Carbohydrates form the main part of almost all normal diets and they generally provide the major source of energy for your body. They are called ‘carbo-hydrates’ because chemically they consist of carbon, hydrogen and oxygen. The main foods containing them are bread, flour, rice, sugar and potatoes. There are many different kinds of carbohydrate, but often distinction is made between relatively small sugars (mono- or disaccharides) and big polysaccharides, to which starch and cellulose belong.
Most of the carbohydrates you eat must be broken down into smaller, soluble forms before they can be absorbed from the digestive tract into the bloodstream. Glucose and fructose are two small and simple sugar carbohydrates which do not need to be broken down by digestion. Glucose occurs naturally in grapes and other sweet fruit, onions, unripe pota-toes and honey (which is about 35 per cent glucose). Because it is so rapidly absorbed by the body glucose is beneficial for invalids. It also provides a readily available source of energy for children, sportsmen and others who use energy fast and need plentiful sup-plies.
Glucose, fructose and another sugar, galactose, are formed by the digestion of larger carbohydrate units, and are all simple sugars known as monosaccharides. After they are absorbed into the bloodstream they are carried to the liver. The liver contains enzymes that convert fructose and galactose into glucose, so most of the sugar leaving the liver in the bloodstream is in the form of glucose. This blood sugar is distributed throughout the body and passes into individual cells where it is ‘burned’ to release the energy that powers the chemical reactions of living tissues.
Excess glucose is removed from the blood and converted into an insoluble starchlike material called glycogen, which is stored as an energy reserve in the liver and muscles. This ‘reservoir’ of stored energy can be turned back into glucose again, under the control of hormones, when the blood sugar level falls too low. If this glucose-controlling mechanism goes wrong the condition diabetes mellitus may result.
Whereas monosaccharide sugars do not need breaking down into smaller units during digestion, all other carbohydrates have to be converted to monosac-charides before they can be absorbed into the body. For example, commercial white sugar is sucrose, a disaccharide that consists of two monosaccharide sugar units chemically linked. Sucrose occurs natu-rally, mixed with fructose and glucose, in almost all fruits and vegetables; maple syrup and molasses are mainly sucrose. Another disaccharide, maltose, is formed during the sprouting of plants. This is the sugar that gives malt extract its sweetness. Lactose, or milk sugar, is a disaccharide found in the milk of mammals. Human milk contains between six and seven per cent lactose and cow’s milk about five per cent. Lactose is split during digestion into glucose and galactose by the action of the enzyme lactase.
Starches are long chains of several hundred glucose units linked into a complex network. Starch is found in all cereals and breads, dried beans, peas, potatoes and many other roots and tubers. At the beginning of this century 90 per cent of the carbohydrate eaten in the Western world was in the form of starch; now it is only 40 per cent, while the other 60 per cent is in the form of sugars. Nutritionists suggest this trend should be reversed; starchy foods are now thought to be more valuable than sugars because they contain vitamins and proteins as well as carbohydrates.
Excess and deficiency
If you eat more carbohydrates than your body needs, the excess is converted into fat and stored in the adipose (fatty) tissues of your body. It is therefore very easy to become fat simply by eating more carbo-hydrates than you need. But, although most people eat adequate or excessive amounts of carbohydrate, there are individuals who reduce their intake to such an extent that they suffer from carbohydrate deficiency. A person who does this feels tired due to a lack of energy. The glycogen stores in the liver are soon used up. To maintain the blood glucose at a normal level, fat and proteins have to be converted to glucose. Lack of carbohydrates interferes with the metabolism of other foodstuffs and the condition of keto-acidosis may result. In addition, milk production in the nursing mother may be disturbed. When sugar and other carbohydrate foods are consumed constantly, as when children chew and suck endless amounts of sugary foods and sweets, the bacteria present in the mouth have a ready source of energy. They grow, producing acids that dissolve the tooth enamel and cause dental decay (caries). Even the sugar in tea, coffee and other beverages has this effect. Take the sweets and sugar away and there is a lower incidence of dental trouble. For those who find unsweetened coffee, tea, or other drinks impossible to drink, there are commercially produced non-sugar sweeteners such as saccharine, sorbitol and xylitol. Used in place of sugar, these sweeteners can reduce the incidence of dental caries.