什麼是脂肪

源自動植物的不揮發、不溶於水而油膩的物質。在化學構造上，動物油和植物油（參閱oil）是相同的，主要由甘油三酯（甘油和脂肪酸反應生成的酯）組成，只有在溶點和物理狀態（固體或液體）有所不同，全視脂肪酸的飽和情況和碳鏈的長度而定。甘油酯可能只含有少量脂肪酸，也可能多至一百種（在乳脂肪中）。幾乎所有的天然脂肪和油由2碳單位構成，而且僅含碳原子數爲偶數的脂肪酸。天然脂肪如玉米油除了甘油三酯之外，還含有少量化合物，其中包括磷脂、植物類固醇、維生素A、維生素E、蠟、類胡蘿蔔素等，以及許多其他的成分（包括這些東西的分解產物）。在食物中，脂肪來自成熟的種子和水果（玉米、花生、油橄欖和鱷梨），也來自動物的肉、蛋和奶。脂肪的每一單位重量的熱量兩倍於蛋白質和碳水化合物的。在食物中脂肪的消化（通常部分）是由脂來進行，被分解的產物從腸子進入血液，脂肪即以肉眼看不見的小滴隨血液運至使用地點或貯藏地點。脂肪容易因水解而被分解（主要化爲丙三醇和脂肪酸），這是應用在許多工業上的第一步驟。

fat

Any organic compound of plant or animal origin that is not volatile, does not dissolve in water, and is oily or greasy. Chemically, fats from animals and from vegetables (oils) are identical, consisting mainly of triglycerides (esters of glycerol with fatty acids); the differences are in the melting temperature and physical state (solid or liquid) of each, which depends on the saturation of the fatty acids and the length of the carbon chain. The glycerides may have only a few component fatty acids or as many as 100 (in butterfat). Almost all natural fats and oils are constructed from two-carbon units and thus contain only fatty acids with an even number of carbon atoms. Natural fats such as corn oil have small amounts of compounds besides triglycerides, including phospholipids, plant steroids, tocopherols (vitamin E), vitamin A, waxes, carotenoids, and many others, including decomposition products of these. Fats in foods come from ripe seeds and fruits (corn, peanuts, olives, avocados) and from animal sources (meat, eggs, milk). They contain more than twice as much energy (calories) per unit of weight as proteins and carbohydrates. Digestion of fats in foods, often partial, is done by enzymes called lipases. The breakdown products are absorbed from the intestines into the blood, which carries microscopic fat droplets reconstituted from digested fats or synthesized in cells to sites of storage or use. Fats are readily broken down, primarily into glycerol and fatty acids, by hydrolysis, a first step for many of their numerous industrial uses. See also lipid.