HUN1201 LECTURE NOTESCHAPTER 4: Carbohydrates: Plant-Derived Energy NutrientsWhat are Carbohydrates?• Macronutrient• Major source of energy for all cells• Carbon, Hydrogen, Oxygen (CHO)• Good sources: • fruits• vegetables• grains• Glucose• The most abundant carbohydrate• Produced by plants through photosynthesis• Simple carbohydrates• Contain one or two molecules• Commonly referred to as sugars• Monosaccharides contain one molecule• Glucose, fructose, and galactose• Disaccharides contain two molecules• Lactose, maltose, and sucrose• Complex carbohydrates• Oligosaccharides contain 3 to 10 monosaccharides • Most polysaccharides consist of hundreds to thousands of glucose molecules • Starch, glycogen, most fibersComplex Carbohydrates• Starch • Plants store carbohydrates as starch• Amylose—straight chain of glucose• Amylopectin—branched chain of glucose• Resistant starch (fiber)—glucose molecules linked by beta bonds arelargely indigestible • Sources: grains, legumes, fruits, vegetables• Glycogen• Storage form of glucose for animals (humans) • Not found in food and therefore not a source of dietary carbohydrate• Stored in the liver and muscles• Fiber• Long polysaccharide chains• Dietary fibers are non-digestible parts of plants • Functional fibers are non-digestible forms of carbohydrates extracted from plants and have known health benefits • Total fiber = Dietary fiber + Functional fiber• Soluble fibers• Dissolve in water; viscous and gel-forming• Fermentable, digested by intestinal bacteria • Associated with risk reduction of cardiovascular disease and type 2 diabetes• Examples: pectin, gum, mucilage• Found in citrus fruits, berries, oats, beans• Insoluble fibers• Not water-soluble, nonviscous• Cannot be fermented by bacteria in the colon• Promote regular bowel movements, alleviate constipation, and reducediverticulosis • Examples: lignins, cellulose, hemicelluloses• Good sources: whole grains, seeds, legumes, fruits, and vegetablesCarbohydrate Digestion• Salivary amylase • Enzyme that begins carbohydrate digestion in the mouth• Breaks carbohydrates down to maltose• Carbohydrate digestion does not occur in the stomach• Stomach acids inactivate salivary amylase• Most chemical digestion of carbohydrates occurs in the small intestine• Pancreatic amylase• Enzyme produced in the pancreas and secreted into the small intestine• Digests carbohydrates to maltose• Additional enzymes in the microvilli digest disaccharides to monosaccharides• Maltase• Sucrase• Lactase• Monosaccharides are absorbed into the cells lining the small intestine and then enter the bloodstream• All monosaccharides are converted to glucose by the liver• Glucose circulating in the blood is our primary energy source• Excess glucose is stored as glycogen in the liver and muscles• We do not have the enzymes necessary to digest fiber• Bacteria in the large intestine can break down some fiber• Most fiber remains undigested and is excreted in the fecesBlood Glucose Regulation• Blood glucose must be closely regulated• Hormones control blood glucose levels:• Insulin• Glucagon• Epinephrine• Norepinephrine• Cortisol• Growth hormoneInsulin• Secreted by beta cells of the pancreas• Stimulates glucose transporters (carrier proteins) to help take glucose from the blood across the cell membrane• Stimulates the liver to take up glucose and convert it to glycogenGlucagon• Secreted by alpha cells of the pancreas• Stimulates the liver to convert glycogen to glucose• Stimulates gluconeogenesis, the production of glucose from amino acids Epinephrine and Norepinephrine• Secreted by the adrenal glands and nerve endings when blood glucose is low• Increase glycogen breakdown in the liver, releasing glucose into the blood• Responsible for our “fight-or-flight” reactions to dangerBlood Glucose Regulation• Secreted by the adrenal glands to act on the liver, muscle, and adipose tissue• Cortisol: increase gluconeogenesis, decrease muscle glucose use• Growth hormone • Decrease muscle glucose uptake• Increase fatty acid mobilization • Increase liver glucose output• A food’s potential to raise blood glucose• Foods with a high glycemic index cause a sudden surge in blood glucose Increase insulin, Decrease blood glucose• Foods with a low glycemic index cause low to moderate fluctuations in blood glucose • Not always easy to predict • Food’s absorption rate varies with type of carbohydrate, preparation methods, and its fat and fiber content • Most foods are eaten in combination in a mealGlycemic Load• Determines the effect of a food on a person’s glucose response • Grams of carbohydrates in a food are multiplied by the glycemic index• Glycemic index and glycemic load remain controversial • Evidence of health benefits is weakValue of Lower Glycemic Index• Less fluctuations in blood glucose• Risk reduction for heart disease and colon cancer• High fiber intake helps to improve fat levels in the blood (higher HDL and lower LDL) • Foods with lower glycemic index:• Beans, fresh vegetables, whole wheatThe role of carbohydrates:• Energy• Red blood cells use only glucose for energy • Both carbohydrates and fats supply energy for daily activities• Glucose is especially important for energy during exercise• Ketosis• Fat breakdown during fasting forms ketones• Excess ketones increase blood acidity and cause ketoacidosis• Sufficient energy from carbohydrates prevents ketone production as alternate energy sourceCarbohydrates Spare Protein• Gluconeogenesis occurs when a diet is deficient in carbohydrate• The body will make its own glucose from protein • Amino acids from these proteins cannot be used to make new cells, repair tissue damage, support the immune system, or perform any of their other functionsComplex Carbohydrates Have Health Benefits• Fiber• May reduce the risk of colon cancer• Helps prevent hemorrhoids, constipation, and other intestinal problems• May reduce the risk of diverticulosis • May reduce the risk of heart disease• May enhance weight loss• May lower the risk of type 2 diabetesHow much carbohydrate?• (RDA): 130 grams/day • to supply adequate glucose to the brain• (AMDR): 45% to 65% of daily calories• Focus on fiber-rich carbohydrate foodsSimple
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