Carbohydrates PDF

Chapter 4-Carbohydrates This document details the role of carbohydrates in human health. It is based on chapter 4 of the textbook. Where applicable the section number e.g. 4.1, figure number, or pg number is included on a slide, so you can easily find the portion of the textbook from which the infor...

Chapter 4-Carbohydrates This document details the role of carbohydrates in human health. It is based on chapter 4 of the textbook. Where applicable the section number e.g. 4.1, figure number, or pg number is included on a slide, so you can easily find the portion of the textbook from which the information on the slides is found. The document is divided into 6 sections: 1) Chemistry of Carbohydrates 2) Carbohydrates in Foods 3) Digestion, Absorption, and Metabolism 4) Blood Glucose Regulation and Diabetes 5) Carbohydrates & Health 6) Non-nutritive Sweeteners 1  (1) Chemistry of Carbohydrates This first section describes the chemistry of carbohydrates. After this section, be sure that you can: 1. describe which monosaccharides make up sucrose, lactose, and maltose 2. distinguish between hydrolysis and condensation 3. explain the difference between digestible and indigestible polysaccharides 4. describe the two categories that make up total fibre 5. distinguish between soluble and insoluble fibre 6. explain the difference between glycogen, amylopectin, and amylose 3  4.2 Photosynthesis: Plants biosynthesize glucose using the energy of the sun Energy from sunlight used to convert carbon dioxide and water to the monosaccharide glucose Glucose can then be converted to starch which is a polymer of glucose (polysaccharide) or other monosaccharide, disaccharides and polysaccharides Fig. 4.6 4 Chemistry of Carbohydrates: There are many monosaccharides. Here are three common one monosaccharides most relevant to our discussions. Derived from lactose. Animal source carbohydrate: milk Fig 4.5 5  Disaccharides are 2 monosaccharide units covalently bonded 2 glucose units 1 galactose and 1 glucose unit The most common disaccharides are shown here. Fig 4.7 1 glucose and 1 fructose unit 6 Hydrolysis breaks down the linkage or covalent bond between sugars, with the addition of water. Condensation forms a linkage or covalent bond between sugars, with the release of water. 7 Fig 4.8 4.2 Oligosaccharides Contain 3 to 10 monosaccharide units Commonly found in beans Fermentation of these oligosaccharides in the colon cause the flatulence often experienced with the consumption of beans 8 Section 4.2 4.2 Polysaccharides Contains more than 10 monosaccharide units Polysaccharides fall into 2 main categories Digestible Indigestible Starches: Dietary fibre Amylose Resistant starch Amylopectin (Cannot be broken (Are broken down by down by digestive digestive enzymes and absorbed) enzymes) 9  4.2 Total Fibre falls into 2 Dietary Fibre categories Functional Fibre Fibre isolated from Fibre in intact foods plant sources Both dietary fibre and functional fibre can be a mixture of: Insoluble fibre, which does not dissolve in water and Soluble fibre, which absorbs water and can form viscous solutions 10 Examples of Cellulose is one example of dietary Polysaccharides of fibre. Other forms of glucose: dietary fibre are composed of other monosaccharides beside glucose. Glycogen: form of Amylopectin is a branched Fig 4.9 polysaccharide stored polysaccharide; in the liver. Amylose is a linear polysaccharide 11 2) Carbohydrates in Foods Now that we have briefly reviewed the chemical composition of carbohydrates, this next section looks at carbohydrates in foods. After this section be sure that you can: 1. distinguish between unrefined and refined carbohydrates 2. Define “free sugars” 3. Explain the importance of the comparison made between a kiwi and a soft drink 4. Describe the 3 major components of a kernel of whole grain 5. Explain how 100% whole grain bread differs from 100% whole wheat bread 12 Carbohydrates are also categorized as 4.1 Carbohydrates in Whole Foods unrefined or refined. e.g. fruits, vegetables, whole grains, legumes (beans, peas, lentils) “Unrefined” “Refined” Carbohydrates Carbohydrates as ingredients in highly processed foods White flour, starches (cornstarch, potato starch) Free sugars: Added sugars: monosaccharides and disaccharides added during processing e.g. glucose, fructose, sucrose Monosaccharides and disaccharides in Fig 4.2 honey, syrups, juices 13  Total sugars refers to Total Sugars sugars intrinsic to foods e.g. fruits contain sugars and free sugars. Sugars intrinsic Free Sugars to food (monosaccharides and disaccharides) Added Sugars in honey, Sugars syrups, juices e.g. sucrose 14 Free sugars in foods can often go by many names. It is important to recognize that these ingredients are all various forms of monosaccharides or disaccharides. The many names for sugars: all the ingredients listed below are sources of monosaccharides or disaccharides Sugar (sucrose) Invert sugar (1:1 mixture of glucose and fructose) Glucose Dextrose (same as glucose) Maltose or malt sugar Lactose Corn syrup (glucose in syrup form) Honey Molasses Fruit juice concentrates Glucose/fructose or high fructose corn syrup ( a mixture of glucose and fructose in syrup form) 15 This table compares sugar-containing kiwi with a soft drink. Both have similar kcalorie and sugar content, but the kiwi is much more nutrient-dense also containing fibre, potassium, vitamin C and other nutrients. The soft drink provides nothing but kcalories. For this reason, Canada’s Food Guide recommends drinking water and eating fresh fruit. Fig 4.4 16 Whole grain refers to the kernel of cereal. When we consume the entire kernel, containing the endosperm, bran and germ, we are consuming a whole grain product. Examples of whole grains are shown here. Pseudocereals, are not technically grains, but are nutritionally similar and consumed like grains. Fig 4.3 17 Whole Grains Council Stamps Food products that contain whole grains can voluntarily put this stamp on their label. This stamp acts as a signal to consumers that the product contains whole grains. Pg. 121 18  These breads are different: 100% Whole grain wheat bread 100% Whole wheat bread Made from flour that contains: Made from flour that contains Endosperm the whole grain: Bran Endosperm – contains starch Germ is removed because it contains oil and spoils easily Bran –contains fibre Some nutrients are missing, as a result of missing germ, but still a good source of Germ - contains vitamin E fibre Pg 121 White bread Made from white flour that contains endosperm only; bran and germ removed Fortified with iron, thiamin, niacin, riboflavin to replace some but not all nutrients removed when bran and germ removed Low in fibre, vitamin E 19 How are Canadians doing with respect to fibre intake? Dietary Reference Intake for dietary fibre is an Adequate Intake (AI) AI is assigned to a nutrient when there is not enough research to determine an EAR How to interpret the intake results for a nutrient with an AI: If 50% or more of the population is consuming above the AI, the population intake is adequate If less than 50% of the population is consuming above the AI, the population intake may or may not be adequate Results suggest that adult Canadians’ intake of fibre may or may not be adequate, as less than 50% are consuming above the AI, in all age categories shown. 20 3) Digestion, Absorption, and Metabolism of Carbohydrates After looking at the types of carbohydrates and carbohydrate- containing foods that Canadians eat, this next section discusses their digestion, absorption, and metabolism After reading this section be sure that you can: 1. distinguish between the functions of salivary amylase and pancreatic amylases 2. explain how lactose intolerance develops 3. describe how both soluble and insoluble fibre reduces constipation 4. describe how soluble fibre slows the absorption of nutrients 5. Explain what is meant by cellular respiration and list the four major steps in the metabolism of glucose and the formation of ATP 21 Digestion of Carbohydrates Fig 4.12 22 Brush border enzymes include: Sucrase-breaks down sucrose Maltase-breaks down maltose Lactase-breaks down lactose All human infants produce lactase to process lactose in human milk Many individuals lose the ability to produce sufficient quantities of lactase as they age, leading to lactose intolerance Lactose intolerance Because lactose cannot be digested when lactase is not present, lactose enters the colon where it is fermented by microflora, causing intestinal gas and bloating Lactose intolerance varies in severity Many can tolerate lactose in small quantities e.g. in small doses or in fermented milk products, where most but not all lactose is converted to lactic acid 23  4.3 Indigestible Carbohydrates: Insoluble and soluble fibre have different physiological effects. Both are beneficial. Insoluble Fibre Soluble Fibre Does not dissolve in water Some is fermented by microflora Not fermentable by microflora to short chain fatty acids Increases fecal bulk → promotes Absorbs water → softens fecal GI motility → reduced bulk → reduced constipation constipation e.g. psyllium-containing foods e.g. bran-containing & whole-grain foods Consume fibre with water 24  Role of Fibre Dietary fibre, especially soluble fibre dilutes the contents of the stomach and small intestine and “slows” the absorption of nutrients as shown (green dots) An example, of where this process is important, is the reduction blood glucose levels, which will be discussed later in this chapter Fig 4.14 25 Glucose Metabolism Fig 4.16 After absorption glucose is delivered to cells in the body, where it can be used as a source of energy. Glucose is broken down to carbon dioxide and water. In the presence of oxygen, this happens with the associated production of the high energy compound ATP (adenosine triphosphate), which is used to fuel many cellular processes. This process is called cellular respiration. 26 Fig 4.16 27 Fig 4.16 28 Fig 4.16 29 4) Blood glucose regulation and diabetes Now that we have discussed the digestion, absorption, and metabolism of glucose, we will focus on one very important aspect of the glucose utilization in the body and that is the regulation of blood glucose. After this section be sure you can: 1. Explain the shape of a typical blood glucose curve 2. Compare and contrast the action of insulin and glucagon 3. define gluconeogenesis and glucogenic amino acids 4. explain the mechanism by which soluble fiber reduces glycemic response 5. explain how glycemic index is measured 6. explain why it is important to treat gestational diabetes 7. describe “Just the Basics” and how glycemic index can be incorporated into food choices for the management of diabetes 8. interpret the results of the study described in the required reading 30 4.5 Glycemic response: Blood glucose response curve Shown opposite is a blood glucose response curve. It shows the rise in blood (or plasma) glucose that occurs after a meal. The characteristics of the blood glucose curve, the rate of rise, its magnitude and its duration is called the glycemic response. The rise in blood glucose immediately after a meal stimulates the release of the hormone insulin from the pancreas. Insulin stimulates the uptake of glucose from the blood into the cells, causing the blood glucose to return to normal levels. In healthy individuals blood glucose levels returns to fasting levels in about two hours. 31  The action of insulin Fig 4.22 After a meal, blood glucose rises and stimulates the secretion of insulin from the beta-cells of the pancreas 1. In the pancreas, glucose (in a multi-step process), stimulates the synthesis of insulin, so there is always a supply of the hormone in the beta-cell 2. In the liver, insulin stimulates the uptake of glucose for the synthesis of glycogen and other metabolic processes 3. In the muscle, insulin stimulates the uptake of glucose as a source of energy to support physical activity and for the synthesis of muscle glycogen 4. In adipose tissue, insulin stimulates the uptake of glucose where it is used in the synthesis of lipids 32 The action of glucagon When carbohydrates have not been consumed for several hours, blood glucose levels begin to decline. 1. In the pancreas, the drop in blood glucose stimulates the release of glucagon, from the alpha cells of the pancreas. 2. In the liver, glucagon stimulates the breakdown of glycogen. 3. In the liver, glucagon also stimulates gluconeogenesis, the synthesis of glucose from amino acids (See next slide) 4. Both steps 2 & 3 return blood glucose levels to normal. 33  Fig 4.17 Gluconeogenesis: When blood glucose levels drop, protein is broken down. Some of the amino acids in protein are glucogenic amino acids, meaning they can be converted to glucose, a process called gluconeogenesis. Ketogenic amino acids are converted to Acetyl CoA and used as a source of energy or for the synthesis of fatty acids. 34 Ketone formation Fig 4.18 When carbohydrates (and hence oxaloacetate-red arrow) is present, acetyl Co-A, which is formed from the breakdown of fatty acids can enter the citric acid cycle. When carbohydrate intake is low or absent, such as during fasting, acetyl Co-A cannot enter the citric acid cycle and is diverted into metabolic pathways that produce ketones. 35  4.5 Glycemic response varies with the food consumed. Fig 4.19 Shown here are two glucose response curves. The rise in blood glucose is more rapid in the absence of soluble-fibre (red line) (1). This steep rise stimulates the release of high levels of insulin which in turn causes the large drop in blood glucose (2), compared to the fibre rich meal (blue line) where the rise was not as high, so less insulin is released and the decline is more gradual. This gradual response is considered the healthier response because it does not stress the pancreas as much as steep increases in blood glucose, which require more insulin secretion. 36 4.5 Glycemic Index (GI) The reason that the glycemic response of a high soluble- fibre rich meal is more blunted than a low-fibre meal is shown opposite. Foods high in soluble fibre absorbs water and form a viscous solution that dilutes glucose (green dots) in the intestine. This slows the release of glucose from the food in the digestive tract and into the blood. This effect, of food on blood glucose, is characterized by the glycemic index. 37 Definition of Glycemic Index A ranking of the effect, on blood glucose, of a food of a certain carbohydrate content relative to an equal amount of carbohydrate from a reference food, usually 50g of glucose or white bread containing 50 g of digestible (available) carbohydrate. Determining Glycemic Index A person is asked to consume a test food and the glucose response curve is obtained, by measuring blood glucose at various time points. This is repeated with the reference food and the index is the ratio of the two areas under the glucose response curves. Area 2 (from glucose) has a value of 100, and the index for other foods is expressed relative to this. The glycemic index is the average result obtained from 10 healthy individuals. 38 Effect of GI on Nutrient Absorption: Low glycemic index foods are believed to slow the rate at which glucose is absorbed from the digestive tract. This is similar to the previous description of the effect of soluble fibre. The factors that influence whether a food has a low glycemic index include, but are not limited to fibre content. In fact some high-fibre foods, e.g. whole wheat bread, have a moderately high glycemic index. The way in which the food is processed, the presence of other components such as fats and proteins, and the physical and chemical properties of the food may all influence its glycemic index. Low glycemic index foods can be helpful in the prevention or treatment of Fig 4.20 diabetes. 39 Diabetes Diabetes is a disease characterized by high blood glucose levels and is linked to the inability of the body to produce sufficient insulin, the inability of the body to respond to insulin or a combination. Three are three main types of diabetes: Type-1-Diabetes This disease is characterized by the body’s inability to produce insulin due to an autoimmune disease that destroys the pancreas. It is treated with insulin injections. About 10% of diabetes cases are type-1-diabetes. Gestational Diabetes This form of diabetes occurs during pregnancy. If untreated in can result in a high birthweight baby, which may cause complications during delivery. Babies born to mothers with gestational diabetes may also be at increased risk of obesity and type-2- diabetes later in life. Mothers who develop gestational diabetes are also at increased risk of developing type-2-diabetes later in life. Gestational diabetes can be treated, reducing the risk to both mother and infant. Type-2-Diabetes The most common type of diabetes representing 90% of cases. 40 Type-2-Diabetes Type-2-diabetes is characterized by high blood glucose levels and results from a combination of: insulin resistance in muscle, liver, and adipose tissue reduced insulin secretion by the pancreas. Insulin resistance: refers to the state when the muscle and adipose tissue do not respond to stimulation by insulin to take up glucose. As a result glucose does not readily enter muscle or adipose tissue but instead remains in the blood, elevating blood glucose levels and causing damage to blood vessels which increase the risk of heart disease In the insulin-resistant liver, gluconeogenesis, which is normally suppressed by insulin, continues, contributing to elevated blood glucose In the early stages of insulin resistance, the pancreas responds to the lack of responsive to insulin, by liver, muscle and/or adipose tissue, by increasing insulin secretion. This initially results in a lowering of blood glucose levels, but eventually the pancreas becomes exhausted and secretion of insulin is insufficient to maintain normal blood glucose levels. 41 The figure opposite illustrates the Fig 4.23 changes in blood glucose levels after an overnight fast and after a meal. The normal blood glucose levels are shown in blue. As insulin resistance develops and/or pancreatic function begins to decline, blood glucose rises to the pre-diabetes state. Treatment of pre-diabetes, with lifestyle changes, can often successfully normalize blood glucose levels. If however insulin resistance and pancreatic function continue to worsen blood glucose levels rise to levels that meet the definition of type-2-diabetes. 42  Risk factors for type-2-diabetes One of the major risk factors for the development of type-2-diabetes is obesity. When we discuss the Consequences of Obesity later in the term, how obesity contributes to the development of diabetes will be described. Type-2-diabetes normally develops during adulthood, typically after age 40, but, because of the rise of obesity among children and teens, it is being seen increasingly in this population and in younger adults. 43 Treatment of Type-2-Diabetes The main steps in the treatment of type-2-diabetes include: Weight loss – obesity is a major risk factor Exercise – exercise is effective because it promotes the uptake of glucose by muscle tissue, independent of the need for insulin Diet- dietary patterns that are generally well-balanced are effective for type-2-diabetes Medication – medication in combination with the three steps above can be effective 44 Dietary Patterns Effective in the Prevention or Treatment of Type-2- Diabetes: A number of dietary patterns have been linked to reduced risk for type- 2-diabetes. Generally, dietary patterns that are nutritionally balanced, with an emphasis on whole grains, fruits and vegetables and plant-based proteins are suitable dietary patterns. The Mediterranean diet has been widely studied and has been shown to be effective in lowering blood glucose levels. 45 Dietary Patterns Effective in the Prevention or Treatment of Type-2- Diabetes: Canada’s Food Guide also promotes a nutritious diet. 46 Dietary Patterns Effective Fig 4.25 in the Prevention or Just the Basics Treatment of Type-2- Diabetes: Diabetes Canada (formerly the Canadian Diabetes Association) has developed its own plate-based dietary pattern called Just the Basics. It is similar, but not identical to Canada’s Food Guide. 47  Fig 4.26 Diabetes Canada also recommends selecting low and medium glycemic index foods, like those listed opposite. Including at least one with each meal helps to slow glucose absorption. This results in lower blood glucose levels and less demand on the pancreas to secrete insulin, helping in the prevention and or treatment of diabetes. 48 Class Activity:Textbook Required Reading To read about the impact of both GI and dietary fibre on the development of type-2-diabetes see the required reading for this chapter: Critical Thinking: Dietary Fibre, Glycemic Index, and Type-2-Diabetes 3e: 161 Be sure you can answer the questions in the textbook, that are included with the reading. 49 Carbohydrates and Health In addition to blood glucose regulation and diabetes management, carbohydrate-containing foods play a role in number of other health- related issues that will be discussed in this section. After reading this section be sure you can: 1. describe the role of carbohydrate containing foods in weight management 2. explain what is meant by a sustainable diet, with respect to weight management 3. explain the relationship between soluble fibre and blood cholesterol levels 4. indicate how added sugars influence serum triglycerides 5. distinguish between tumour initiator and tumour promoter 6. describe how dietary fibre might reduce cancer risk 50 4.6 Carbohydrates and Weight Loss Good carbs Bad carbs Low GI carbs may help Sugar-sweetened beverages are directly associated with obesity in systematic reviews of observational modestly with weight loss studies, probably due to the ease with which one can overconsume kcalories from beverages. Slower absorption rates This is why Canada’s Food Guide recommends drinking extend the feeling of water. fullness, which may reduce Sugar-sweetened beverages, such as soft drinks, are food intake sweetened with high fructose corn syrup. There is some concern that fructose metabolism promotes the synthesis of fat in the body, but this may only occur at doses higher than what most people consume. This remains an area of controversy 51 4.6 Carbohydrates and Weight Loss: Comparing High-Carb vs Low Carb diets High carb VS low carb diets Low carb diets tend to result in faster initial weight loss (first 6 months) but by one year, there is no significant difference between the two diets Healthy weight management requires sustainability, i.e. diets that can be maintained long term without compromising health, so diets should be selected that are nutritionally-balanced and match the personal preferences of the individual 52 4.6 Carbohydrates and Cardiovascular Disease: Good Carbs: Foods high in soluble fibre help lower blood cholesterol levels. Bile acids, which aid in the digestion of fats, are biosynthesized from cholesterol, in the liver. In the small intestine the some of the bile acids are recycled as shown by the blue arrow in figure (a). Soluble fibre tends to bind bile acids, causing more of the bile acids to be excreted and less to return to the liver. Because lower amounts of bile acids return to the liver, new bile acids have to be synthesized from cholesterol. Because cholesterol is used in bile acid synthesis, levels in the blood decline. 53 4.6 Carbohydrates and Cardiovascular Disease: Bad Carbs: Foods high in free sugars may raise serum triglycerides, a risk factor for cardiovascular disease. Blood lipids will be discussed in more detail, in chapter 5. 54  Fig 4.31 4.6 Carbohydrates and Cancer Cancer is the result of mutations in a cell’s DNA that cause it to grow uncontrollably. A single mutated cell can grow into a tumour and cells from that tumour can break free and spread to other parts of the human body, a process called metastasis. Compounds that cause cancer are called carcinogens. Dietary carcinogens can be tumour initiators because they initiate mutations or tumour promoters because they promote the growth of established tumours. 55 Colon Cancer and Dietary Fibre Studies have shown that dietary fibre can reduce the risk of colon cancer Possible mechanisms of action: Fibre reduces contact time between intestinal cells and fecal contents, which may contain carcinogens Fibre undergoes, fermentation by microflora, to produce short chain fatty acids, which act as a source of energy for cells of the colon and help maintain their function High-fibre foods also contain beneficial compounds such as antioxidants and other phytochemicals which may help reduce cancer risk 56  6) Non-Nutritive Sweeteners (Alternative Sweeteners) Throughout this document you have read about the detrimental effects of free or added sugars. One alternative to sugars is non- nutritive sweeteners. These are sweeteners that, unlike sugars, contribute little or no kcalories to a food. The safety of some of these compounds is controversial. After reading this section be sure you can: 1. Describe the safety issues and controversies associated with non-nutritive sweeteners 2. Compare and contrast aspartame and sucralose 3. Indicate Canada’s Food Guide’s advice on non-nutritive sweeteners 57 4.7 Sugar Substitutes or Non-nutritive Sweeteners fall into two categories: Artificial Sweeteners Sweet compounds extracted from plants Aspartame Steviol glycosides Neotame Monk fruit extract Advantame Thaumatin Sucralose Stevia plant Acesulfame K Saccharin & Cyclamate Sugar Alcohols 58 Sugar Substitutes Safety concerns Animal studies and human observational studies suggest that use of sugar substitutes are associated with weight gain, insulin resistance and dysfunction of the microflora Human RCTs usually conclude that sugar substitutes do NOT have harmful effects More research is needed More RCTs of better quality and longer duration More study of individual sweeteners E.g. observational studies often group all artificial sweeteners together but they function by different mechanisms 59 Some common non-nutritive sweeteners Aspartame Sucralose Aspartame is a dipeptide of aspartic acid and Sucralose is a molecule of sucrose with -C- methylated phenylalanine When consumed it is hydrolyzed in the small Cl groups in place of several -C-H groups intestine to aspartic acid, phenylalanine, two amino The presence the chlorine atoms prevents acids that are present in all proteins, and a minute amounts of methanol the absorption of the sweetener. Instead Aspartame is a very intense sweetener and is used it is excreted in feces. in foods in milligram amounts, so it contributes essentially no kcalories to the food Some studies have suggested that the Individuals who have phenylketonuria (PKU), a sweetener, because it enters the large genetic disorder that prevents the proper metabolism of phenylalanine, need to avoid intestine, may disrupt microflora, a topic phenylalanine of ongoing research. All products that contain aspartame indicate on the label: Aspartame contains phenylalanine Concerns about the safety of aspartame have been around for decades but the majority of scientific studies confirm its safety 60 Canadian Dietary Guidelines address the topic of sugar substitutes by indicating their use is not essential. GUIDELINE 2: CONSIDERATIONS Sugary drinks, confectioneries and sugar substitutes Processed or prepared Sugary drinks and confectionaries should not be foods and beverages that consumed regularly. contribute to excess Sugar substitutes do not need to be consumed to reduce the intake of free sugars. sodium, free sugars, or Publically funded institutions saturated fat undermine Foods and beverages offered in publically funded healthy eating and should institutions should align with Canada’s Dietary Guidelines. not be consumed regularly. Alcohol There are health risks associated with alcohol consumption. 61 This concludes our discussion of carbohydrates. 62