Carbohydrates are compounds which contain carbon, hydrogen and oxygen. They are chemically known as “saccharides“, as carbon, hydrogen and oxygen form saccharide groups. They are classified, based on the number of saccharide groups in them:
- Disaccharides, and
Monosaccharaides contain one sugar unit, disaccharides two, and polysaccharides three or more. Polysaccharides are often referred to as complex carbohydrates because they are typically long multiple branched chains of sugar units.
Monosaccharides cannot be simplified as they are the simple form of carbohydrates and hence is the absorbable form of carbohydrate in our body. In human nutrition only hexoses are of importance. The 3 monosaccharides of hexose group are:
- Fructose, &
Glucose (Dextrose): Also known as “Grape Sugar”, glucose is an aldose sugar. It is a white crystalline substance easily soluble in water with a sweet taste. It is readily absorbed from the stomach. It is present in fruits and honey.
Fructose (Levulose): Also known as “Fruit Sugar” is a ketosugar. It is sweeter than glucose. Fructose enters the cells without the aid of insulin and so it is recommended for diabetics. Honey is the richest source of fructose (30-40%). Fructose on reduction yields a mixture of sorbital and mannitol.
Galactose: It is not present in nature. It is present only in milk sugar lactose; on hydrolysis lactose yields galactose and glucose.
Disaccharides are complex sugars with two saccharide groups and are formed by the condensation of two monosaccharides. On hydrolysis or by digestion they are split into monosaccharides. They are water soluble, diffusible and crystallisable. They vary in their sweetness. Commonly found disaccharides are:
- Maltose, &
Sucrose: Also known as “Invert Sugar”, is the commonest form of sugar in the diet. Either by hydrolysis or by enzyme action sucrose is simplified into glucose and fructose. It is present in sugarcane, beetroot, and in many fruits and vegetables, and honey.
Maltose: Also known as “Malt Sugar”. It is a disaccharide which contains two molecules of glucose. Starch is converted to maltose before breaking down into glucose. It is present in sprouted grains. Cereal grains are rich in maltose.
Lactose: Also known as “Milk Sugar” and contains glucose and galactose. It is easily digestible but is not as sweet as cane sugar.
They are large complex molecules of monosaccharides linked together. Several hundreds of glucose units are linked together in polysaccharides. They are insoluble in water. Starch, dextrins, pectin, glycogen and cellulose are the common forms of polysaccharides.
Starch: Plants store carbohydrates in the form of starch and it is the main source of nourishment for human race. Cereal grains, seeds, roots like potato, tapioca, yam and plantain contain considerable amount of starch. On cooking, starch absorbs water and it swells and ruptures.
Different sources of starch behave differently. Maize starch and corn starch are better “thickening agents” than rice or wheat starch. All starches are broken down into glucose in the digestive system.
Dextrins: When starch is partially broken into fragments either by digestion or by acids they are called dextrins. They are like starch itself. Dextrin is broken down into maltose.
Pectin: It is a polysaccharide with no nutritional significance. But it is useful in the preparation of jam and jelly and thus it contributes to the palatability of foods.
Glycogen: It can be described as the animal starch as it is in this form animal store carbohydrate in the body. Glycogen is stored in the liver and muscles. This is the intermediate form of energy for the body. About 350 g of carb is stored as glycogen in the body.
Cellulose: It is an insoluble, indigestible polysaccharide. More than 3,000 glucose units are there in cellulose but it is not of human utilization. Cattle can digest cellulose.
Even though it is not of much food value but it provides bulk to the diet and thus helps movements in the large intestine. It prevents constipation, reduces cholesterol levels, and hence helps in weight loss too. A high fiber diet can help one to get rid of obesity as it delays digestion and contributes satiety to obese people.
Sugar Alcohol: These are 3 sugar alcohols available commercially. They are sorbitol, mannitol and dulcitol. Sorbitol is used by diabetic patients. Mannitol is produced from mannose, a monosaccharide which does not occur free in nature.
Liquid Glucose: It is a complex carbohydrate predigested by hydrolysis of starch into tri-, tetra-, penta- or higher saccharides as desired. Its viscosity and sweetness can be changed by varying the extent of hydrolysis with acids or specific enzymes. It has one-fifth the sweetness of fructose and one-third that of dextrose. It can be dried, powdered or demineralized to suit any requirements.
Liquid glucose and dextrose are absorbed twice as fast as sucrose and ten times as fast as fructose.
Sources of Carbohydrate
Carbohydrates form the staple food in the tropics. The best sources of carbohydrates are cereals like rice, wheat, millets like ragi, maize, roots & tubers like potato, tapioca, sweet potato, yam and colocasia, pulses, sugar and jiggery; honey, fruit and vegetables are the other sources of carbohydrates. Banana, apple, plantain and dried fruit are also good sources of carbohydrates.
Carbohydrates are the cheapest and most important source of energy for the vast majority of people. One gram of carbohydrate gives 4 kcals and an average Indian diet contains about 300 gm of carbohydrate.
Requirements of Carbohydrate
Carbohydrates are interchangeable with fats for energy requirements. Humans convert amino acids of proteins and glycerol of fats to glucose.
Consequently, there are no specific dietary recommendations for carbohydrate, provided adequate calories are supplied. However, a person taking low-calorie diet requires a minimum of about 50-75 g a day of carbohydrate to prevent ketosis.