef for the standard man and 217 grams (7.66 ounces) for the standard woman or by 181 grams (6.3 ounces) of roast turkey for the standard man and 149 grams (5.2 ounces) for the standard woman. In the United States and most of the other affluent developed countries the typical diets contain amounts of protein well over the RDA.

对最佳生长和良好健康的最低每日蛋白质的需要量只能进行大概的估计。目前，蛋白质推荐的每日摄取量是，标准70公斤（154镑）的男性每日蛋白质推荐摄取量为56克（2盎司），标准58公斤（132镑）的女性每日蛋白质的推荐摄取量为46克（1.6盎司）。例如，265克（9.33盎司）的瘦牛肉可为标准男性，和217克（7.66盎司）的瘦牛肉可为标准女性提供这些蛋白质的数量，或181克（6.3盎司）的烤火鸡也可为标准男性和149克（5.2盎司）的烤火鸡可为标准女性提供这些蛋白质的数量。在美国和大多数富裕的国家，典型的饮食包含的蛋白质数量远超过了推荐的每日摄取量。

The daily recommended allowance of protein is increased by several factors, including rapid growth in childhood, pregnancy, breast feeding, surgery, and severe illness. Persons who weigh more need more protein. Physical activity, though, does not affect protein requirements. However, when too few calories are supplied by carbohydrates and fats, then body protein must be burned for fuel. There is no evidence that eating more than the RDA of protein benefits health. It probably does no harm in healthy persons, although it may be harmful to persons with liver or kidney disease. In such persons the liver or kidneys may not do a good enough job of breaking down and removing the potentially harmful waste products that result when protein is metabolized.

每日推荐的蛋白质摄取量是由若干因素增加的，包括儿童期的快速成长、怀孕、母乳喂养、手术情况以及严重的疾病。体重更重的人就需要更多的蛋白质，虽然，体力活动并不影响蛋白质的需求。然而，当由碳水化合物和脂肪提供的热量太少时，那么身体的蛋白质必然被作为燃料来燃烧。尚无证据表明，摄取多于推荐的日摄取量的蛋白质会有益健康。它可能对健康人并无什么伤害，尽管它对患肝脏或肾脏疾病的人可能有害。在这样的人群中，肝脏和肾脏无法很好地分解和清除在代谢蛋白质时所产生的潜在有害废物。

Protein deficiency is often associated with deficiencies of calories, vitamins, and minerals. But when severe protein deficiency dominates, a characteristic disease, called kwashiorkor, occurs. The symptoms of kwashiorkor, which appears most often in young children, include anemia, edema, loss of skin pigment and hair, and retarded growth. Milder degrees of sustained protein deficiency may cause retarded growth, weight loss, slow healing of wounds, increased susceptibility to disease, and other problems.

蛋白质缺乏通常与缺乏热量、维生素和矿物质有关。但当出现严重的蛋白质缺乏时会出现一种被称为恶性营养不良病的典型疾病。恶性营养不良病的症状，最常出现在年幼儿中，它包括贫血、水肿、皮肤色素和毛发减少以及生长迟缓。较轻度的持续蛋白质缺乏可能导致生长迟缓、体重减轻、伤口愈合缓慢、增加疾病的易感性以及其它问题。

CARBOHYDRATES

Carbohydrates are an important source of calories in most diet, and they are the major source in the typical diets of less developed countries. In the form of glucose, carbohydrates enter many of the most important energy-producing cycles in the body’s cells. If carbohydrate intake exceeds energy needs, moderate amounts are stored in muscle and liver in the form of glycogen, a starch-like substance. Larger amounts are converted to fat and stored in that form.

碳水化合物是大多数饮食中热量的重要来源，而且它们是欠发达国家典型饮食中的主要来源。以葡萄糖的形式，碳水化合物进入身体细胞许多最重要的产生能量的循环。如果碳水化合物的摄入量超出了能量需要，适量的碳水化合物就会以糖原的形式，一种像淀粉的物质储存在肌肉和肝脏中。更多的碳水化合物会转化为脂肪，并以这种形式储存起来。

Much as proteins consist of chains of amino acid subunits, carbohydrates consist of chains of simple sugar units, such as glucose, fructose, and maltose. Such simple sugars are called monosaccharides, because they contain only one sugar unit (saccharide). Substances such as sucrose that contain two linked simple sugar units are called disaccharides, and substances such as starch that contain many linked simple sugars are called polysaccharides.

非常像蛋白质由氨基酸亚基链组成一样，碳水化合物是由单糖单位链组成，诸如葡萄糖、果糖和麦芽糖。这种单糖被称为单糖类，因为它们只包含一个糖单位（糖类）。像蔗糖这样包含两个连接单糖单位的物质被称为二糖，而像淀粉这样包含许多连接单糖的物质被称为多糖。

Nutritionally, the most useful sources of carbohydrates are the complex foods such as unrefined grains, vegetables, and fruit. Such foods provide protein, vitamins, minerals, fats and oils, and bulk, in addition to calories. In contrast, many high-calorie carbohydrate sources, such as table soft drinks, and candy, have little nutritional value except for the calories themselves. For this reason some nutritionists call such calories “empty calories.” In recent decades, there has been a marked increase in calories in the American diet.

在营养方面，最有用的碳水化合物来源是复合食物，诸如未精制的谷物、蔬菜和水果。除了热量，这样的食物还提供蛋白质、维生素、矿物质、脂肪和油以及纤维素。相比之下，许多高热量的碳水化合物，诸如餐桌上的软饮料和糖果，除了热量本身外少有营养价值。由于这个原因，一些营养学家称这样的热量为“空热量”。在最近几十年里，在美国人的饮食中热量有明显的增加。

FATS AND OILS

Fats and oils are the most concentrated form of food energy. For most animals the advantage of storing excess energy in the form of body fat for future needs is evident. For civilized man, however, there is little or no benefit in being overweight, while the harm both to physical and emotional health may be great.

脂肪和油类

脂肪和油类是食物能量最集中的形式。对大多数动物来说，很明显，以身体脂肪的形式储存多余的能量来为未来需要提供优势。然而，对于文明人来说超重几乎没有什么好处，而对身心健康的损害可能很大。

The chief dietary fats are triglycerides, which are formed by the union of a glycerol molecule, an alcohol molecule, and THREE fatty-acid molecules. A fatty acid consists of a straight chain of carbon atoms to which are attached hydrogen atoms, and, at one end of the chain, an acid group (COOH).

饮食的主要脂肪是由一个甘油分子、一个酒精分子和三个脂肪酸分子结合形成的三酸甘油酯。脂肪酸由附着氢原子的碳原子直链组成，在该链的一端是一个酸基(COOH)。

Fatty acids that contain as many hydrogen atoms as they can hold are said to be saturated, as is a fat molecule made up entirely of saturated fatty acids. Saturated fats are the predominant fats in most foods derived from animals. If some hydrogen atoms are missing from the carbon chain of a fatty acid, the fatty acid is unsaturated. It is called monounsaturated if a single pair of hydrogens is missing and polyunsaturated if more than one pair are missing. If a fat contains some unsaturated fatty acids, the fat, too, is said to be unsaturated.

包含尽可能多氢原子的脂肪酸被称为饱和脂肪酸，如同完全由饱和脂肪酸组成的脂肪分子一样。饱和脂肪是源自大多数动物食物的主要脂肪。如果脂肪酸的碳链缺少了一些氢原子，该脂肪酸就是不饱和的。如果缺少了单对氢，则被称为单不饱和脂肪，而如果缺少不止一对则被称为多不饱和脂肪。如果脂肪包含一些不饱和脂肪酸，该脂肪也被称为不饱和脂肪酸。

Polyunsaturated fats are predominant in most vegetable oils, with several important exceptions. Olive oil is mostly monounsaturated; cocoa butter (chocolate fat) is comparable to animal fats in degree of saturation; and coconut oil, commonly used in nondairy cream substitutes and dessert toppings, is one of the most saturated of all food fats.

除了几个重要的例外，多不饱和脂肪在大多数植物油中都占有主导地位。橄榄油主要是单不饱和脂肪；可可脂（巧克力脂肪）在饱和度上可与动物脂肪相比；而椰子油，通常用于不含奶的奶油替代品和甜点配料，是所有食物脂肪中最饱和的脂肪之一。

At room temperature, highly saturated fats, such as butter and other animal fats, tend to be solid, while polyunsaturated fats, such as vegetable oils, tend to be liquid. Such liquid fats can be converted into solid forms, such as pastry shortening, by a process of hydrogenation. Hydrogenation increases the saturation of fatty acids by adding hydrogen atoms.

在室温下，像奶油和其它动物脂肪这类高饱和的脂肪都趋向于固态，而多不饱和脂肪，比如植物油则趋向液态。这类液态脂肪可转换成固态形式，诸如通过氢化过程的酥皮糕点。通过添加氢原子，氢化可增加脂肪酸的饱和状态。

Two polyunsaturated fatty acids, linoleic acid and arachidonic acid, are the only fats known to be essential for humans. The minimum requirement for these fatty acids will be met if they provide on the order of 1%--2% of the total calories in the diet. This amount is easily provided by most diets, since there are large amounts of linoleic acid in polyunsaturated vegetable oils, and since the body can synthesize arachidonic acid from linoleic acid. Small amounts of arachidonic acid occur in animal fats.

两种多不饱和脂肪酸，亚油酸和花生四烯酸，是已知的对人类至关重要的唯一脂肪。在饮食中如果它们能提供约占总热量的1%--2%，就能满足对这些脂肪的最低需求。大多数的饮食能很容易地提供这个数量，因为在多不饱和的植物油中含有大量的亚油酸，并且由于身体能够从亚油酸中合成花生四烯酸。在动物的脂肪中也含有少量的花生四烯酸。

The distinction between saturated and polyunsaturated fats has been important in planning diets ever since scientists discovered that high levels of blood cholesterol in the blood are linked to atherosclerotic heart disease and strokes. Saturated fat in the diet tends to raise blood cholesterol; polyunsaturated fat tends to lower it.

自从科学家发现血液中的高血胆固醇与动脉粥样硬化心脏病和中风有关以来，在饮食规划中区别饱和脂肪与多不饱和脂肪已变得很重要。饮食中的饱和脂肪常常会提高血液的胆固醇；而多不饱和脂肪常常会降低胆固醇的含量。

                                                                     RICHARD N. PODELL, M. D., M. P. H.

                                                                        Overlook Hospital, Summit, N. J.

                                                                               理查德·N. 波德尔，医学博士，

           公共卫生学硕士

                                                                                     眺望医院，新泽西州SUMMIT

 

                                                                                       2023年7月28日译

（译者注：词条部分位列《大美百科全书》1985年版，第20卷，第564页至568页）

待续部分：VITAMINS维生素；MINERALS矿物质；MALNUTRITION营养不良；DISEASES OF OVERNUTRITION营养过剩的疾病；VEGETARIANISM素食主义；CONTROVERSIES IN NUTRITION对营养的争议。