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    كربوهيدرات من مقرر ال physiology

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    Zoologist
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    تاريخ التسجيل : 12/03/2009
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    كربوهيدرات من مقرر ال physiology

    مُساهمة  Zoologist في الأحد فبراير 28, 2010 6:07 am



    Carbohydrates and Nutrition


    1. Introduction

    There have been major advances in the understanding of how carbohydrates influence human nutrition and health in recent years. Progress in scientific research has highlighted the diverse functions of carbohydrates in the body and their importance in the promotion of good health. In fact, there is so much good news that it is time to take a closer look at carbohydrates.
    2. Importance of carbohydrates

    Carbohydrates take the form of sugars, oligosaccharide s, starches and fibres and are one of the three major macro-nutrients which supply the body with energy (fat and protein being the others). There is now good evidence that at least 55% of our daily calories should come from carbohydrates.
    Whereas it is important to maintain an appropriate balance between calorie intake and expenditure, scientific studies suggest that:
    A diet containing an optimum level of carbohydrates may help prevent body fat accumulation;
    Starch and sugars provide readily accessible fuel for physical performance;
    Dietary fibre, which is a carbohydrate, helps keep the bowel functioning correctly.
    Apart from the direct benefits of carbohydrates for the body, they are found in a wide range of foods which themselves bring a variety of other important nutrients to the diet. For this reason it is recommended that carbohydrates be supplied from diverse food sources to ensure that the overall diet contains adequate nutrients.
    It is also important to remember that carbohydrates contribute to the taste, texture and appearance of foods and help to make the diet more varied and enjoyable.
    3. What are carbohydrates?

    The building blocks of all carbohydrates are sugars and they can be classified according to how many sugar units are combined in one molecule. The table below shows the major types of dietary carbohydrates.
    CLASSIFICATION OF DIETARY CARBOHYDRATESMonosaccharides Glucose, fructose, galactose DisaccharidesSucrose, lactose, maltosePolyolsIsomalt, sorbitol, maltitolOligosaccharide sMaltodextrins, fructo-oligo-saccharidesPolysaccharides Starch: Amylose, amylopectinPolysaccharides Non-starch: Cellulose, pectins, hydrocolloids
    3.1. Sugars

    Glucose and fructose are simple sugars or monosaccharides and can be found in fruits, berries, vegetables and honey. When two simple sugars combine, they form disaccharides. Table sugar or sucrose is a combination of glucose and fructose and occurs naturally both in sugar beet, sugar cane and fruits. Lactose is the main sugar in milk and dairy products and maltose is a disaccharide occurring in malt.
    Polyols are so-called sugar alcohols. They do occur naturally but most are made commercially by the transformation of sugars. Isomalt is the most commonly used polyol and is derived from sucrose. Polyols are sweet and can be used in foods in a similar way to sugars although they can have a laxative effect when eaten in large quantities.
    3.2. Oligosaccharide s

    When 3-9 sugar units combine they form oligosaccharide s. Maltodextrins contain up to 9 glucose units and are produced commercially by the partial hydrolysis (or breakdown) of starch. They are less sweet than mono- or disaccharides. Raffinose, stachyose and fructo-oligosaccharide s are found only in small amounts in certain pulses, grains and vegetables.
    3.3. Polysaccharides

    More than 10 and sometimes even up to several thousand sugar units are needed to form polysaccharides . Starch is the main energy reserve in root vegetables and cereals. It comprises long chains of glucose and occurs as granules whose size and shape vary according to the plant in which it is contained.
    Non-starch polysaccharides are the main components of dietary fibre. They include; cellulose, hemicelluloses, pectins and gums. Cellulose is the major component of plant cell walls and consists of several thousand glucose units. The separate components of dietary fibre have different physical structures and properties.
    4. Carbohydrates In The Body

    The main function of carbohydrates is to provide energy, but they also play an important role in:
    The construction of the body organs and nerve cells
    The definition of a person's biological identity such as their blood group
    4.1. Energy source and storage

    Starches and sugars are the main energy-providing carbohydrate sources and supply 4 kilocalories (17 kilojoules) per gram.
    Simple sugars are absorbed directly by the small intestine into the bloodstream, where they are then transported to their place of use. Disaccharides are broken down by digestive enzymes into simple sugars. The body also needs the help of digestive enzymes to break down the long chains of starches into their constituent sugars which are then absorbed into the bloodstream.
    The human body uses carbohydrates in the form of glucose. Glucose can also be converted to glycogen, a polysaccharide similar to starch, which is stored in the liver and the muscles and is a readily available source of energy for the body. The brain needs to use glucose as an energy source, since it cannot use fat for this purpose. It is for this reason that the level of glucose in the blood must be constantly maintained above at optimum level. Glucose may come directly from dietary carbohydrates or from glycogen stores. Several hormones, including insulin, work rapidly to regulate the flow of glucose to and from the blood to keep it at a steady level.



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