The Carbohydrates Sugars Starches and Fibers Chapter 4 Lecture 2 Dr Alisha Rovner University of Delaware Nutrition Concepts NTDT200 Fall 2013 Class Overview Digestion and Absorption of Carbohydrates Glucose in the Body Sugars Alternative Sweeteners Class Objectives Summarize carbohydrate digestion and absorption Explain how the body maintains its blood glucose concentration Describe how added sugars can contribute to health problems Do Carbohydrates Cause Weight Gain No Calories cause weight gain Excess calories from carbohydrates are not any more fattening than calories from other sources Despite the claims of low carb diets a high carbohydrate diet does not promote fat storage by enhancing insulin resistance In fact a recent study published in the JADA found that consuming a lowcarbohydrate diet was associated with an increased likelihood of being overweight or obese The lowest risk was found in those who consumed about half of their calories from carbohydrates Carbohydrate Digestion Ultimate goal is to to break down sugars and starches into small molecules the body can use Large starch molecules require extensive breakdown Disaccharides need to be broken down once Monosaccharides don t need to be broken down Carbohydrate Digestion Carbohydrate digestion begins in the mouth Salivary enzyme amylase hydrolyzes starch to shorter polysaccharides and to the disaccharide maltose Food is in the mouth for a short time so very little carbohydrate digestion takes place there Carbohydrate Digestion Stomach Swallowed bolus mixes with stomach s acid and protein digesting enzymes Acid continues to break down starch but doesn t contain enzymes to digest carbohydrates Fibers linger and delay gastric emptying which provides a feeling of fullness and satiety Carbohydrate Digestion Small intestine Site of most carbohydrate digestion Pancreatic amylase a carbohydrate digesting enzyme continues breaking down polysaccharides to shorter glucose chains and maltose Final step takes place on the outer membranes of intestinal cells Specific enzymes break down specific disaccharides Now all polysaccharides and disaccharides have been broken down to monosaccharides Carbohydrate Digestion Large intestines Within 1 4 hours after a meal all sugars and most of the starches have been digested Only fiber remains in the digestive tract Fibers attract water which softens stools What type of fiber helps alleviate constipation A Soluble B Insoluble Carbohydrate Absorption Nutrient absorption takes place in the small intestines Glucose and galactose enter the cells lining the small intestines by active transport Fructose is absorbed by facilitated diffusion In the liver galactose and fructose are converted to glucose Absorption of Monosaccharides Lactose Intolerance Normally intestinal cells produce enough lactase enzyme that breaks down lactose for digestion and absorption Lactase activity Highest immediately after birth with age About 30 of people retain enough lactase to digest and absorb lactose throughout adulthood Lactose intolerance Condition that results from inability to digest the milk sugar lactose Symptoms include bloating abdominal discomfort and diarrhea May develop when intestinal villi are damaged by diseases medications prolonged diarrhea and malnutrition Prevalence of Lactose Intolerance Prevalence varies among ethnic groups 80 Southeast Asians 80 Native Americans 75 African Americans 50 Hispanics 20 Caucasians 10 Northern Europeans Lactose Intolerance Dietary changes Manage lactose consumption rather than restriction total elimination usually not necessary Most people can consume 6 grams of lactose 1 2 cup milk without symptoms Milk products that have been treated with an enzyme that breaks down lactose ie Lactaid Non dairy foods can contain lactose too Lactose Free Diet Complete lactose free diet can be difficult because lactose is found not only in milk milk products but many nondairy foods Those on strict lactose free diet need to avoid foods that include milk milk solids whey milk liquid and casein milk protein Need to check medications too since some contain lactose as a filler Lactose in Selected Foods Carbohydrate Metabolism Main goal supply cells with glucose for energy Storing glucose as glycogen Liver stores about 1 3 of supply and releases glucose into bloodstream as needed Muscle also stores glucose but keeps most of it for themselves Brain maintains small amount but only provides an emergency energy reserve Carbohydrate Metabolism Carbohydrate Metabolism Fat can t be converted to glucose to any significant extent When person doesn t consume enough carbohydrates ketone bodies provide an alternate fuel source Dangerous because this disturbs acid base balance Carbohydrate Needs To spare body protein and prevent ketosis the body needs at least 50 100 grams of carbohydrates a day Dietary recommendations are that people select abundantly from carbohydrate rich foods to provide for considerably more The Constancy of Blood Glucose Body must maintain blood glucose within a certain range Blood glucose homeostasis regulated by 2 hormones 1 Insulin Moves glucose from blood into cells 2 Glucagon Brings glucose out from storage when necessary Maintaining Blood Glucose Homeostasis The Constancy of Blood Glucose Balancing within the normal range 70 100 mg dL Diabetes Insulin is either inadequate or ineffective Type 1 diabetes pancreas fails to produce insulin pancreas doesn t function properly Type 2 diabetes cells fail to respond to insulin caused by being overweight The Constancy of Blood Glucose In healthy people blood glucose rises after eating and then falls back into the normal range Glycemic response refers to how quickly glucose is absorbed how high blood glucose rises and how quickly it returns to normal Slow absorption a modest rise is blood glucose and a smooth return to normal are desired low glycemic response Glycemic Index GI Method of classifying foods according to their potential to raise blood glucose General classifications Low GI Legumes milk products Moderate GI Whole grains fruits High GI Processed foods made from refined flours eg snack foods breads ready to eat cereal Health Effects of Sugars In moderation sugar can add pleasure to meals without harmful effects In excess sugar can contribute to nutrient deficiencies by supplying energy without nutrients and it can contribute to dental caries tooth decay Added sugars Sugars and syrups used as an ingredient in the processing and
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