How The Body Uses Carbohydrates

Carbohydrates are sources of energy for vital metabolic processes and also are constituents of cellular substances such as nucleic acids, and are enzyme cofactors and structural components of cell walls and cell membranes.

The digestive system breaks down the carbohydrates into simple sugars and ultimately absorbs glucose into the bloodstream. Any excess glucose is converted into glycogen which is then stored around the body. Thus an excess of carbohydrates can lead to an increase in body fat and a gain in weight.

Carbohydrates are conveniently classified into three major groups polysaccharides (complex carbohydrates), monosaccharides and disaccharides (simple carbohydrates). Complex carbohydrates contain many more nutrients than simple carbohydrates. These complex carbohydrates, such as whole wheat bread, potatoes and rice have a high nutrient density and are a good source of dietary fiber, and should be a major ingredient in any persons diet. Between 55% and 75% of calories should come from carbohydrates.

They have a low fat content, but are usually consumed with meat, cheese, oil or butter, all of which are very fattening. It is a good idea to try and limit the amount of fatty foods and protein rich foods combined with carbohydrates. For instance a pasta dish with a garlic, chili or tomato sauce is far healthier than a similar pasta covered in a cream, cheese or meat topping.

Simple carbohydrates are usually refined foods such as white sugar, white bread or cola. These foods contain little else besides energy (ie. empty calories). Any food that does not have a large variety of nutrients is considered to have a low nutrient density. Foods that have low nutrient densities should be limited to small portions and only eaten as a special treat if the bulk of your diet consists of nutrient dense, natural, wholefoods (eg. bananas, fruit juice, nuts, granola, beans and green vegetables).


Monosaccharides are simple sugars (glucose, fructose, and galactose) that do not need to be further digested to be absorbed. The most important dietary monosaccharide is glucose, also called dextrose. It constitutes about 0.1% of the blood of mammals and is essential to life. Glucose, either free or combined with other molecules, is probably the most abundent organic compound. It is the ultimate hydrolysis product of starch and cellulose.

Fructose, also known as fruit sugar, occurs free, along with glucose and sucrose, in many fruits, vegetables, and honey.


Are sugars formed from two monosaccharides. Ordinary cane sugar (sucrose) is a disaccharide composed of glucose and fructose monosaccharide units. Sucrose is the most important disaccharide, it occurs in all photosynthetic plants where it appears to serve as an easily transported energy source. Its two main commercial sources are sugar beets and sugar cane. The juices, which contain about 20% sucrose, are put through a rather extensive purification process to remove impurities.

Maltose is a disaccharide of two glucose molecules and is found in beer and cereals. It is formed by the action of an enzyme from malt on starch, further hydrolysis of maltose, catalyzed by the enzyme maltase (from yeast) gives only glucose. Lactose is the sugar present in milk, human milk contains five to eight percent and cow's milk, four to six percent. It is composed of one molecule of glucose and one of galactose.


Starch, glycogen, cellulose and most types of fiber are polysaccharides. Starch is the reserve carbohydrate in many plants and comprises large percentages of cereals, potatoes, corn, and rice. Under the microscope, the appearance of the granules of starch from these different sources varies both in shape and size. Chemically, however, they are similar.

Complete hydrolysis of starch yields glucose but partial hydrolysis gives maltose as well. Partial hydrolysis of starch transforms it into dextrins, polysaccharides of smaller molecular weight than starch. They are more readily digested than starch and are used, mixed with maltose, in infant foods. A dried mixture of dextrins, maltose, and milk is the preparation used for making malted milk. Dextrins are sticky when wet and are used in manufacturing mucilage (gum) for postage stamps and envelopes. In laundries, starched materials become stiff and shiny due to the transformation of the starch to dextrins by the heat of the iron.

When starch is ingested, it is hydrolyzed enzymatically in a stepwise fashion. Initiated in the mouth by the enzyme amylase, present in saliva, hydrolysis is continued by additional amylase in the pancreatic juices. The maltose produced in this way is, in turn, hydrolyzed to glucose with the aid of an enzyme present in the intestines. The glucose is absorbed from the intestines into the blood and transported to the liver, muscles, and other sites where it is converted to another glucose polymer, glycogen, and stored.

Glycogen, the reserve carbohydrate of animals, is found mainly in the liver and muscles. Glycogen helps maintain the proper amount of glucose in the blood by removing and storing excess glucose derived from ingested food or by supplying it to the blood when it is needed by the body cells for energy.

Cellulose is the main structural material of plant life, being the chief ingredient of cell walls of cotton, wood pulp, straw, corn cobs, and many other materials. Fiber includes a variety of carbohydrates and other components.

The chemical linkages in starch and glycogen can be split by the human intestinal enzymes, but those of polysaccharides found in fiber are indigestible, although some fiber components can be broken down by enzymes released by intestinal bacteria to short-chain fatty acids that can be reabsorbed.

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