Carbohydrates have been an important constituent of the human diet and the subject of many a discussion on nutrition for a long time. Recognised as essential nutrients that provide energy to tissues, they play a number of other physiological roles as well.
No discussion on obesity and diets is complete without a mention of carbohydrates. In fact, there are a number of low-carb or no-carb diets which, as the names suggest, restrict the intake of carbohydrates.
Carbohydrates are the most abundant of the major classes of biomolecules. They serve as energy stores (for instance starch and glucogen) with readily-available energy utilising glucose. They are important structural components in the cells of plants (as cellulose) and animals (as cartilage and chitin).
Carbohydrates and their derivatives perform a number of key functions in immune systems, fertilisation, pathogenesis, blood clotting and the development process.
Examples of high-carb foods are bread, beans, milk, cereal grains, potatoes, cookies, soft drinks, cakes, etc. They are commonly found in the form of sugars, dietary fibres and starches.
There are basically two types of carbohydrates: simple carbohydrates and complex carbohydrates. Fructose, glucose and common sugar (also called sucrose) are examples of simple carbohydrates, while oligosaccharides, starch and dextrins are examples of complex carbohydrates. Complex carbohydrates are preferred to simple ones.
Of late, the concepts of glycaemic index (GI) and glycaemic load (GL) have been developed. Foods containing different types of carbohydrates have been grouped on the bases of GI and GL to facilitate the selection of a diet.
In a telephonic conversation with F&BNews, Dr K Bhaskarachary, scientist, food chemistry, National Institute of Nutrition (NIN), Hyderabad, and secretary, Indian Dietetic Association, busted a few myths about carbohydrates and explained how the nutrient is perceived in India.
When asked whether Indians are not aware about bad carbs, he said, “There is no such thing as a 'good' carbohydrate or a 'bad' carbohydrate. Carbohydrates are an important source of energy and therefore necessary to stay fit and healthy. In fact, about 55 per cent of the body's total energy comes from carbohydrates.”
“Lifestyle diseases, which are on the rise in India, are the result of a poor (read: not balanced) diet and the lack of physical activity. Indian diets are healthy. It's actually the way foods are processed that makes the difference,” he said.
Monosaccharides such as glucose, fructose and galactose are the building blocks of the carbohydrates. They combine with two or more of the same or a different monosaccharide to form another carbohydrate.
Disaccharides contain two monosaccharides. Examples are maltose, which contains two units of glucose; sucrose, which contains one unit each of glucose and fructose; and lactose (the sugar present in milk), which contains one unit each of glucose and galactose.
Trisaccharides contain three monosaccharide units.
Oligosaccharides contain between two and ten units of monosaccharides. In fact, disaccharides and trisaccharides are also oligosaccharides.
Raffinose (a trisaccheride of galactose, fructose and glucose, present in such vegetables as beans, broccoli and cabbage) and stachyose (a tetrasaccharide made up of two units of galactose and one
unit each of glucose and fructose, present in soya and other beans) are examples of oligosaccharides.
Fructooligosaccharide (known by its abbreviation FOS) is another oligosaccharide. It mainly contain fructose units and is prepared from inulin, a polymer of natural fructose found in chicory and Jerusalem artichokes), either by the process of chemical degradation or enzymatic degradation.
Polysaccharides are larger carbohydrates. Examples of polysaccharides are starch and cellulose. They are both glucose polymers, but starch contains alpha 1-4 linkages of glucose units and cellulose contains beta 1-4 linkages of glucose units.
Cereals, pulses, many unripe fruits and most vegetables are sources of starch.
Human beings cannot hydrolyse beta linkages of cellulose. That is the reason why starch can be digested by humans while cellulose cannot. But animals that ruminate, such as cows and goats, have microbes in their rumens that are capable of hydrolysing cellulose, which is why they can eat cellulosic materials such as grass and leaves.
Many polysaccharides used in the food industry are derived from beans, vegetables, trees, microbes, etc. Examples are gum Arabic (acacia), guar gum, gum karaya, locust bean gum, pectin, hemicellulose, xanthan and dextran. These are large polymers and cannot be digested by humans. They have applications in food products as thickeners, stabilisers and texture modifiers.
Pectin and others also form gels. Some like chicle gum and modified or synthetic gums are used in chewing gums and bubble gums. These are not digestible either. They become a part of dietary fibre along with cellulose and push food down the gastro-intestinal tract, thus preventing constipation and colon cancer.
Only monosaccharides (such as glucose, fructose and galactose) can be absorbed by the small intestine. Larger carbohydrates, including disaccharides, need to be digested or hydrolysed into monosaccharides to facilitate their absorption.
Food is consumed in the form of fruits and vegetables, which are either eaten raw or cooked; grain products such as bread and chapatti; dairy products; and others. Fruits and vegetables contain some starch and sugars, grain products contain starch, dairy products contain lactose and certain kinds of foods contain added sugar.
When these foods are eaten, they are first acted upon by the salivary amylase and then by the pancreatic amylase. These break down bigger starch molecules into maltose, maltotriose and some glucose.
Then the enzymes at the intestinal brush border act upon them, converting them into glucose. The enzymes in the small intestines digest disaccharides such as sucrose and lactose into the respective monosaccharides, which are then absorbed in the small intestine.
Cellulose and other non-digestible carbohydrates such as raffinose, stachyose, FOS, some varieties of gum polysaccharides and limit dextrins, left after the hydrolysis of amylopectin fraction of starch, may pass through the intestine undigested, but in the large intestine some partial breakdown may occur due to bacteria.
Of the monosaccharides that are absorbed, glucose is delivered to different tissues or cells, which used it for their energy needs.
Fructose and galactose are converted into glucose in the liver, but they can also be used for energy.
Glucose may be converted into other necessary carbohydrates, such as ribose, deoxyribose, glucosamine, etc. and may also help in the synthesis of non-essential amino acids.
The excess is converted into glycogen or fatty acids and stored in the body. The blood glucose level is affected when glucose is absorbed from the intestine, when glucose is consumed by various cells or tissues for the