Food and nutrition.pdf

Function of Food 91diet are mainly in the form of starches and various sugars. Carbohydratescan be divided into three groups:• Monosaccharides, e.g. glucose, fructose, galactose;• Disaccharides, e.g. sucrose, lactose, maltose;• Polysaccharides, e.g. starch, glycogen (animal starch), cellulose.MonosaccharidesThe simplest carbohydrates are the monosaccharides, or simplesugars. These sugars can pass through the wall of the alimentary tractwithout being changed by the digestive enzymes. The three most commonare glucose, fructose and galactose.Glucose, sometimes also called dextrose, is present in fruit, sweetpotatoes, onions and other plant substances. It is the substance into whichmany other carbohydrates, such as the disaccharides and starches, areconverted by the digestive enzymes. Glucose is oxidized to produceenergy, heat and carbon dioxide, which is exhaled in breathing.Because glucose is the sugar in blood, it is most often used as anenergy-producing substance for persons fed intravenously. Glucosedissolved in sterile water, usually in concentrations of 5 or 10 per cent, isfrequently used for this purpose.Fructose is present in honey and some fruit juices. Galactose is amonosaccharide that is formed, along with glucose, when the milk sugarlactose is broken down by the digestive enzymes.DisaccharidesThe disaccharides, composed of simple sugars, need to be convertedby the body into monosaccharides before they can be absorbed from thealimentary tract.Examples of disaccharides are sucrose, lactose and maltose. Sucroseis the scientific name for table sugar (the kind that is used, for example,to sweeten tea).It is most commonly produced from sugar cane but is also producedfrom beets. Sucrose is also present in carrots and pineapple. Lactose isthe disaccharide present in human and animal milk. It is much less sweetthan sucrose. Maltose is found in germinating seeds.PolysaccharidesThe polysaccharides are chemically the most complicatedcarbohydrates. They tend to be insoluble in water, and only some can beused by human beings to produce energy. Examples of polysaccharidesare starch, glycogen and cellulose.Starch is an important source of energy for humans. It occurs in cereal