Definition

Carbohydrates or hydrates of carbon are organic compounds made up of carbon , hydrogen and oxygen . They are the major components of most plants . Carbohydrates with five to seven carbon  atoms are called simple sugars and they play an important role in nutrition . Fructose Galactose

Classification

Carbohydrates are separated into two categories- simple and complex

Simple sugar -

Monosaccharides-

The monosaccharides are the building blocks of carbohydrates.  They are further divided into aldoses and ketoses

Dietary sources of monosaccharides--

Glucose and fructose are the common monosaccharides found in nature .

 Glucose :

Glucose is the human body's primary source of energy . It is found in fruits such as grapes and oranges and is mostly present as starch in plant foods . Anything we consume needs to be converted to glucose for the body to utilize it as a source of energy .

Fructose:

The sweetest natural sugar is found in fruits and honey.  It is 1.8 times as sweet as sucrose (table sugar or cane sugar) and is used as a sweetener in citrus drinks. 

Galactose:

It does not occur alone in nature.  It is linked to glucose to make lactose (milk sugar) a disaccharide.

Disaccharides -

Disaccharides or double sugars include sucrose, maltose and lactose. 

Dietary Sources of Disaccharides-

Sucrose:

Extracted from sugar cane and sugar beets.  It is the chemical name for granulated or table sugar or refined sugar.  Glucose and fructose linked together is sucrose.  also occurs in many fruits and vegetables. 

Maltose:

Two glucose units linked together is maltose.  It occurs in germinating grains and also in the commercial production of malt (manufacture of beer).

  Lactose:

Found naturally only in milk (about 5%) and also in other milk products such as cottage cheese and cream.  Milk when fermented gives lactic acid, which is used in setting curds and cheese.

Oligosaccharides -

Oligosaccharides contain 3–10 monosaccharide units . They are formed during the breakdown of starch ( long chains of hundreds of thousands of glucose linked together ) , e.g. raffinose and stachyose .

Complex carbohydrates

Starch -

It is composed of many monosaccharide units linked together.  They are also called as polysac charides, e.g.  starch, fibre and dextrin.

  Dietary Sources of Starch

Hundreds of thousands of glucose units linked together form starch.  It is found only in plant food and is a storage form of energy.  It is mainly found in rice, corn, potato, sago, arbi, pulses and legumes.  Bananas are a rich source of starch, which is converted into sugar when they ripen.  Hence, they are a good source of energy. 

Dextrin-

When starch is broken down into smaller units, it forms dextrin.  They are more easily digested.  This is the reason for dextrin being used in the commercial preparation of infant foods.  They are produced when heat acts on starch, e.g.  toasting of bread. 

Dietary fiber -

It is found only in plant foods.  It is divided into soluble and insoluble fibers.  Fibers such as pectin and gums are soluble in water and fibers such as celluloses and hemicelluloses are insoluble.  Fiber cannot be digested by the human body and hence is eliminated in the faeces. 

Health Benefits of Dietary Fiber

Dietary fiber gives a feeling of fullness or satiety.  It relieves constipation and prevents many degenerative diseases such as colon cancer.  It helps in reducing bad cholesterol in dyslipidaemia patients.  For diabetic patients, dietary fiber adds bulk to the diet without raising calories and also helps in the slow absorption of glucose.  It is recommended to consume 30 g of fiber per day as a part of balanced diet for a normal adult.

Sources of Dietary Fiber

Dried beans, peas and nuts, whole grains, raw vegetables, fruits, oats, barley and soya bean are all rich sources of fiber and must be included in every meal as a part of balanced diet. 

Glycogen

Body stores excess carbohydrates in the form of glycogen in the liver and muscles.  When the body needs a source of energy, like during prolonged fasting, the body converts the stored glycogen, which is quickly converted to glucose, and is utilized by the body. 

CALORIFIC VALUE

One gram of carbohydrate on digestion yields 4 kcal of energy.  For example, a teaspoon of sugar, which is 5 g, yields 5 g x 4 kcal = 20 kcal of energy.

FUNCTIONS OF CARBOHYDRATES •

Carbohydrates provide the major source of energy for the body . Each gram of carbohydrate yields 4 kcal of energy and 60-70 % of the daily requirement of energy comes from carbohydrates . When adequate carbohydrates are not supplied by the diet , either in the case of starvation or prolonged fasting , or by following fad diets , protein is used as an alternative source of energy . One gram of protein also yields 4 kcal , but protein should be spared for tissue building , growth and cell repair . Hence , it is essential to consume adequate carbohydrates in the diet to prevent protein from being broken down as a source of energy . This function of carbohydrates to spare protein from being utilized as a source of energy is called protein - sparing action . When the diet does not have adequate carbohydrates due to fad diets ( Atkins diet ) , protein and fat are broken down as a source of energy ( 1 g of fat yields 9 kcal ) . Large amount of ketones are formed leading to ketosis and muscle wasting . • Brain uses only glucose for its functioning and which is also the primary source of energy for the central nervous system . Hence , hypoglycaemia ( low blood sugar ) is considered more dangerous than hyperglycaemia ( high blood sugar ) . Carbohydrates provide the cheapest source of energy . India is a country where cereals are consumed mainly . Sadly , many people still live below the poverty level and they mainly depend on roots and tubers and cereals for their energy requirements . • Dietary fibre or roughage is provided by carbohydrates which provide numerous health benefits

DIGESTION, ABSORPTION AND STORAGE

Digestion of Carbohydrates

in the mouth:

Carbohydrates may be in various forms in different types of food.  Starches pres ent in rice, roots and tubers form a major component of our daily meal in our Indian diet.  The digestion of starch begins in the mouth where the enzyme salivary amylase splits starch into smaller units such as oligosaccharides and disaccharides.

  In the stomach:

When the food bolus reaches the stomach, it is acted upon by the gastric juice and the food is passed into the small intestine. 

In the small intestine

: Pancreatic amylase, the enzyme present in the small intestine, continues the digestion.  The food is passed into the brush border where it is acted upon by their respec tive enzymes and is broken down into monosaccharide (glucose), which is readily absorbed by the body.  The final products of digestion are glucose, fructose and galactose.  These are absorbed in the small intestine.

Absorption

Glucose, the product of digestion, is absorbed from the intestinal lumen by diffusion and also by active transport.  Glucose is transported from the intestinal cells into the capillaries by diffusion. 

Storage

Excessive glucose is converted into glycogen in the liver and muscles and is used when required.

METABOLISM OF CARBOHYDRATES

Metabolism is a chemical process in the body that helps the food to be used in an optimal way to provide energy for all vital functions and biological processes in the body Carbohydrates play a major role in energy metabolism as it is involved in all major  metabolic pathways in the body.  Glucose, which is the end product of digestion, is the only way by which the body uses carbohydrate as a source of energy.  Oxidation occurs through glycolysis and Krebs cycle.  Energy is released in the form of adenosine triphos phate (ATP). 

DEFICIENCY OF CARBOHYDRATES

Daily meal should contain not less than 100 g of carbohydrates per day.  When there is a deficiency of carbohydrates, the body finds alternate sources of energy.  Severe deficiency causes protein - energy malnutrition (PEM) in children which is common in developing countries like India.

OVERCONSUMPTION

Excessive carbohydrates lead to increase in body weight and obesity.  Excess carbohydrates in the diet is stored as fat in the adipose tissue.  There is a growing concern globally that there is an increase in childhood obesity due to increased consumption of junk food and less physical activity