CARBOHYDRATES
Most of the energy our body needs comes from carbohydrates. Carbohydrates are mainly found in vegetables. They are the main source of food in the world as they are easy to obtain and less costly. Nonetheless, the general population has a tendency to think of carbohydrates as fattening products. Because of this, the consumption of starchy carbohydrates and fibre has gone down drastically in western countries, increasing on the other hand the consumption of sugar.
Carbohydrates (or glucids) are organic compounds composed of carbon, oxygen and hydrogen. The number of carbons varies but only hexoses (with six carbons) and pentoses (with five) and their polymers (union of several carbons) are nutritionally important.
Classification
Carbohydrates are classified according to the number of saccharide units (a carbohydrate simplest form); monosaccharides can breakdown into a simpler form, disaccharides can breakdown into two monossacharide molecules, oligosacchararides produce 3 to 10 units and polysaccharide from 10 to 10000 monosaccharide units.
a) Monosaccharides or simple sugars
Hexoses (glucose, fructose and lactose) are of nutritional value. Pentoses (ribose, xylose and arabinose) are part of nucleic acids.
- Glucose (or dextrose) is found in fruits and in honey. It is the main end product of the rest more complex carbohydrates. This type of sugar is found in the blood, and used by all body tissues (being the only source of possible energy for the central nervous system). It is stored in the liver and muscle in the form of glycogen.
- Fructose (or levulose) is the sugar found in fruits, and it is also found in honey. It is the sweetest of all sugars.
- Galactose is produced from lactose in the milk.
b) Disaccharides
These are formed by two monosaccharide molecules, one of which is always glucose.
- Sucrose is a common sugar obtained from beets and sugar canes.
- Maltose can be found in some cereals such as barley, but it is mainly found in starches.
- Lactose is the sugar form found in milk.
c) Polysaccharides
Nutritionally important polysaccharides are formed from unions of molecules of glucose. They are less soluble and more stable than simple sugars. Starches, dextrines, and glycogen are totally digestible whilst fibre is not digestible.
- Starch is the great reserve of glucose in vegetables. It is present in cereals, tubercles, and legumes. To be digested in the human intestinal tract it needs to be previously boiled (it causes diarrhoea if eaten raw).
- Glycogen is the carbohydrate reserve in animals and the first and leading source of available glucose. It is stored in the liver and muscle (about 340 gm).
- Fibre: Fibre is understood to include all those polysaccharides that are not starch from the cell wall of plants. Depending on their solubility in water, the different types of fibre are classified into insoluble (cellulose and hemicelluloses, found mainly in fruits, vegetables, and cereals), and soluble (pectines found in fruits, betaglycogens, legume gums, and mucilages from the cuticle of cereals). Most foods contain a mixture of soluble fibre and insoluble fibre.
Digestion, Absorption And Metabolism Of Carbohydrates
The ingestion of cooked starch starts in the mouth where amylase produced by salivary glands begins to digest starch on its way to the intestine. Complete digestion is achieved thanks to the pancreatic amylase, which degrades starch into disaccharides. Disaccharides from starch and those ingested with the rest of foods are subsequently degraded into monosaccharides in the intestine by the action of lactase, saccharase and maltase. Monosaccharides are absorbed in the intestine before they enter the bloodstream. Part of the ingested starch is not degraded and utilised by intestinal bacteria.
The passage of glucose coming from the intestine into the bloodstream triggers the secretion of insulin by the pancreas. Thanks to the insulin, glucose is transported to the liver, muscle and fat tissue to be used or stored.
Unused glucose is immediately used and transformed into glycogen in the liver. Glycogen is the main and first source of glucose during fasting. When glycogen deposits are full (about 300 gm), excess glucose is transformed into triglycerides (fats) and accumulated into fat tissue.
During fasting periods, because blood glucose level decreases, the secretion of insulin by the pancreas also decreases, thus favouring the transformation of glycogen into glucose, which results in elevated blood glucose levels. When glycogen reserves run out, the low level of insulin transforms fat triglycerides and certain protein amino acids into glucose, maintaining blood levels.
Functions Of Carbohydrates
- Energy generation: Carbohydrates are the main source of energy in the organism. Each gram provides approximately 4 kilocalories.
- Saving function: Glucose is indispensable to maintain the integrity of nervous tissue, being the only source of energy usable by the brain. A minimal amount of carbohydrates (approximately 80 grams per day) is necessary to avoid the protein destruction and ketogenesis (production of ketones from fats).
- Plastic function: Carbohydrates are part of some tissues; for instance, pentoses are part of nucleic acids, others are part of membranes in blood capillaries or nerve tissue and others make up connective tissue.
- Function of diet fiber: The effects of fiver on the gastrointestinal function are varied. These functions depend on their solubility and other physical and chemical properties such as viscosity, capacity for water retention, interaction with biliary salts, and capacity to be used by the intestinal flora (adequate maintenance of intestinal balance).
Sources Of Carbohydrates
Most carbohydrates that we consume come from vegetables. Cereal grains are the main source of starch. Even though glycogen is stored in muscles and in the liver, carbohydrates only provide small amounts of glycogen. Among animal products that constitute a source of carbohydrates, milk and its derivates are one of the most important and also the only source of lactose.
Fiber is found in fruits, vegetables, legumes, dry fruits, nuts, and cereals. The most concentrated source of fiber is found in whole cereal grains (specially in wheat). Fruits and vegetables contain less carbohydrates due to their high water content.