This is the collective name given to starches, glycogen, sugars and dietary fibres which can be converted to glucose and used as fuel for energy. Chemically speaking, carbohydrates consist of various combinations of carbon, hydrogen and oxygen, and are classified into three main groups, according to their complexity. Least complex are the monosaccharides and disaccharides (also known as the sugars), and most complex are the polysaccharides (also known as starch or complex carbohydrate).
Sources and types of carbohydrate
Carbohydrates come from plants, which store them as their chief source of energy. Water, minerals and nitrogen in the soil are taken up by the plant’s roots and sent to the leaves. With the aid of chlorophyl, these nutrients combine with carbon dioxide and sun energy to form sugars via photosynthesis.
The most common of all the sugars is sucrose, or ordinary table sugar. Like lactose and maltose, sucrose is a disaccharide, which simply means a combination of two simple sugars, or monosaccharides. Common sugars and their sources are shown in the table. As the table shows, all disaccharides have a glucose component. A daily supply of glucose is needed for all the body’s metabolic functions.
Starches are found in grains, roots, vegetables and pulses (legumes), encased within plant cells. Cooking softens and ruptures the plant cell to make the starch available for enzymes to work on in our intestines. Common starch foods include rice, potatoes, bread and other products made from flour, cereals, and pasta.
The end product of starch digestion is also mainly glucose. Starches may be fully or partially digestible, or completely indigestible, depending on the type.
Carbohydrates are broken down into simple sugars by the processes of digestion. Saliva contains an enzyme which begins this process. Pancreatic enzymes continue it, and enzymes in the wall of the small intestine complete the process, breaking down disaccharides into simple sugars.
Once disaccharides and starches have been broken down into simple sugars by the digestive processes, these simple sugars (or monosaccharides) enter the bloodstream and travel to the liver. Glucose can be directly used to make energy, and can also be turned by the liver into glycogen, a form of stored carbohydrate which is mostly sent to the muscles. Any remaining excess glucose is converted to fat.
Fructose cannot be used to make energy. The liver makes some of it into glycogen for its own use, but most of the fructose in a western diet is in excess of human requirements and is turned into fat.
Within 30 minutes to one hour after a meal, the blood glucose (also known as blood sugar) peaks.
High blood glucose (blood sugar) levels stimulate the pancreas to produce the hormone insulin. Insulin allows muscle and fat cells to extract glucose from the blood. Once in these cells it enters a part of the cell known as the mitochondrion, where it goes through successive chemical changes that result in the production of the body’s pure energy source, ATP (adenosine triphosphate).
|Disaccharides||Main food sources||Converted to|
|Sucrose||Cane and beet sugars, maple syrup||Glucose and fructose|
|Lactose||Milk and milk products||Glucose and galactose|
|Maltose||Malt products||Glucose and glucose|
|Glucose||Fruits, honey, corn syrup||Energy|
|Fructose||Fruits, honey||Mostly converted to fat|
|Galactose||Does not occur in free form in food||Glucose|
|Sorbitol||Fruits, vegetables, dietetic products||Glucose and carbon dioxide|