CARBOHYDRATES

CARBOHYDRATES

Carbohydrates are synthesized by all green plants using solar energy, water  from the soil and carbon dioxide from the air. This complex process is called photosynthesis, the prefix photo indicates the importance of sunlight in this process. Plants are thus the primary source of food in the world.

Carbohydrates contain carbon, hydrogen and oxygen. The suffix hydrate indicates that water and oxygen occur in the same proportion as in water.

The members of the simplest class of carbohydrates have a single unit--- monosaccharide (mono or one, saccharine or sugar containing). Glucose is an example of this class. The disaccharides contain two sugars linked together to form a chain. Cane or beet sugar (sucrose), milk sugar (lactose), and maltose (malt sugar) are members of this class. Carbohydrates made up, of long chains of sugars are called polysaccharides (poly—many). Among them are starch, dextrin, glycogen, cellulose, hemicelluloses, pectin, plants gums and mucilage’s, presents the types and sources of carbohydrates occurring in nature.

Simple carbohydrates include mono and disaccharides. These are small molecules, which dissolve in water and are absorbed very quickly in the body. In contrast, starches and dietary fibers are very large, complex molecules, containing several hundred small sugar units and are absorbed slowly. Therefore polysaccharides are referred to as complex carbohydrates. Individual members of each of these classes (starch, pectin’s, gums, mucilage, cellulose, hemicelluloses, lignin) differ in the type of small units they contain and in the way these units are  united in the molecule.

Glucose, the most common monosaccharide, is present in honey, fruits and corn syrup. After absorption of food it is transported through blood and hence is present in blood as an easily available source of energy.

Sucrose is the ordinary sugar available in the grocery store and used in beverages and food preparations as a sweetener. As you can note from the table, it is mainly manufactured from cane sugar in the tropics and sugar beets in the temperature regions. It is also present in molasses, honey, fruits and vegetables. Sucrose is hydrolyzed to a mixture of equal amounts of glucose and fructose by action on digestive enzymes or when boiled with acid (Such as citric acid from fruits). This mixture of glucose and fructose is called invert sugar and it is used in preparation of candies and icings.

Maltose does not occur in free form in nature. It is formed during sprouting of grains or in digestion of starch by action of enzymes. It contains two glucose molecules. Lactose is the sugar inmilk. It yields glucose and galactose on hydrolysis by lactose. It is less soluble than sucrose and maltose and less sweet than glucose. Persons, who have lactase insufficiency, cannot utilize lactose and hence have to restrict their intake of milk products. Children, who are born without the liver enzyme lactase, cannot digest milk and have to be fed soya milk instead. 

Table: Forms and Sources of Carbohydrates

Carbohydrates	Main food sources	Remarks
Polysaccharides
Glucose & hemi-cellulose	Stalks & leaves of vegetables	Indigestible
	Outer covering  of seeds	Indigestible
Pectin	Fruits	Indigestible
Gums & mucilage	Plant secretions & seed exudates	Indigestible
Starch & dextrin	Grains, legumes & tubers	Digestible
Glycogen	Meats & Sea-foods	Digestible
Disaccharides
Sucrose	Cane & beet sugar, molasses	Digestible
lactose	Milk & milk products	Digestible
Maltose	Malt products, some breakfast cereals	Digestible
Mono saccharine
Glucose	Fruits, honey, corn syrup	Digestible
Fructose	Fruits, honey	Digestible

Starch is found in cereal grains, legumes and in tubers. Glycogen is found in  meat and seafood. These are digestible polysaccharides.

Cellulose and hemicelluloses, pectin, gums and mucilage are indigestible polysaccharides. Celluloses are found in stalks and leaves of vegetables and outer coverings of seeds. Pectin are present in fruits and gums and mucilage are part of plant exudates and seeds.  

     Table: Comparative sweetness of sugars

Sugar	Sweetness value
Fructose	173
Invert sugar	130
Sucrose	100
Glucose	74
Galactose	32
Maltose	32
Lactose	16

FUNCTIONS AND FOOD SOURCES OF CARBOHYDRATES

FUNCTIONS AND FOOD SOURCES OF CARBOHYDRATES

FUNCTIONS OF CARBOHYDRATES: Carbohydrates have many important functions in the body:

1. The primary function of carbohydrates in the body is to supply energy. Each gram of carbohydrate, as starch and sugar, provides 4 kcal/g. Carbohydrates are a source of readily available energy, which is needed for physical activities as also the work of the body cells. The brain and the central nervous system are dependent on the constant supply of glucose from the blood to meet their energy needs.
2. Carbohydrates act also as reserve fuel supply in the form of glycogen, stored in muscles and liver. Then total amount of glycogen in the body is over 300 g. But it must be maintained by regular intake of carbohydrates at frequent intervals, so that the breakdown of fat and protein tissue is prevented.  
3. Carbohydrates serve other special functions in the body. Carbohydrates provide chemical framework, which combine with the nitrogen to synthesize non-essential amino acids in the body.
4. Carbohydrates and their derivatives work as precursors of important metabolic compounds. These include nucleic acids, the matrix of connective tissue and galactosides of nerve tissue.
5. Lactose, the milk sugar, provides galactose needed for brain development. It aids absorption of calcium and phosphorous, thus helping bone growth and maintenance.
6. Lactose forms lactic acid in the intestinal track due to the action of the bacteria (lactobacilli) present there. These lactobacilli synthesize some of the B-complex vitamins. It aids, bacteria (lactobacilli) present to suppress the activities of putrefactive bacteria and protects us from their undesirable effects.
7. Carbohydrates are an important part of some compounds, which increase our resistance to infection (immunopolysaccharides). Ribose, a five carbon sugar, is an essential part of DNA and RNA. Carbohydrates are a part of important compounds, which are components of nervous tissue (galactolipid), heart value, cartilage, bone and skin (chondroitin sulfate).
8. Carbohydrates are needed for ensuring complete normal metabolism of fats, thus preventing acidosis.
9. Carbohydrates are needed to prevent dehydration. A low carbohydrate diet causes loss of water from tissues as also electrolytes ( especially sodium and potassium) in the urine and can lead to involuntary dehydration.
10. Dietary fiber acts like a sponge and absorbs water. It helps smooth movement of food waste through the digestive tract and the soft, bulky stools are comfortably eliminated.

FOOD SOURCES OF CARBOHYDRATES: Carbohydrates are synthesized by plants and occur in several forms. Starch is found in plant seeds (cereals and legumes) and roots and tubers. These foods are the primary energy sources in the human dietary.

The intake of complex carbohydrate in the form of cereals, dals and legumes, in normal Indian dietary is sufficient. Cereals, which are the staple food in the Indian dietary, contribute major part of the energy  and proteins and some minerals and vitamins. Dals and legumes provide significant amounts of proteins, iron and B-vitamins, in addition to energy. Potatoes, yams, jack fruit are good source of starch in the diet. Vegetables and fruits provide dietary fiber, minerals and vitamins. Milk is the only animal food, which is the source of a very important sugar—lactose--- in the diet.

Table: Carbohydrate Content of Foods

Food	Carbohydrate		Calories/ unit
	g / 100g	g / unit
Rice, rice flakes, rawa, vermicelli, etc.	75-78	19	86-88
Wheat flour (atta), jowar	69-72	17-18	82-87
Dals	58-60	15	84-87
Tapoica or cassava	18	19	80
Banana	27	22	95
Potato, colocasia, ehiku, peas, yam, jackfruit	16-22	8-11	40-49
Mango	17	9	37
Orange, guava, pear,apple, carrot, onion, cluterbeans	7-13	3-6	24-30
Leafy and other vegetables	1-4	1-2	6-17
Milk	5	7	100
Sugar	100	5	20

Sugars are found in fruits, the percentage is about three in melons, ten in citus fruit and guava and seventeen in mango. Thus fruits are an excellent snack food as they are a dilute source of quick energy and other nutrients.

Sugars found in plant sap or juice (sugar-cane and beetroot) is extracted to give pure 100 per cent sugar. It must be used judiciously, as it provides only energy. The intake of foods high in added sugars, such as soft drinks, syrups and sweets needs to be controlled to avoid obesity.