Carbohydrates: A Comprehensive Guide - PDF

Carbohydrates Dr Linda Ibrahim Abuhwaileh  There are practical reasons for the universal use of carbohydrates in diets. The yield of cereals, the primary source of carbohydrates, is high per unit area. Therefore, they are widely available and are an economic source of energy. They are...

Carbohydrates Dr Linda Ibrahim Abuhwaileh  There are practical reasons for the universal use of carbohydrates in diets. The yield of cereals, the primary source of carbohydrates, is high per unit area. Therefore, they are widely available and are an economic source of energy. They are easily packed and have a long shelf-life in dry storage. They are mild flavoured and combine well with other foods. Carbohydrate foods are easy to prepare.  Nature, Composition and Classification Carbohydrates are synthesised by all green plants using solar energy, water from the soil and carbon dioxide from the air. This complex process is called photosynthesis, the prefix photo indicates the importance of sunlight in this process. Plants are thus the primary source of food in the world.  Carbohydrates contain carbon, hydrogen and oxygen. The suffix hydrate indicates that water and oxygen occur in the same proportion as in water. The members of the simplest class of carbohydrates have a single unit— monosaccharide (mono or one, saccharide or sugar containing). Glucose is an example of this class.  The disaccharides contain two sugars linked together to form a chain. Cane or beet sugar (sucrose), milk sugar (lactose), maltose (malt sugar) are members of this class.  Carbohydrates made up, of long chains of sugars are called polysaccharides (poly—many). Among them are starch, dextrins, glycogen, cellulose, hemicelluloses, pectins, plant gums and mucilage's. Table 4.1: Forms and Sources of Carbohydrates Carbohydrates Main Food Sources Remarks Polysaccharides Cellulose and hemicelluloses Stalks and leaves of vegetables Indigestible Outer covering of seeds Indigestible Pectins Fruits Indigestible Gums and mucilages Plant secretions & seed exudates Indigestible Starch and dextrins Grains, legumes & tubers Digestible Glycogen Meats and scafood Digestible Disaccharides Sucrose Cane and beet sugar, mollasses Digestible Lactose Milk and milk products Digestible Maltose Malt products, some breakfast cereals Digestible Monosaccharides Glucose Fruits, honey, corn syrup Digestible Fructose Fruits, honey Digestible Galactose Milk Digestible  Simple carbohydrates include mono- and disaccharides. These are small molecules, which dissolve in water and are absorbed very quickly in the body. In contrast, starches and dietary fibres are very large, complex molecules, containing several hundred small sugar units and are absorbed slowly. Therefore polysaccharides are referred to as complex carbohydrates. Individual members of each of these classes (starch, pectins, gums, mucilages, cellulose, hemicelluloses, lignin) differ in the type of small units they contain and in the way these units are united in the molecule.  Glucose, the most common monosaccharide, is present in honey, fruits and corn syrup. After absorption of food it is transported through blood and hence is present in blood as an easily available source of energy.  Sucrose is the ordinary sugar available in the grocery store and used in beverages and food preparations as a sweetener. As you can note from the table, it is mainly manufactured from cane sugar in the tropics and sugar beets in the temperate regions. It is also present in molasses, honey, fruits and vegetables. Sucrose is hydrolysed to a mixture of equal amounts of glucose and fructose.  Maltose does not occur in free form in nature. It is formed during sprouting of grains or in digestion of starch by action of enzymes. It contains two glucose molecules.  Lactose is the sugar in milk. It yields glucose and galactose on hydrolysis by lactase. It is less soluble than sucrose and maltose and less sweet than glucose. Persons, who have lactase insufficiency, cannot utilise lactose and hence have to restrict their intake of milk and milk products. Children, who are born without the liver enzyme lactase, cannot digest milk and have to be fed soya milk instead.  Starch is found in cereal grains, legumes and in tubers. Glycogen is found in meats and seafood. These are digestible polysaccharides. Cellulose and hemicelluloses, pectins, gums and mucilages are indigestible polysaccharides. Celluloses are found in stalks and leaves of vegetables and outer coverings of seeds. Pectins are present in fruits and gums and mucilages are part of plant exudates and seeds.  Properties Sugars are soluble in water, are sweet to taste and are hygroscopic (absorb water from the atmosphere). As they are hygroscopic, they need to be stored in air tight containers. Sugars vary in their solubility in water; sucrose is more soluble than glucose. Carbohydrates, can be ranked according to their sweetness. Fructose is the sweetest and the most soluble sugar. It is followed by sucrose, glucose, dextrin and lactose. The relative sweetness of sugars is indicated in Table 4.2. Table 4.2: Comparative Sweetness of Sugars Sugar Sweetness Value Fructose 173 Invert sugar 130 Sucrose 100 Glucose 74 Galactose 32 Maltose 32 Lactose 16  Starches are not sweet, but bland in flavour. Tender corn is sweet, as it matures it becomes less sweet as sugars are converted to starch. Raw fruits contain starch, which changes to sugars during the ripening process with development of sweet taste. Functions Carbohydrates have many important functions in the body: The primary function of carbohydrates in the body is to supply energy. Each gramme of carbohydrate, as starch or sugar, provides 4 kcal/g. Carbohydrates are a source of readily available energy, which is needed for physical activities as also the work of the body cells. The brain and the central nervous system are dependent on the constant supply of glucose from the blood to meet their energy needs.  Carbohydrates act also as reserve fuel supply in the form of glycogen, stored in muscles and liver. The total amount of glycogen in the body is over 300g. But it must be maintained by regular intake of carbohydrates at frequent intervals, so that the breakdown of fat and protein tissue is prevented. Carbohydrates serve other special functions in the body. Carbohydrates provide chemical framework, which combine with the nitrogen to synthesise non-essential amino acids in the body.  Carbohydrates and their derivatives work as precursors of important metabolic compounds. Lactose, the milk sugar, provides galactose needed for brain development. It aids absorption of calcium and phosphorus, thus helping bone growth and maintenance.  Lactose forms lactic acid in the intestinal track due to the action of the bacteria (lactobacilli) present there. These lactobacilli synthesise some of the B-complex vitamins. It aids, bacteria (lactobacilli) present to suppress the activities of putrefactive bacteria and protects us from their undesirable effects.  Carbohydrates are an important part of some compounds, which increase our resistance to infection (immunopolysaccharides). Ribose, a five carbon sugar, is an essential part of DNA and RNA. Carbohydrates are a part of important compounds, which are components of nervous tissue (galactolipid), heart valve, cartilage, bone and skin (chondroitin sulfate).  Carbohydrates are needed for ensuring complete normal metabolism of fats, thus preventing acidosis. Carbohydrates are needed to prevent dehydration. A low carbohydrate diet causes loss of water from tissues as also electrolytes (especially sodium and potassium) in the urine and can lead to involuntary dehydration. Dietary fibre acts like a sponge and absorbs water. It helps smooth movement of food waste through the digestive tract Utilisation in the Body Starch is partly hydrolysed by (salivary amylase) in the mouth to dextrose and maltose. Starch and dextrin are further hydrolysed to maltose by amylase in the small intestine. Maltose, sucrose and lactose are further broken down to glucose, fructose and galactose (simple sugar units) by the enzymes maltase, sucrase and lactase. Carbohydrates The digestion process begins with chewing the food in the mouth. The enzyme ptyalin (salivary amylase) starts the digestion of starch in the mouth. It hydrolyses starch to dextrins, isomaltose and maltose in neutral or alkaline pH in the mouth. The food tastes sweet due to these products of hydrolysis.  The activity of amylase continues in its movement from the mouth to the upper part of the stomach. But as soon as the food mass comes in contact with hydrochloric acid secreted there, this action ceases. Very little digestion of carbohydrate occurs in the stomach as the pH is unfavourable.  The food mixed with gastric juice forms a semi-fluid mass called chyme. It takes about three to five hours to form chyme. Small portion of chyme are released through the pyloric sphincter into the duodenum, the first part of the small intestine. The small intestine is so named because of the small diameter of its tube. It is about 20 ft long. Most of the digestive activity takes place in its three compartments namely the duodenum, the jejunum and ileum.  Carbohydrate digestion occurs almost completely in the small intestine, mainly in the duodenum. Pancreatic amylase breaks starches into maltose and dextrins. The maltase from mucosal cells breaks down maltose to glucose. The brush border, on the surface of the epithelial cells lining the intestines, is the site of this enzyme action.  The enzymes sucrase, lactase, maltase, and isomaltase, found on the outer cell membranes of the intestines, act on the sugars sucrose, lactose, maltose and isomaltose respectively. The monosaccharides formed—glucose, galactose and fructose—pass through the mucosal cell and via the capillary into the blood stream. These are carried to the liver by the portal vein.  Place of Products of Reaction Action Enzyme Optimum pH Substrate Mouth Salivary amylase 7.0 Cooked starch Dextrins, maltose (Ptyalin) Stomach Pepsin (protease) 2.0 Proteins Polypeptides Rennin 6.0–6.5 Milk, casein Calcium caseinate Lipase 7.0 Emulsified fats Fatty acids, glycerol Small Pancreatic Juice Intestine Trypsin (Protease) 8.0–9.0 Proteins Polypeptides, some amino- acids Lipase 7.0 Fats Di and monoglycerides, fatty acids, glycerol Amylase 7.1 Starch Maltose Intestinal Juice Peptidases 8.0 Peptones, Amino acids (Erepsin) Polypeptides Sucrase 5.0–7.0 Sucrose Glucose, fructose Maltose 6.7–7.2 Maltose Glucose (2 molecules) Lactase 5.4–6.0 Lactose Glucose, galactose  Some glucose is stored in the liver and muscles as glycogen, the rest is transported to tissues to be used for their activities. Fructose and galactose are converted to glucose in the liver. Cellulose, hemicellulose, lignin and other forms of carbohydrate, which are collectively known as fibre, are not split by human amylases. These are excreted in the faeces.  The glucose formed by the digestion of the starch and sugar is absorbed mainly into the blood through the walls of the intestine and carried to the liver. The glucose thus absorbed helps to maintain the glucose level in blood and the glycogen stores in the muscle and the liver. Whenever we need energy, the glycogen is broken down to glucose which is oxidised and the energy produced is used by the body. Any excess glycogen is converted to fat.  In normal persons, the blood sugar level is maintained at a constant level, which is 70 to 100 mg per 100 ml, under fasting conditions. A number of hormones regulate the reactions, and ensure the maintenance of normal blood sugar level. These hormones are insulin, glucagon, epinephrine, glucocorticoids, thyroxine and growth hormones.  Fibre reduces transit time and binds some minerals such as calcium, iron, zinc, etc. Soluble fibre binds bile acids and cholesterol and helps carry these out of the body. There is no recommended dietary allowance for fibre. However, nutrition researchers and dieticians suggest that fibre intake be increased to 25 grammes or more per day.  Polyols, also known as sugar alcohols, are a type of carbohydrate that have a chemical structure similar to both sugars and alcohols. They are used as low-calorie sweeteners and are found naturally in some fruits and vegetables, but they can also be manufactured for use in food products.  Characteristics Sweetness: Polyols are less sweet than sucrose (table sugar), with sweetness levels ranging from about 25% to 100% that of sucrose. Calories: They provide fewer calories than regular sugar, generally about 1.5 to 3 calories per gram compared to 4 calories per gram for sugar. Dental Health: Unlike sugar, polyols do not contribute to tooth decay and are often used in sugar-free gum and dental products. Blood Sugar Levels: Polyols have a lower glycemic index compared to regular sugar, making them a better option for people with diabetes as they have less impact on blood glucose levels. Uses Food Industry: Commonly used in sugar-free and low-calorie food products such as candies, chewing gum, ice cream, baked goods, and diabetic-friendly foods. Pharmaceuticals: Used in some medications and syrups to provide sweetness without increasing the risk of cavities.  Health Considerations Digestive Issues: Consuming large amounts of polyols can lead to digestive issues such as bloating, gas, and diarrhea, particularly in individuals with irritable bowel syndrome (IBS) or those who are sensitive to FODMAPs (fermentable oligo-, di-, mono-saccharides, and polyols). Moderation: It is generally recommended to consume polyols in moderation to avoid gastrointestinal discomfort Recommended Dietary Allowance A minimum of 100g carbohydrates are needed in the diet to ensure the efficient oxidation of fats. Most diets supply more than this amount. If the carbohydrate foods are consumed in excess of the body’s need, the excess is converted into fat and is stored as reserve. No daily allowance has been fixed for carbohydrates. If the proteins supply about 10 per cent of the calories, fat 20 per cent, then carbohydrates must supply the remaining 70 per cent calories. Clinical Problems: Obesity It is very easy to take sweets, candy, soft drinks, etc., in excess of one’s needs. Most of these sweets contain a lot of fat also, which is a concentrated source of calories. When the energy intake exceeds expenditure, the excess is deposited as fat. Over a period of time, overweight and obesity can occur. Obesity is known to be a predisposing factor for a number of health problems.  Study Questions Why are cereals used as staple food in India? Why do we need to consume sufficient amounts of carbohydrates in our diet? List the properties of sugars. What are the functions of carbohydrates in our body? Can we say that the sources of carbohydrates are the glycogen found in meats? Explain your answer What is dietary fibre? Why is fiber important? What is the difference between prebiotics and probiotics, and why are they important? Write down the disadvantages of consuming polyols.