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Carbohydrates provide us with the energy that we need to carry out day-to-day functions. They make up 45-65% of our total energy intake a day. Lack of carbohydrate consumption can cause muscle cramps, fatigue, poor mental functions, and decreased resistance to diseases and ailments. Sources of carbohydrates include grains (rice, cereal), fruits (apples, oranges, bananas), starchy vegetables (peas, potatoes, corn), and dairy products (milk, cheese, yogurt).

The following are the various functions carried out by carbohydrates:
  • Providing energy.
  • Regulating bowel movements.
  • Maintaining hormone balance in the body.
  • Stimulating the functioning of the immune system.
  • Generating antioxidants.
  • Helping in regulation of sugar levels.
  • Preventing the breakdown of amino and fatty acids.
  • Sparing proteins.
  • Supplying fiber.
Carbohydrates are mainly divided into two groups: simple carbohydrates and complex carbohydrates. The two vary nutritionally, and are broken down in different ways during digestion. Based on their chemical composition, they are further divided in 4 major classes. Let us take a look at all the types of carbohydrates and their functions.

Based on Structural Complexity

Simple Carbohydrates
Simple carbohydrates have a 'simple' molecular structure and are made of 1 or 2 sugar molecules. Simple carbs can be digested very quickly due to their simple structures, and tend to elevate the blood sugar quickly. On consumption, these sugars are directly absorbed in the blood in the form of glucose. Glucose provides instant energy as it reaches the different parts of the body via blood. Although these carbs provide quick energy, they need to be consumed in moderation to prevent rapid swings in blood sugar. They increase the storage of fats which, if not burned or used up, can lead to obesity and other health issues.

Refined sugar, dairy sugar, fruits, refined flour, etc. are all sources of simple sugars. They are also present in natural foods like fruits, vegetables, milk, and other dairy products. In addition to these, honey, molasses brown sugar, soft drinks, sweets, snack bars, corn syrup, maple syrup, biscuits, cakes, chocolate, etc., are also rich sources of simple sugars.

Complex Carbohydrates
Complex carbohydrates are made up of long chains of sugar molecules. These take longer to break down into their elementary form and hence, require more time for digestion compared to simple carbohydrates. This slows the breakdown process and thus, supplies us with constant energy for a longer duration. Since these carbohydrates require more time for conversion, they are constantly used up by the body. Therefore, the sugar which is converted to fat, is not stored in a large quantity, unlike simple carbohydrates. Complex carbohydrates are of two types:

▶ Starch
Starch is a form of carbohydrate that is made up of long, complex chains of simple sugars. Starch must be broken down through digestion before your body can use it as a glucose source. Some starches actually have a higher glycemic index than some sugars. In this sense, they are not "complex" for very long at all. Since most starchy foods are rapidly broken down into sugar, people who are sensitive to sugar should avoid most starchy foods. Examples of starchy complex carbohydrates are corn, bread, cereal, rice, beans, pasta, wheat, grains, potatoes, etc.

▶ Dietary Fiber
These are indigestible foods that cannot be broken down into sugars. They are low on calories, and help to carry out the digestion process smoothly. They also help regulate blood glucose levels, as well as lower cholesterol levels, and promote regular digestion and excretion of waste. Whole grains and many fruits and vegetables, including dark leafy greens and orange-colored fruits and vegetables, are rich in fiber.

Complex carbs are healthier than simple carbs and hence, should make up a major part of our diet. They provide us with a more controlled release of blood sugar (and the corresponding insulin response), and generally provide better nutritional value.

Based on Chemical Composition

Carbohydrates are the most easily available and the largest set of compounds found on Earth. Based on their chemical composition, there are four major classes of carbohydrates. These are described in brief as follows.
These are the basic compounds with a cyclic structure consisting of carbon, hydrogen, and oxygen in the ratio of 1:2:1. 'Mono' refers to single and saccharides means sugar. Glucose, fructose, galactose, arabinose, and xylose are common types of monosaccharides.

▶ Chemical Properties
  • Monosaccharides are aliphatic aldehydes or ketones.
  • They contain one carbonyl group and one or more hydroxyl group.
  • They commonly contain either five carbons (pentose) or six carbons (hexose).
▶ Physical Properties
  • Crystalline
  • Soluble in water
  • Sweet-tasting

Disaccharides contain two sugar molecules. Ideally, they come under oligosaccharides (since oligosaccharides contain two to ten sugar molecules). However, due to their comparatively simple chemical structure, disaccharides are classified as a different class of carbohydrates. Common disaccharides are sucrose, lactose, maltose, trehalose, and cellobiose.

▶ Chemical Properties
  • A disaccharide is formed when two monosaccharides bond together (by glycosidic bond) by a condensation reaction and release one molecule of water.
  • They are further classified into reducing and non-reducing disaccharides. In reducing disaccharides, the monosaccharide has a free hemiacetal unit.
  • However, in non-reducing disaccharides, the components bond through an acetal linkage between their anomeric centers and do not have a free hemiacetal unit.
▶ Physical Properties
  • Crystalline
  • Water-soluble
  • Sweet-tasting
  • Sticky

These are carbohydrates with more than two basic types of sugar molecules, usually between three and ten basic units. Their main role in the body is to store glucose. Raffinose and stachyose are the main types of oligosaccharides which consist of repetitive chains of fructose, galactose, and glucose.

▶ Chemical Properties
  • Oligosaccharides are carbohydrate polymers with a low molecular weight.
  • They are covalently bonded with glycosidic bonds due to condensation reaction or dehydration synthesis.
  • Many oligosaccharides bond with proteins and lipids on the outer cell surface for cellular recognition of extracellular signal molecules.
▶ Physical Properties
  • Crystalline
  • Water-soluble

▶ Chemical Properties
  • Polysaccharides have a high molecular weight.
  • They are further divided into homopolysaccharides and heteropolysaccharides.
  • Homopolysaccharides contain the same monosaccharides, whereas heteropolysaccharides contain more than one type of monosaccharides.
  • Common homopolysaccharides include starch, cellulose, and glycogen.
  • Pectin, hemicellulose, and gums are common heteropolysaccharides.
▶ Physical Properties
  • Not water-soluble
  • Not crystalline
  • Not sweet
  • Form colloidal suspensions instead of solutions

Disclaimer: This article is for informative purposes only and does not attempt to replace the advice offered by an expert on the subject.