Role of carbohydrates in human nutrition


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The term carbohydrate covers a large number of diferent compounds, all composed of carbon, hydrogen, and oxygen atoms.

Role of carbohydrates in human nutrition

  1. 1. Role of Carbohydrates in Human Nutrition Uploaded by Md. Atai Rabby 1
  2. 2. Different forms of carbohydrates in food • The term carbohydrate covers a large number of diferent compounds, all composed of carbon, hydrogen, and oxygen atoms. • Classification of Carbohydrates: • Simple Carbohydrates: Monosaccharides and Disaccharides – Monosaccharides: Hexose (glucose, fructose, and galactose) – Disaccharides: Sucrose, Lactose, Maltose, Mannose • Complex Carbohydrates: Polysaccharides – Starch – Glycogen – Dextrin – Cellulose Polysaccharides are further designated as digestible (starches) or indigestible (fibers). 2
  3. 3. Carbohydrate as a Dietary Essential • You need some carbohydrate daily as a source of glucose. How much carbohydrate is enough? • Requirements for Carbohydrate • Intakes of less than 50 g of carbohydrate/day are associated with adverse health effects. • These symptoms are similar to those that develop in starvation. • There is an unexplained loss of large amounts of both sodium and water, which explains the rapid loss of body weight found in people eating a carbohydrate-free diet. The loss of sodium is followed by a loss of potassium, which usually leads to weakness. • At the same time, protein of the body (muscles) begins to break down, which leads to muscle wastage, further weakness, and further weight loss. 3
  4. 4. Carbohydrate as a Dietary Essential….. • Even more seriously, when carbohydrate is absent, the use of fat to provide energy becomes hindered in the final series of chemical steps normally involved in oxidizing the fat. • This leads to the accumulation of chemical intermediates of fat oxidation, known as ketones. • This state, known as ketosis, is associated with mild disturbances of cellular function and is an early indication of insufficient carbohydrate availability in the body. • As the ketones build up, they begin to be found as abnormal components of blood and urine. Also, because they change the hydrogen ion balance (or acid-base balance) of tissues, the ketones begin to interfere with the normal functioning of various body processes. 4
  5. 5. Carbohydrate as a Dietary Essential….. • People reaching this stage are said to be suffering from ketosis, which usually produces symptoms of fatigue and dehydration and a loss of stamina. • All these undesirable effects of a carbohydrate-free diet are soon corrected by the return of carbohydrate to the diet – a clear indication that carbohydrate really is a dietary essential. • Carbohydrates are also used in the synthesis of various metabolically active complexes. Glycoproteins are important components of cellular membranes. Glycolipids, such as sphingolipids and gangliosides, have roles at receptor sites on cells and in synaptic transmission. • Mucopolysaccharides have important water-holding or binding properties in many sites of the body; they occur in basement membranes and in intercellular cement and form an integral part of cartilage, tendon, skin and synovial fluid. 5
  6. 6. Recommended Carbohydrates Intake • Intake of available carbohydrate (starch and sugar) that might be ideal to support long-term health: 55 to 60 percent of total Kcal. Further, added sugar be used in moderation (< 10% of Kcal), more should come from complex carbohydrates and naturally occurring sugars in foods. • A person consuming 2000 kcal diet a day should therefore have 1100 to 1200 kcal energy from carbohydrate, or about 275 to 300 grams. • Most of this would be starch, with its accompanying fiber, some would be the naturally occurring sugars of fruits, vegetables, and milk. • Estimates of the minimum amount of carbohydrate needed by an adult are in the range of 150-180 g carbohydrate per day. 6
  7. 7. Trends in Carbohydrates Use • Because most diets contain at least 200 g of carbohydrate/day, a lack of carbohydrate is unlikely to reach the level where it causes serious problems. • Also, because carbohydrate-rich foods are the cheapest source of energy they tend to be used in amounts that ensure al least a minimum intake, especially among those with low incomes. • There is a big difference, however, between eating at least the minimal amount of carbohydrate and eating the optimal amount. • Fiber is not counted in computing energy from carbohydrate; we do not know enough yet about its energy contributions, but they are probably not large in comparison with those from starch and sugar. 7
  8. 8. Fiber • Fiber is an indigestible carbohydrate. It is taken in as part of many foods, but most of it simply passes through the digestive tract because we do not possess the enzymes needed to break it down into a form that can be absorbed into the blood. • However, some bacteria in the large intestine can degrade some components of fiber, releasing products that can be absorbed into the body and used as an energy source. • Two categories of fiber are found in food: – crude fiber and – dietary fiber. • The major component of crude fiber is the polysaccharide called cellulose. 8
  9. 9. Fiber… • Several other carbohydrates and related compounds called pectins, hemilluloses, and lignins are found in plant foods and are also resistant to digestion in the human digestive tract. These, together with cellulose, are known collectively as dietary fiber. • This is a general term that describes any material in food that remains undigested in the intestine. The cellulose that is called crude fiber is just a part of dietary fiber as whole. • Dietary fiber can also be classified into soluble and insoluble fiber, depending on how it reacts with water. • Fiber Intake: With all the uncertainties, it is probably true to say that about 20 – 30 g of dietary fiber daily is a desirable intake. 9
  10. 10. Soluble Fiber • Soluble fiber is found in fruits, some legumes, and such grains as oats, rye, and barely. It dissolves in water to form a gel. This gel forms within the digestive tract and serves to slow the rate at which food passes through the small intestine. This in turn increases the rate of absorption of nutrients from the food. • Which is why home remedies for diarrhea often included eating fruits rich in the soluble fiber pectin, such as apples and pears. • Pharmaceutical manufacturers also add pectin to antidiaahea medications. • Anyone who has made marmalade, jam, or jelly has probably used the soluble fiber pectin as a thickening agent. 10
  11. 11. Insoluble Fiber • Insoluble fiber: composed of cellulose, hemicellulose, and lignins – is found in vegetables and wheat bran. It tends to absorb water and increase in bulk, greatly contributing to the volume of stools or feces. The associated water softens the stools. • This causes insoluble fiber to increase gastrointestinal motility, or in other words speed the movement of material through in GI tract, especially the colon. Bacteria will not digest insoluble fiber. • In other words, insoluble fiber (like cellulose) is valued for its ability to prevent or relieve constipation. 11
  12. 12. Health Benefit of Fiber • Soluble fiber is known to have cholesterol lowering effects because of its promotion of increased fecal excretion of bile acids, slower cholesterol and lipid absorption, and increased production of short-chain fatty acids by fermentation in the large intestine. • Insoluble fiber softens the stool and increases bulk, and thus effectively prevents constipation, which appears to play a role in the development of colon cancer and diverticulitis. • Theoretically, the longer potentially carcinogenic substances contained in feces remain in the colon, the more opportunity they have to damage the intestinal lining. 12
  13. 13. Health Benefit of Fiber…. • How do dietary fibers help to reduce the incidence of colon cancer and diverticulosis? • The link between a lack of dietary fiber and the onset of colon cancer has been attributed to various effects of fiber – namely: – Changes in the populations of microorganisms in the GI tract – The decreased binding of intestinal bile acids – The increased time for which food residue remains in the colon – Decreased stool weight and volume and – Decreased frequency of defecation • Scientists postulate that the microorganisms encouraged by a diet low in dietary fiber enhance the formation of cancer-causing substances (carcinogens). These microorganisms may also prevent or at least limit the breakdown of carcinogens that are normally destroyed by the microorganisms that thrive when there is more dietary fiber in the diet. 13
  14. 14. Health Benefit of Fiber…. • The main alternative theory suggests that the beneficial effect of dietary fiber results from its speeding of the passage of feces through the large intestine. • The logic behind this idea is that faster passage means less time for any carcinogens present to be in contact with the intestinal wall. Also, the bulk and water of the feces may dilute the carcinogens to a nontoxic level. • Some fibers bind bile and carry it out of the body; this is also thought to reduce cancer risk. • Diverticulosis is a condition in which the muscles of the colon weaken, forming small pouches that can become painfully inflamed (diverticulitis) if food residue gets trapped within them. It often occurs in people who are chronically constipated. 14
  15. 15. Health Benefit of Fiber…. • Diverticulosis is associated with a weakening of the intestinal wall caused by the pressure from hard stools. The weakened intestinal wall then develops small outpouchings in which fecal material becomes trapped. Diverticulitis is an inflammation of the intestinal wall that develops when these outpouchings become irritated or infected. • Different Fibers, Different Effects • Not all the fibers have similar effects. For example, wheat bran, which is composed mostly of cellulose, has no cholesterol lowering effect, whereas oat bran and the fiber od apple (pectin) do lower blood cholesterol. • Fibers that form gels in water (pectin and guar) prolong the time of transit of materials through the intestine, whereas insoluble fibers (cellulose) tend to reduce the time. 15
  16. 16. Hazards of Too Much Fiber • There is now concern that some people may be consuming too much fiber, which can lead to a condition known as irritable bowel syndrome. • It is also possible that too much fiber may cause decreased absorption of some mineral elements (magnesium, calcium, zinc, and iron), either by binding to them or by simply speeding the passage of food through the intestinal tract and reducing the opportunity for the mineral elements to be absorbed. • At present it is thought that 20 to 30 g of dietary fiber per day maximizes the benefits of fiber while minimizing the possible adverse effects of either excessive or insufficient fiber consumption. 16
  17. 17. Glycemic Effect of Foods • The glycemic effect of a food is the effect that food has on a person’s blood glucose and insulin response – how fast and how high the blood glucose rises, and how quickly the body responds by bringing it back to normal. • The concept of glycemic index is a measure of the ability of a carbohydrate to contribute to the concentration of blood glucose. • High glycemic index means that the dietary carbohydrate elevates blood glucose faster and to a higher level than a carbohydrate of lower glycemic index. • Diets with a low glycemic index have been shown to have various health benefits, including reduction of blood lipids and improved blood glucose control in diabetic subjects. 17
  18. 18. Classification of Starch according to Digestibility • Rapidly digestible starch: digested within 20 minutes • Slowly digestible starch: digested between 20 and 120 minutes • Resistant starch: not digested • Rapidly digestible starch (RDS) is readily available to digestive enzymes and digested rapidly and completely in the small intestine. Example: freshly cooked starchy food. • Slowly digestible starch (SDS) digested completely in the small intestine, but slowly. They have a specific internal structure that delays digestion and conversion into glucose. Example: mostly raw cereals. • Resistant starch is the name given to the components of dietary starch that are resistant to the normal enzymatic digestion process in the small intestine. 18
  19. 19. Starch according to Digestibility….. • There are three type of resistant starch: – Physically inaccessible (RS1) – Resistant granules (RS2) – Retrograded (RS3) • They originated in three possible ways: – The physical structure of the food may prevent access to digestive enzymes if the starch is surrounded by fat. – The nature of the cell walls around the starch granules may impede digestion. – Where the starch has become retrograded by heating and cooling, the enzymes are no longer able to break the bonds. • It must be remembered, however, that digestibility is variable and probably dependent on the composition of the meal. 19
  20. 20. Resistant Starch • Resistance starches that escape digestion in the small intestine become available for fermentation in the colon by the bacterial flora. • The result of this process is an increase in fecal mass owing to the multiplication of the bacteria, production of short-chain fatty aids (acetic, propionic and butyric acids) and a decrease in colonic pH. • In addition, CO2, H2 and some CH4 are produced. These contribute to a sensation of bloating and flatulence. • It has been estimated that between 20 and 30% of the potential energy contained in the resistant starch becomes available to the body in the form of short-chain fatty acids absorbed from the colon. 20
  21. 21. Which class of starch is nutritionally the most desirable form and why? • SDS is the most desirable form of starch, as it digested slowly thus keep the body’s blood glucose levels relatively steady throughout the day. • The delayed release of glucose into the blood stream from foods containing SDS helps to prevent the rapid rise in blood glucose levels that can occur after a meal high in simple sugars or rapidly digested carbohydrate. • A slower rise in blood glucose level support the more efficient movement of glucose into cell and may prevent elevated blood insulin levels, which can contribute to the development of diabetes. • SDS has been recognized to provide control energy release, more prolonged glucose release and helps in the management of diabetes. 21
  22. 22. Non-Starch Polysaccharides • Constituents of NSP – Cellulose – Non-cellulosic polysaccharides: Hemicelluloses, pectins, beta-glucans, gums and mucilages • In the mouth, high-fiber foods generally require more chewing. This slows down the process of eating and stimulates an increased flow of saliva. The saliva contributes to the volume of the swallowed food bolus. • Once in the stomach, the fiber-rich food tends to absorb water and the soluble component starts to become viscous. Both of these changes delay stomach emptying. In the small intestine, the soluble fiber travels slowly because of increased viscosity; this prolongs the period of time available for the absorption of nutrients. 22
  23. 23. Non-starch Polysaccharides…. • The fiber may also bind some divalent ions in the small intestine, making them unavailable for absorption at this point. • Once in the large intestine, the soluble fiber becomes a food source for the growth and multiplication of the bacteria flora. The consequences of this are exactly the same as described above for resistant starch. • Thus, both resistant starch and soluble NSPs contribute to increasing bulk in the large intestine, and the production of fatty acids and gases. • Insoluble fiber, which has reached the colon largely unchanged, swells by water holding, and adds further to the volume of the colonic contents. The feces, therefore, are both bulkier and softer because of the increased water content. 23
  24. 24. What happens if I do not have enough blood glucose? • Because both too little and too much glucose are harmful, blood levels are maintained within a narrow range through the opposing action of the hormones insulin and glucagon. • When blood glucose levels are high, insulin helps the cells absorb glucose, thereby lowering the blood glucose level. • When blood glucose levels fall beneath the acceptable limit (hypoglycemia), the pancreas releases glucagon, which stimulates degradation of liver glycogen to glucose. • If the fuel crisis persists and the glycogen supply has been depleted, the body will begin to form glucose from protein and glycerol by gluconeogenesis. 24
  25. 25. Hypoglycemia • If the blood glucose level falls much too low, to about 60 mg/dl, a condition called hypoglycemia (too little glucose in the blood) results. • Hypoglycemia is not a disease, but a symptom by which various diseases can be diagnosed (just as hyperglycemia is a symptom of diabetes). • In extreme cases both hypoglycemia and hyperglycemia can cause people to fall into a coma and die. • Reactive Hypoglycemia: A mild form of hypoglycemia, sometimes called reactive hypoglycemia, occurs from time to time in almost everyone. Sometimes this is simple because a person has waited too long between meals. People who get up and get going without eating breakfast are running on an empty fuel tank. Not surprisingly, they report a high incidence of reactive hypoglycemia, as a result of insufficient food. 25
  26. 26. Spontaneous Hypoglycemia • In rare instances people develop a serious condition known as spontaneous or fasting hypoglycemia, characterized by chronically low blood glucose levels. This condition results from an underlying disease state, such as stimulation of excessive insulin production by a pancreatic tumor. No matter how much people with spontaneous hypoglycemia eat, their blood sugar remains too low. • If they go for a long period without food, their blood sugar may drop so low that their energy-starved brain loses consciousness. • Clearly this is a life-threatening condition that requires immediate medical treatment. 26
  27. 27. What happens if I have high blood glucose? • Hyperglycemia: If the blood glucose level rises too high, above 200 mg/dl, a condition called hyperglycemia (too much glucose in the blood) results. • In people suffering from diabetes mellitus, hyperglycemia can occur. • How can diabetes be controlled through diet? • The key to treating diabetes is to keep blood glucose at a constant and moderate level using a carefully planned diet and, if necessary, medication. • A diet rich in complex carbohydrates and containing non-starch polysaccharides is recommended in the management of diabetes. 27
  28. 28. How can diabetes be controlled through diet? • Using a menu planning guide known as the Diabetic Exchange Lists, dietitians help clients with diabetes develop a meal plan that specifies how much of which types of food (exchanges) can be eaten at each meal and snack. • A diet rich in complex carbohydrates but with small amounts of sugar is the key to controlling blood glucose levels. • Diabetic Exchange Lists separate individual foods into six lists: milk, vegetable, fruit, bread, meat, and fat. The milk, starch, and meat lists are further divided to identify high-, medium-, and low-fat food choices. The reason for dividing foods into these lists is that food items vary with respect to their carbohydrate, protein, fat, and kcalorie content. 28
  29. 29. How can diabetes be controlled through diet? • The foods in each Exchange List have roughly the same amounts of nutrients and kcalories, which makes it easy to exchange one food for another within a given list. • For example, one slice of bread and ½ cup of cooked rice are considered to be nutritionally equivalent (contains 15 g carbohydrate, 3 g protein, trace fat and 80 kcal). • Because each Exchange List contains a wide variety of foods, people with diabetes have considerable leeway in developing their own menus. 29
  30. 30. Blood glucose • A blood glucose level of about 90 mg/dl is referred to as the fasting glucose level because it is the level that is maintained during fasting. • Glucose is the main source of energy for the central nervous system (CNS), which used about 140 g (9 Tbsp) of glucose per day. Red blood cells need about 40 g (3 Tbsp) of glucose per day. • Carbohydrate Effect on Brain Function • Prolonged hypoglycemia in brain results in histologically demonstrable and functionally irreversible damage to brain tissue. • Recent research has indicated that the kind and amount of carbohydrate in the diet can influence brain function and behavior. 30
  31. 31. Carbohydrate Effect on Brain Function • When carbohydrate intake is high, comprising 70% to 80% of total kcal, the brain produces more serotonin. • When produced in excess, serotonin causes a carving for carbohydrate and therefore increased consumption of carbohydrate, which in turn stimulates the production of yet more excess serotonin. • This make a person sleepy and sluggish. 31
  32. 32. Some Aspects of the Role of Carbohydrate • Protein Sparing Action of Carbohydrates • The Need for Carbohydrates for the Oxidation of Fats • Carbohydrates as a Source of Energy for Muscular Work • Special Functions of Carbohydrates in Liver • Synthesis of Ribose from Glucose • Carbohydrate and Dental Health 32
  33. 33. Protein Sparing Action of Carbohydrates • The body uses mainly carbohydrates as a source of energy when they are adequately supplied in the diet, thus sparing protein for tissue building, since meeting the energy needs of the body takes priority over other functions like growth. • If the diet does not supply adequate calories, the dietary protein is oxidized as a source of energy. There is also breakdown of tissue proteins to a greater extent. • This function of carbohydrates serving as a source of energy and preventing dietary protein from being oxidized and preventing excess tissue protein breakdown in calorie deficiency in called “protein sparing action of carbohydrates”. • For example, the daily urinary N excretion of 135 mg in an adult rat during fasting is reduced to 93 mg when 12 g of glucose is fed to the rat. Glucose has spared about 33% of the body protein broken down in the fasted rat. 33
  34. 34. The Need for Carbohydrates for the Oxidation of Fats • In the oxidation of fats, the acetyl CoA formed from the oxidation of fatty acids reacts with oxaloacetic acid (formed from carbohydrate breakdown product – pyruvic acid or formed from the aspartic acid) to form citric acid which is oxidized through the TCA cycle back to oxaloacetic acid through a series of reactions. • If adequate amounts of oxaloacetic acid are not available, acetyl CoA is formed in large amounts from the oxidation of fatty acids and a part of it is converted into ketone bodies (acetoacetic acid and β-hydroxy butyric acid) which accumulate in blood and tissues and produce ketosis. 34
  35. 35. Carbohydrates as a Source of Energy for Muscular Work • Carbohydrates are the major source of energy for muscular work. During muscular contraction glycogen is broken down to lactic acid. The process is known as glycolysis. • During the recovery period lactic acid is first oxidized to pyruvic acid and then to acetyl CoA, which in turn is oxidized through TCA cycle to CO2 and H2O thus producing energy for muscular work. • As a source of energy, carbohydrates are more important during moderate or severe muscular exerction as in athletics. 35
  36. 36. Special Functions of Carbohydrates in Liver • These include detoxifying action and a regulating influence on protein and fat metabolism. The liver of well-fed animals is rich in glycogen. Such a liver is more resistant to certain poisons such as carbon-tetrachloride, alcohol or arsenic and toxins of bacteria than the livers of under-fed animals with low glycogen reserves. • Synthesis of Ribose from Glucose • The pentose-ribose is present in RNA and many nucleotides. It is formed in the body from glucose by the pentose pathway and is incorporated in RNA and nucleotides. 36
  37. 37. Carbohydrate and Dental Caries • Dietary carbohydrate is implicated as a major cause of dental caries, or tooth decay. • High intake of dietary sugars  Multiplication of oral bacteria  Production of acid  Low oral pH  Demineralization of teeth  Dental Caries. • Carbohydrate as Glycoproteins • Carbohydrates are found in the form of polysaccharide chains attached to proteins such as glycoproteins. As such they are important constituents of tissue antigens (in the cell membrane) and secreted proteins. 37
  38. 38. Individual Sugars • Glucose: Glucose (also known as dextrose, blood sugar, and grape sugar) found naturally in fruits, vegetables, and honey. These foods, however, provide us with an average of only 18 g of glucose per day, which is only a small fraction of the glucose used daily in the body. • The rest is provided by the breakdown of more complex carbohydrates, by the conversion of other monosaccharides, or by the metabolism of certain amino acid. • Sorbitol: a derivative of glucose that is formed when glucose is chemically “reduced” by the addition of one hydrogen atom. • Sorbitol occurs naturally in such fruits as apples, pears, and peaches and also in several vegetables. 38
  39. 39. Sorbitol • It is not actively absorbed from the gastrointestinal tract and is absorbed at about one third the rate of glucose absorption. • This means that eating food rich in sorbitol allows blood glucose levels to remain above the fasting level for a longer time than does eating food correspondingly rich in glucose. Thus eating sorbitol may delay the onset of hunger. • For this reason, sorbitol is an ingredient in some foods designed for use in weight-reducing diets and has been used clinically as a non-insulin-stimulating carbohydrate. • Unfortunately, sorbitol also causes flatulence and diarrhea in some people, which considerably reduces its acceptability. • Sorbitol is also widely used as a sweetener in chewing gums because it is less likely to promote the formation of dental cavities than is sucrose. 39
  40. 40. Fructose • Fructose (also known as levulose or fruit sugar) occurs in many different fruits. It also makes up one tird of the sugar in honey. • Insulin is not needed to allow fructose to be transported into cells; so fructose does not cause problems of high blood sugar levels in persons with diabetes. This has led to the idea that diets high in fructose might be a suitable way for those with diabetes to get their supplies of carbohydrate. • Fructose is less likely to be cariogenic (tending to cause tooth decay) than other sugar-based sweeteners. Tooth decay, technically known as caries, is produced when bacteria in the mouth act on carbohydrate to produce acid. • The level of acid production depends on what carbohydrates are available in the mouth, with fructose resulting in a relatively low level. 40
  41. 41. Galactose • Galactose, the third main monosaccharides of nutritional relevance, occurs only as a result of the breakdown during digestion of the disaccharide lactose. • Studies using laboratory animals have linked excessive galactose consumption with the development of cataracts. • The levels used, however, indicated that this was unlikely to present any problem to humans apart from the few people with a genetic defect of metabolism known as galactosemia, which leads to excessive amounts of galactose in the blood. • In galactosemia, the concentration of galactose-1-phosphate uridyl transferase is very low. Carbohydrate intake must be controlled when such enzyme deficiencies occur. • If the intake of galactose is not restricted, mental retardation results. 41
  42. 42. Sucrose • Sucrose is the most common disaccharide. The familiar white crystals or powder of sucrose is simply known as “sugar”, which is 100% sucrose. • Brown sugar, with its more distinctive flavor, is 97% sucrose and is made by partially refining sucrose or by adding some molasses to white refined sugar. 42
  43. 43. Lactose • Lactase convert lactose into glucose and galactose. • Milk, our only source of lactose, is also our most dependable source of calcium. • In most mammals, intestinal lactase activity is high at birth, declines to low levels during childhood, and remains low in adulthood. • Lactase deficiency may also arise as a secondary condition, resulting from damage to the intestinal mucosa by some other disease process, such as malnutrition, HIV infection and parasitic infestations. • The low levels of lactase in the intestine (lactase insufficiency) are associated with intolerance to milk and other milk products containing lactose (lactose intolerance). • People with lactose intolerance experience a lot of discomfort if they eat large amounts of lactose-containing dairy products. 43
  44. 44. Lactose • They can eat fermented dairy products, such as cheese and yogurt, prepared by fermentation because the fermentative microorganisms convert the original lactose into lactic acid. • Most infants, including those who become lactase insufficient in later life, do have the appropriate levels of enzymes to thrive solely on milk without any problems. The intolerance to lactose in people with lactase insufficiency develops later, in early childhood. • The availability of the enzyme lactase, purified from yeasts and molds, has made it possible for lactose-intolerant people to drink milk or milk products without concern. • The enzyme, which is available in tablet or powder form, is taken before or with lactose-containing foods or is mixed with the food before it is consumed. 44
  45. 45. Digestion and Absorption of Carbohydrate • Before carbohydrate can fulfill its role in the body, it must be converted into monosaccharide units that can be absorbed by the intestinal mucosal cells and transported to the blood. • The required changes are brought about by the action of certain enzymes found in the mouth and the intestine and by the acidity of the stomach contents. • In the mouth, salivary α-amylase, or ptyalin, which is present in saliva, partially breaks down starch. It catalyzes the hydrolysis of most α(1-4) linkages but not the final or “terminal” α(1-4) linkages of starch chains, the α(1-4) linkages next to branching points, or the α(1-6) linkages responsible for branching. 45
  46. 46. Digestion and Absorption of Carbohydrate…. • The eventual end products of α-amylase digestion (salivary and pancreatic) are maltose, maltotriose, and α-limited dextrins (branched polymers containing an average of eight glucose units). • In stomach, the food particles mixed with gastric secretions (contain no specific enzyme for the breakdown of carbohydrates), but the HCl in stomach stops the action of salivary amylase. The action of α-amylase is inhibited because the optimal pH value of the enzyme is 6.7. • Before the food mixed compltely with the gastric acid secretion, as much as 20% to 30% of the starch may have changed to maltose. • In the small intestine the conditions change from acidic to alkaline, and another potent α-amylase released from the pancreas acts to continue the digestion of starch and dextrins. 46
  47. 47. Digestion and Absorption of Carbohydrate…. • Finally enzymes of small intestine containing α-limited dextrinase, glucoamylase, maltase, lactase, and sucrase, which are able to split α-limited dextrins, maltotriose, maltose, lactose, and sucrose into their respective monosaccharide units. • The monosaccharides glucose, galactose, and fructose produced by this process of digestion are rapidly absorbed across the intestinal wall to the blood in the capillaries that drain into the portal vein. The portal vein carries the monosccharides and other absorbed nutrients to the liver. • When absorbed, monosccharides reach the liver, and fructose and galactose are converted into glucose, which is then carried in the blood to the rest of the body. 47
  48. 48. Summary of Carbohydrate digestion, absorption, and circulation • Source Enzyme Products • Mouth Salivary α-amylase Hydrolyzes α(1-4) linkages, • (Salivary glands) (alkaline pH) producing α-limited dextrins, maltotriose and maltose • Stomach: Hydrochloric acid (acid pH) • Small Intestine Pancreatic α-amylase Hydrolyzes α(1-4) linkages, • (Pancreas) (alkaline pH) producing α-limited dextrins, maltotriose and maltose • Intestinal Wall Maltase, Sucrase,Hydrolyzes maltose & • (intestinal mucosa) Lactase maltotriose to glucose • α-limited dextrinase Sucrose to glucose & fructose • Absorption (alkaline pH) Lactose to galactose & glucose • Goes to blood Starch to glucose • Hepatic circulation to the liver  Glucose  Systemic circulation 48
  49. 49. Utilization of Absorbed Carbohydrates in the Body • The utilization of carbohydrates in the body can be discussed under two heads: – Formation and breakdown of glycogen – Oxidation of glucose 49
  50. 50. How does Processing Affect Carbohydrates? • Processing tends to reduce the nutrient density of carbohydrate- rich foods. • Sugar is extracted from sugarcane and sugar beets through a process that purifies the sucrose molecule. This is accomplished by removing all of the starch, fiber, minerals, and vitamins. Not surprisingly, the resulting refined sugar is considered ‘empty calories’. • Like sugar and starch, whole grains are refined to make white flour. This process, which removes the fiber-rich bran and the vitamin-rich germ, leaves mainly the powdery, white starch of the endosperm. • Law in USA requires the manufacturers of any flour or flour product to add back three of the B-complex vitamins (thiamin, riboflavin and niacin) plus the minerals iron at the same levels in which they were present before refining. Not 50
  51. 51. How does Processing Affect Carbohydrates? • Replaced, however, are fiber and many other vitamins and minerals. The result is a major source of kcalories that is much lower in nutrient density than the whole grain fro9m which it was derived. • If animals store glucose in their tissues as glycogen, why are not meats a good source of dietary carbohydrate? • First of all, the amount of glycogen that animals, or for that matter humans, can store is relatively small. The entire body contains only a couple of hours’ worth of energy in the form of glycogen. • Second, any glycogen that was present in an animal’s tissues is depleted when the animal is fasted or frightened before slaughter. 51
  52. 52. Changes in Carbohydrate during Cooking • The effects of heating on carbohydrates are predominantly beneficial in that it break down the cell walls, solubilizes starch and makes it more easily digested. • The heat during cooking cause the starch within the cells to swell. This bursts the cell walls, and the starch and other nutrients within become accessible to digestive enzymes. 52
  53. 53. Book References • 1. Human Nutrition by Helen A. Guthrie and Mary F. Picciano, McGraw Hill Publisher, 1995. • 2. Understanding Nutrition by Eleanor N. Whitney and Eva M. N. Hamilton, West Publishing Company, Latest edition. 53