生物自环境中摄取营养物质,在体内进行同化作用和异化作用,就是生命存在的必要条件。人类为了维持生活,也必须由外界摄取食物,以供组织生长及各项活动所需的物质和能量。这好比农作物生长需要肥科、火车运行需要煤炭一样。日常食物中含醣(主要为淀粉)、蛋白质、脂肪、维生素、无机盐及水等物质。其中前三类物质分子构造比较复杂,且多不溶于水,蛋白貭并有种特异性,能引起过敏反应;此类物貭必须在消化道内先转变成分子构造比较简单,能溶于水及每种特异性的物貭后方能被吸收。若未经转变,则不能为组织所利用。所以营养良否不仅取决于食物质量的优劣,还要看消化 Biological intake of nutrients from the environment, in vivo assimilation and alienation, is the necessary conditions for the existence of life. In order to sustain life, human beings must also take food from the outside world for organizing the material and energy needed for their growth and activities. This is like the need for fertilizer to grow crops, and the trains need coal. Daily food sugars (mainly starch), protein, fat, vitamins, inorganic salts and water and other substances. Among them, the former three kinds of substances are relatively complex in molecular structure and are mostly insoluble in water. Protein 貭 is also kind-specific and can cause anaphylactic reaction. Such substances must be converted into molecules in the digestive tract first, Each specific object 貭 rear can be absorbed. If not changed, can not be used by the organization. Therefore, nutrition is good or not depends not only on the quality of food, but also depends on digestion