A carbohydrate is an organic compound that consists only of carbon, hydrogen, and oxygen, usually with a hydrogen:oxygen atom ratio of 2:1 (as in water); in other words, with the empirical formula Cm(H2O)n. (Some exceptions exist; for example, deoxyribose, a component of DNA, has the empirical formula C5H10O4.)
Nineteenth century scientists thought the compound to be a hydrate of carbon and hence the name ‘carbohydrate’. However, this is not strictly true. Structurally it is more accurate to view them as polyhydroxy aldehydes and ketones.
Carbohydrates perform numerous roles in living organisms. Polysaccharides serve for the storage of energy (for example, starch and glycogen), and as structural components (for example, cellulose in plants and chitin in arthropods).
The 5-carbon monosaccharide ribose is an important component of coenzymes – for example, adenosine-5'-triphosphate (ATP), flavin adenine dinucleotide (FAD), and nicotinamide adenine dinucleotide (NAD+) – and the backbone of the genetic molecule, ribonucleic acid, known as RNA. The related deoxyribose is a component of deoxyribonucleic acid (DNA).
Saccharides and their derivatives include many other important biomolecules that play key roles in the immune system, fertilisation, preventing pathogenesis, blood clotting, and development.
In food science and in many informal contexts, the term carbohydrate often means any food that is particularly rich in the complex carbohydrate starch – such as cereals, bread, and pasta – or simple carbohydrates, such as sugar, found in confectionary, jams, preserves and desserts.
Complex carbohydrates should make up a large proportion of the diet because they tend to be more nutritious and release energy slowly. They also contain B vitamins, which are important for energy production, iron (essential for making red blood cells to transport oxygen to cells) and fibre; essential for a healthy gut.
Simple carbohydrates are quickly released into the blood stream. They can be of use when the body needs a quick injection of energy, either before or during exercise.
In order to carry out its daily physiological functions and maintain a constant temperature – despite exposure to a myriad of different environments – the body requires a constant source of energy.
Carbohydrates are an important energy source in the human diet. They generally supply about 45 percent of the energy requirement in developed countries and up to 85 percent in developing countries.
Carbohydrates are some of the cheapest, most efficient and most readily available sources of food energy since they are the main constituents of the foods that are the easiest to produce and can be obtained throughout the world, namely in the form of cereal grains, legumes and stem tubers such as potatoes. Key carbohydrates that are important in nutrition include sugars, starches, dextrin and glycogen.
The functional properties of carbohydrates in food include:
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providing nutrition
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adding flavour and colour production
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imparting sweetness
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supplying texture
Nutritionists generally accept the fact that humans don’t need more than 10-12 percent of their daily Calorie/kilocalorie intake from protein, and less than 30 percent of their daily kilocalorie intake from fat. Subsequently, intake of carbohydrate should equate to 55 percent of a person’s daily kilocalorie total.
The average human diet in the Western world has, historically, contained 40-80 percent of their energy as carbohydrate. However, as average financial incomes have increased so has the fat content of the diet, while carbohydrate content especially starches, has decreased.
Starch is the major plant polysaccharide that is readily digested in the intestine and thus serves as a source of carbohydrates.
In the human body, carbohydrates support the immune system and assist processes such as growth and blood clotting. Their primary role, however, is to supply energy to every cell of the body. Carbohydrates are the preferred energy source for the nervous system. The brain and nervous system as a whole are very sensitive to the concentration of glucose in the blood. Blood sugar concentration or blood glucose levels are important for a feeling of well being. Normally the body maintains the blood glucose level at a reference range – a set of values used by a health professional to interpret a set of medical test results from blood samples – between about 3.6 and 5.8 mM (mmol/L, i.e., millimoles/liter), or 64.8 and 104.4 mg/dL. The human body naturally tightly regulates blood glucose levels as a part of metabolic homeostasis.
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