Chem 2N - Carbohydrate Metabolism PDF

CHEM 2N - BIOCHEMISTRY CARBOHYDRATE METABOLISM © 2008 Thomson - Wadsworth  The Chemist’s View of Carbohydrates Carbohydrates are made of carbon, hydrogen and oxygen atoms. These atoms form chemical bonds that follow the laws of nature. The Simple Carbohydrates Monosaccharr...

CHEM 2N - BIOCHEMISTRY CARBOHYDRATE METABOLISM © 2008 Thomson - Wadsworth  The Chemist’s View of Carbohydrates Carbohydrates are made of carbon, hydrogen and oxygen atoms. These atoms form chemical bonds that follow the laws of nature. The Simple Carbohydrates Monosaccharrides are single sugars (most are hexoses). ✓ Glucose serves as the essential energy source, and is commonly known as blood sugar or dextrose. ✓ Fructose is the sweetest, occurs naturally in honey and fruits, and is added to many foods in the form of high-fructose corn syrup. ✓ Galactose rarely occurs naturally as a single sugar. The Simple Carbohydrates Disaccharides are pairs of monosaccharides, one of which is always glucose ✓ Condensation reactions link monosaccharides together. ✓ Hydrolysis reactions split molecules and commonly occur during digestion. ✓ Maltose consists of two glucose units. It is produced during the germination of seeds and fermentation. ✓ Sucrose is fructose and glucose combined. It is refined from sugarcane and sugar beets, tastes sweet, and is readily available. ✓ Lactose is galactose and glucose combined. It is found in milk and milk products.  The Complex Carbohydrates Few (oligosaccharides) or many (polysaccharides) glucose units bound/linked together in straight or branched chains.  The Complex Carbohydrates Glycogen ✓Storage form of glucose in the body ✓Provides a rapid release of energy when needed Starches ✓Storage form of glucose in plants ✓Found in grains, tubers, and legumes  The Complex Carbohydrates Dietary fibers provide structure in plants, are very diverse, and cannot be broken down by human enzymes. ✓ Soluble fibers are viscous and can be digested by intestinal bacteria (this property is also known as fermentability). These fibers are found in fruits and vegetables. ✓ Insoluble fibers are nonviscous and are not digested by intestinal bacteria. These fibers are found in grains and vegetables.  The Complex Carbohydrates Fiber Sources ✓ Dietary fibers are found in plant foods. ✓ Functional fibers are health-benefiting fibers that are added to foods or supplements. ✓ Total fiber considers both dietary and functional fibers. Resistant starches escape digestion and are found in legumes, raw potatoes and unripe bananas. Phytic acid or phytate has a close association with fiber and binds some minerals.  Digestion and Absorption of Carbohydrates Carbohydrate Digestion In the mouth, the salivary enzyme amylase begins to hydrolyze starch into short polysaccharides and maltose. In the stomach, fiber delays gastric emptying and provides a feeling of fullness (satiety).  Digestion and Absorption of Carbohydrates Carbohydrate Digestion ✓In the small intestine, pancreatic amylase among other enzymes (maltase, sucrase, and lactase) hydrolyzes starches to disaccharides and monosaccharides. ✓In the large intestine, fibers remain and attract water, soften stools and ferment. © 2008 Thomson - Wadsworth  Digestion and Absorption of Carbohydrates Carbohydrate Absorption ✓Primarily takes place in the small intestine ✓Glucose and galactose are absorbed by active transport. ✓Fructose is absorbed by facilitated diffusion.  Digestion and Absorption of Carbohydrates Lactose Intolerance ✓ Symptoms include bloating, abdominal discomfort, and diarrhea. ✓ Causes include lactase deficiency due to a natural decrease that occurs with aging or damaged intestinal villi. ✓ Prevalence Lowest in Scandinavians and northern Europeans Highest in Southeast Asians and native North Americans  Digestion and Absorption of Carbohydrates Lactose Intolerance - Dietary Changes ✓ Increase consumption of milk products gradually. ✓ Mix dairy with other foods. ✓ Spread dairy intake throughout the day. ✓ Use of acidophilus milk, yogurt, and kefir (fermented products) ✓ Use of enzymes ✓ Individualization of diets ✓ Must be careful that vitamin and mineral deficiencies do not develop  Glucose in the Body A Preview of Carbohydrate Metabolism ✓ The body stores glucose as glycogen in liver and muscle cells. ✓ The body uses glucose for energy if glycogen stores are available. ✓ If glycogen stores are depleted, the body makes glucose from protein. Gluconeogenesis is the conversion of protein to glucose. Protein-sparing action is having adequate carbohydrate in the diet to prevent the breakdown of protein for energy.  Glucose in the Body A Preview of Carbohydrate Metabolism ✓ Making ketone bodies from fat fragments The accumulation of ketone bodies in the blood is called ketosis. Ketosis upsets the acid-base balance in the body. ✓ The body can use glucose to make body fat when carbohydrates are consumed excessively.  Glucose in the Body The Constancy of Blood Glucose ✓Maintaining Glucose Homeostasis Low blood glucose may cause dizziness and weakness. High blood glucose may cause fatigue. Extreme fluctuations can be fatal.  Glucose in the Body The Constancy of Blood Glucose ✓ The Regulating Hormones Insulin moves glucose into the cells and helps to lower blood sugar levels. Glucagon brings glucose out of storage and raises blood sugar levels. Epinephrine acts quickly to bring glucose out of storage during times of stress. ✓ Balance glucose within the normal range by eating balanced meals regularly with adequate complex carbohydrates. ✓ Blood glucose can fall outside the normal range with hypoglycemia or diabetes. © 2008 Thomson - Wadsworth  Glucose in the Body The Constancy of Blood Glucose ✓ Diabetes Type 1 diabetes is the less common type with no insulin produced by the body. Type 2 diabetes is the more common type where fat cells resist insulin. Prediabetes is blood glucose that is higher than normal but below the diagnosis of diabetes. ✓ Hypoglycemia is low blood glucose and can often be controlled by dietary changes.  Glucose in the Body The Constancy of Blood Glucose ✓ Glycemic response is how quickly the blood glucose rises and elicits an insulin response. Glycemic index classifies foods according to their potential for raising blood glucose. Glycemic load refers to a food’s glycemic index and the amount of carbohydrate the food contains. The benefit of the glycemic index is controversial. © 2008 Thomson - Wadsworth  Health Effects and Recommended Intakes of Sugars Sugar poses no major health problem except dental caries. Excessive intakes may displace nutrients and contribute to obesity. Consuming foods with added sugars should be limited. Naturally occurring sugars from fruits, vegetables and milk are acceptable sources.  Health Effects and Recommended Intakes of Sugars Health Effects of Sugars ✓ Foods with added sugars have sugars listed as a first ingredient. ✓ Nutrient deficiencies may develop from the intake of empty kcalories. Just because a substance is natural does not mean it is nutritious. (Example: honey) ✓ Dental caries may be caused by bacteria residing in dental plaque and the length of time sugars have contact with the teeth. © 2008 Thomson - Wadsworth  Health Effects and Recommended Intakes of Sugars Controversies Surrounding Sugars ✓ Excessive sugar intake can contribute to the development of body fat. ✓ Sugar may be able to alter blood lipid levels and contribute to heart disease in some. ✓ There is no scientific evidence that sugar causes misbehavior in children and criminal behavior in adults. ✓ There is a theory that sugar increases serotonin levels, which can lead to cravings and addictions.  Health Effects and Recommended Intakes of Starch and Fibers Health Effects of Starch and Fibers ✓ May be some protection from heart disease and stroke Soluble fibers bind with bile and thereby lower blood cholesterol levels. Fiber may also displace fat in the diet. ✓ Reduce the risk of type 2 diabetes by decreasing glucose absorption ✓ Enhance the health of the GI tract which can then block the absorption of unwanted particles ✓ May protect against colon cancer by removing potential cancer-causing agents from the body  Health Effects and Recommended Intakes of Starch and Fibers Health Effects of Starch and Fibers ✓Promote weight control because complex carbohydrates provide less fat and added sugar. ✓Harmful effects of excessive fiber intake Displaces energy and nutrient-dense foods Abdominal discomfort and distention May interfere with nutrient absorption © 2008 Thomson - Wadsworth  Health Effects and Recommended Intakes of Starch and Fibers From Guidelines to Groceries ✓ Grains – encourage whole grains ✓ Vegetables – starchy and nonstarchy vegetables differ in carbohydrate content ✓ Fruits – vary in water, fiber and sugar content ✓ Milks and Milk Products – contain carbohydrate; cheese is low ✓ Meat and Meat Alternates – meats are low but nuts and legumes have some carbohydrate ✓ Food labels list grams of carbohydrate, fiber and sugar; starch grams can be calculated. © 2008 Thomson - Wadsworth CLINICAL RELEVANCE CLINICAL RELEVANCE CLINICAL RELEVANCE CLINICAL RELEVANCE Hormonal And Hepatic Regulation of Blood Glucose CLINICAL RELEVANCE CLINICAL RELEVANCE CLINICAL RELEVANCE CLINICAL RELEVANCE CLINICAL RELEVANCE CLINICAL RELEVANCE CLINICAL RELEVANCE CLINICAL RELEVANCE Mechanisms of DKA DKA is associated with very low insulin levels and extremely high levels of glucagon, catecholamines, and other counter-regulatory hormones. Increased levels of glucagon and the catecholamines lead to mobilization of substrates for gluconeogenesis and ketogenesis by the liver. Gluconeogenesis in excess of that needed to supply glucose for the brain and other glucose- dependent tissues produces a rise in blood glucose levels. Mobilization of free fatty acids (FFA) from triglyceride stores in adipose tissue leads to accelerated ketone production and ketosis. (CNS, central nervous system.)  CLINICAL RELEVANCE Long-term Complications of DM CLINICAL RELEVANCE Neuropathic ulcers occur on pressure points in areas with diminished sensation in diabetic polyneuropathy. Pain is absent (and therefore the ulcer may go unnoticed until it enlarges). CLINICAL RELEVANCE CLINICAL RELEVANCE