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Carbohydrate is the preferred energy source for many of the body's functions. As long as carbohydrate is available the human brain depends exclusively on it as an energy source.

Carbohydrate shares its fuel providing responsibility with fat. Fat however normally is not used as fuel by the brain and central nervous system and diets high in certain types of fat are associated with chronic diseases. The other energy sources available to the body protein and alcohol offer no advantage as fuels.
Carbohydrates are one of the essential food ingredients, which we all require. We consume carbohydrates in one form or the other. We encounter carbohydrates at every turn of our lives. The paper we use for writing, the cotton clothes we wear and the wooden furniture around us are all made of cellulose.

The sweetening agents in fruits are nothing but simple carbohydrates. Carbohydrates are organic molecules composed of carbon, hydrogen and oxygen.


Types of Carbohydrates

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The dietary carbohydrates include the sugars, starch and fiber. Chemists describe the sugar as
  • Mono saccharides (single sugars)
  • Di saccharides (double sugars)

Starch and fibers are

  • Polysaccharides (compounds composed of chains of mono saccharides units)

All of these carbohydrates are composed of the single sugar glucose and other compounds that are much like glucose in composition and structure.

Three mono saccharides are important in nutrition glucose, fructose and galactose. All three mono saccharides have the same number and kinds of atoms, but in different arrangements.

1. Glucose

Most cells depend on glucose for their fuel to some extent, and the cells of the brain and the rest of the nervous system depends almost exclusively on glucose for their energy. The body can obtain this glucose from carbohydrates. To function optimally the body must maintain blood glucose within the limit that allow the calls to nourish themselves. If blood glucose level falls below normal the person may become fatigued.

Blood glucose homeostasis is regulated primarily by two hormones insulin which moves glucose from blood into the cells and glucagon which brings glucose out of storage when blood glucose falls.

The structure of glucose is shown below.


2. Fructose

Fructose is the sweetest of the sugars. It occurs naturally in fruits, honey and saps. Other sources include soft drinks, ready to eat cereals and other products sweetened with high fructose corn syrup. Glucose and fructose are the most common mono saccharides in nature.

3. Ga lactose

The third single sugar is galactose occurs mostly as a part of lactose, a di saccharides also known as milk sugar. During digestion galactose is freed as a single sugar.

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In disaccharides pairs of single sugars are linked together. Three disaccharides are important in nutrition: maltose, sucrose and lactose. All three have glucose as one of their single sugars.

1. Sucrose

Sucrose is the most familiar of the three disaccharides and is what people generally mean when they speak of "sugar". this sugar is usually obtained by refining the juice from sugar beets or sugarcane to provide the brown, white and powdered sugars available in the supermarket, but it occurs naturally in many fruits and vegetables. When a person eats a food containing sucrose, enzymes in the digestive tract split the sucrose into its glucose and fructose components.


2. Lactose

Lactose is the principal carbohydrate of milk. Most human infants are born with the digestive enzymes necessary to split lactose into its two mono saccharides parts, glucose and galactose. So as to absorb it. Breast milk thus provide a simple, easily digested carbohydrate that meets an infants energy needs; many formulas do too, because they are made from milk.


3. Maltose

The third disaccharide maltose is a plant sugar that consists of two glucose units. maltose is produced whenever starch breaks down - as happens in plants when they break down their stored starch for energy and start to sprout and in human beings during carbohydrate digestion.

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Unlike other sugars which contain the three mono saccharides-glucose, fructose and galactose in different combinations, the polysaccharides are composed almost entirely of glucose. Three types of polysaccharides are important in nutrition; glycogen, starch and fibers.

1. Glycogen

Glycogen molecules which are made of chains of glucose, are most highly branched than starch molecules. Glycogen is found in metals only to a limited extent and not at all in plants. For this reason glycogen is not a significant food source of carbohydrate, but it does play an important role in the body. The human body stores much of its glucose in the liver and muscles. The liver stores the glycogen making it available as blood glucose for the brain to other tissues when the supply runs low.



Starch is a long straight or branched chain of hundreds or thousands of glucose units linked together. These giant molecules are packed side by side in grains such as rice or wheat, in root crops and tubers such as yams and potatoes, and in legumes such as peas and beans. When a person eats the plants, the body splits the starch into glucose units and uses the glucose for energy.


Dietary fibers are the structural parts of plants and thus are found in all plant derived foods-vegetables, fruits, whole grains and legumes. Most dietary fibers are polysaccharides chains of sugars just as starch is but in fibers the sugar units are held together by bonds that human digestive enzymes cannot break. Consequently most dietary fibers pass through the body providing little or no energy for its use.

More topics in Carbohydrates
Monosaccharides Disaccharides
Glycogen Glycosaminoglycans
Bond Order Sugar Alcohol
Glycosidic Linkage Oligosaccharides
Epimer Pectin
Carbohydrate Metabolism Functions of Carbohydrates
Chemical Structure of Carbohydrates Complex Carbohydrates
Classification of Carbohydrates Simple Carbohydrates
Carbohydrates Monomer Polymers of Carbohydrates
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