Lecture 3 - Carbohydrates PDF

Preview

Summary

Lecture 3 - Carbohydrates...

Description

1.

2.

Carbohydrates are the most abundant compounds found in nature a. Carbohydrates (CHO) are composed of carbon, hydrogen, & oxygen b. Main source of fuel in the average human diet i. Typically provide >50% of energy intake c. Dietary sources of carbohydrates include cereals, grains, legumes, fruits, vegetables, dairy, sweeteners and processed foods 2 types of CHO a. Simple i. Monosaccharides 1. Glucose 2. Fructose 3. Galactose 4. Can cross brush border of intestines 5. Name of monosaccharide is based on the number of carbons a. Usually in a ring structure 6. Monosaccharides are not (typically) present in the diet in significant quantities a. Some free glucose and fructose are present in honey b. Processed foods provide the majority of dietary monosaccharides (HFCS) i. Maldextrose ii. Disaccharides 1. Maltose a. Formed primarily from partial hydrolysis of starch 2. Sucrose a. Glucose and fructose b. Most widely distributed of the disaccharides c. Most commonly used in natural sweetener 3. Lactose 4. Trehalose a. Glucose and glucose b. 1,1 Alpha linkage (differs from maltase) c. Lower glycemic response d. Fungi (mushrooms) and shrimp b. Complex i. Slows gastric emptying ii. Contain more fiber iii. Bound carbohydrates 1. Digest slower iv. Oligosaccharides (3-10 sugar units) 1. Raffinose (trisaccharide) 2. Stachyose (tetrasaccharide) 3. Verbascose (pentasaccharides) 4. Beans, peas, bran, whole grains 5. Human digestive enzymes do not hydrolyze oligosaccharides a. Bacteria in lower intestine can digest them v. Polysaccharides (>10 sugar units) 1. Starches are the most common digestible polysaccharides in plants 2. Glycogen is the storage form of carbohydrates in animal tissues 3. Dietary fibers are non-digestible plant polysaccharides

1

a. b.

c.

d.

Naturally occurring in foods i. Functional fibers are added for a specific, beneficial effect Bran, oats, barley, rye, legumes, nuts, root vegetables (carrots, potatoes), cruciferous vegetables (cabbage, broccoli), celery, fruits (strawberries, citrus, apples, raspberries, bananas), allium vegetables (onions, garlic, artichoke) Human enzymes cannot digest the b-glycosidic and a-glycosidic linkages contained in fibers i. Linkages remain intact throughout the digestive processes in the duodenum and jejunum Fibers are either soluble or insoluble in water i. Peel is insoluble 1. Speed up intestinal transit time and increases fecal bulk 2. Foods rich in insoluble fiber: a. whole grains, wheat and corn bran nuts and seeds b. zucchini, celery, broccoli, cabbage, onions, tomatoes, carrots, cucumbers, green beans ii. Inside is soluble 1. Soluble fibers can bind up to several times their weight in water 2. Can produce a viscous, slow-moving gel solution 3. Functions a. Slow down digestion and absorption b. Delays gastric emptying → increases c. d.

satiety Lowers postprandial blood glucose levels Interferes with the absorption of dietary cholesterol and recycling of bile acids → lowers serum cholesterol

iii.

e. Delays nutrient absorption Some fiber can be fermented in the colon (soluble > insoluble) 1. Provides energy (short-chain fatty acids) for colon cells 2. Provides other substances (e.g. nitrogen) for microbial growth 3. Increases water and sodium absorption in colon 4. Acidifies luminal environment a. Reduces bile acid solubility → aids bile excretion → helps control cholesterol levels

e.

b. Protects against colon cancer 5. Favors growth of beneficial colonic bacteria and inhibits growth of unfavorable bacteria 6. Enhances immune function by stimulating production of macrophages Non-fermentable fibers are also important for colonic health i. Detoxify colonic environment

2

1.

f.

Adsorb carcinogens to prevent their interaction with the colonic mucosa 2. Dilute material in the colon Recommended fiber intake

4.

3.

4.

5.

Amylose comprises 20-30% of dietary starch a. Polymer of glucose bound by a(1,4) linkages b. Linear and unbranched (more slowly digested) c. Rich dietary sources are legumes (peas, beans, lentils) d. Typically considered resistant starch 5. Amylopectin comprises 70-80% of dietary starch a. Polymer of glucose bound by a(1,4) and a(1,6) linkages b. Highly branched (more rapidly digested) c. potatoes, rice, bread, pasta, and cereals 6. Glycogen is the storage form of glucose in muscle and liver tissue a. Polymer of glucose bound by a(1,4) and a(1,6) linkages b. Very highly branched i. Enzymes can hydrolyze many glucose molecules simultaneously for quick release of glucose Many tissues rely on glucose as a source of energy a. The adult human brain uses ~120 g/d of glucose b. Tissues dependent primarily on anaerobic glycolysis use ~40 g/d of glucose i. Red blood cells; cornea, lens, regions of the retina; kidney medulla, testis, leukocytes, type II muscle fibers How much carbohydrate do we need? a. The Dietary Reference Intakes recommend that children and adults consume at least 130 grams of carbohydrate per day b. The Accepted Macronutrient Distribution Range for carbohydrates is 45 -65% of total daily calories c. Average intakes by American adults: i. Males: ~300 g/d ii. Females: ~200 g/d iii. This equals ~50% of total calories from CHO. What happens if we do not consume enough carbohydrates? a. Liver glycogen stores are depleted after about 18 hours of fasting b. The body must rely on fatty acids and protein for fuel i. Proteins are catabolized and some amino acids are converted to glucose to aid in the breakdown of fatty acids c. The incomplete breakdown of fatty acids results in formation of ketone bodies d. After fasting for about 2 days, the body enters ketosis (elevated ketone levels in the blood) i. Brain can use ketone bodies for fuel - reduces requirement for glucose

3

ii.

6.

7.

Ketone bodies are acids, and excessive amounts can cause blood to become slightly acidic. If you are otherwise healthy, ketosis itself is not necessarily harmful iii. However, protein from muscle and organs continues to be broken down to make glucose iv. If fasting continues, protein reserves are depleted and death occurs What happens if we eat too much carbohydrates? a. Excess carbons from CHO drive hepatic triglyceride synthesis b. These triglycerides (TAG) are delivered to non-liver tissues via VLDL for storage c. Increase lipogenesis CHO Digestion a. Begins in the mouth by salivary a-amylase b. Stops in the stomach i. Salivary a-amylase is inactivated by low pH c. Continues in small intestine i. Pancreatic a-amylase digests dextrins, amylose, amylopectin and glycogen in the small intestinal lumen ii. Forms maltose and limit dextrins d. Brush border enzymes hydrolyze limit dextrins and disaccharides

e.

1.

Monosaccharides are absorbed through several transporters i. Absorption occurs by facilitated and active transport ii. CHO digestion/absorption is so efficient that nearly all monosaccharides are absorbed by the end of the jejunum

Blood glucose levels are tightly regulated

4

a. b.

A minimum amount of circulating glucose is required to meet the needs of tissues, but too much blood glucose over long periods of time can cause damage to tissues (extremities) Regulated by pancreatic hormones i. Senses the amount of glucose in the blood ii. Synthesizes hormones to maintain glucose within a particular range (60-100 mg/dL) iii. Insulin: “storage hormone” 1. Synthesized and released by beta cells 2. Secreted in response to high blood glucose levels 3. Decreases the amount of glucose in the blood iv. Glucagon: “counter-regulatory” hormone 1. Synthesized and released by alpha cells 2. Secreted in response to low blood glucose levels 3. Increases the amount of glucose in the blood

So far we know: ● Several tissues, including the brain, rely almost exclusively on glucose for energy ● A minimum level of blood glucose is required by the body, but too much glucose in the blood is also undesirable ● Blood glucose levels are tightly regulated ○ Insulin decreases the level of glucose in the blood ○ Glucagon increases the level of glucose in the blood Study Question: Amylase is secreted from the ____ and _____ for CHO digestion. A. chief cells; parietal cells B. salivary glands; stomach C. salivary glands; pancreas D. stomach; pancreas 1. Glucose cannot passively diffuse through cell membranes a. Glucose transporters (GLUT) move glucose across the plasma membrane b. GLUT-1, -2, -3, and -5 do not require insulin to move glucose into cells allow a basal level of glucose entry into cells c. GLUT-4 is insulin dependent (i.e. regulated) 2. GLUT-4 is the primary glucose transporter in muscle, heart, and adipose tissue 3. Insulin causes GLUT-4 to be trafficked from the cytosol to the plasma membrane, allowing glucose to enter

5

the cell

4.

5.

6.

7.

Glucose can be oxidized or stored a. Oxidation of glucose during glycolysis releases energy and generates intermediates used in other metabolic pathways i. Insulin stimulates glycolysis b. Glucose not immediately needed for energy can be stored in muscle and liver as glycogen through the process of glycogenesis i. Glycogen serves as a reserve of quick energy ii. ~75% of body glycogen is stored in and used by muscle (350- 800g) iii. Up to 7% of liver weight is glycogen (70-80g) c. The body’s capacity to store glycogen is finite. When glycogen stores are maxed out, glucose is stored as fat (lipogenesis) d. Insulin stimulates glycogenesis (and lipogenesis) The body can break down glycogen to release the stored glucose a. When blood glucose levels decrease, muscle and liver tissue can break down glycogen in the process of glycogenolysis i. Muscle tissue breaks down glycogen into glucose for oxidation within the muscle tissue ii. The liver breaks down glycogen and releases the glucose into the blood to maintain/increase blood glucose levels b. Glucagon (and nor/ epinephrine) stimulates glycogenolysis Gluconeogenesis forms glucose from non-carbohydrate sources a. When glycogen stores are depleted, the body makes new glucose molecules from noncarbohydrate sources in a process called gluconeogenesis i. Lactate, pyruvate, glycerol, and certain amino acids are precursors for glucose synthesis in the liver ii. Gluconeogenesis is a critical process for maintenance of blood glucose levels iii. Glucagon stimulates gluconeogenesis

Review of definitions

6

a. Glycogenolysis - breakdown of the storage form of glucose b. Glycolysis - oxidation of glucose for energy c. Gluconeogenesis - synthesis of new glucose molecules from noncarbohydrate precursors d. Glycogenesis - synthesis of polymers of glucose for storage in muscle and liver tissue 8. Which tissue(s) can make new glucose molecules from noncarbohydrate precursors? a. Liver 9. Insulin stimulates which of the following processes? a. Glycolysis in muscle b. Glycogenolysis in muscle and liver 10. Glucagon stimulates which of the following processes? a. Glycogenolysis in muscle and liver 11. Glycemic Index a. The rate of glucose absorption is an important factor in blood glucose homeostasis b. The glycemic index is a measure of the increase in blood glucose level during a 2- hour period following the consumption of a defined amount of CHO in a food compared with the same amount of CHO in a reference food (glucose or white bread) c. High glycemic index foods are rapidly digested and absorbed, causing a rapid rise and decline in blood glucose levels d. Low glycemic index foods cause a more gradual rise and fall in blood glucose levels

So far we know: ● We need a minimum level of glucose in the blood to adequately supply our tissues (e.g. the brain) with fuel ● To supply this glucose, we should eat at least 130 g/d of carbohydrates to provide 45-65% of total calorie intake ○ Carbohydrates are absorbed and raise blood glucose levels at varying rates depending on their composition and other properties of the food 1. Hypoglycemia: Low Blood Glucose a. Hypoglycemia is blood glucose below 70 mg/dl i. Occurs most frequently in patients receiving insulin treatment or undernourished or fasted individuals consuming alcohol b. Symptoms begin around 55 mg/dl i. Adrenergic symptoms are mediated by epinephrine release in response to hypoglycemia 1. Shakiness, nervousness or anxiety, sweating, chills and clamminess, rapid/fast heartbeat ii. Neuroglycopenic symptoms are due to impaired delivery of glucose to the brain 1. Usually develop due to a gradual decline in blood glucose, often to levels below 40mg/dl 2. Headache, confusion, including delirium, lightheadedness or dizziness,

7

blurred/impaired vision, slurred speech, lack of coordination, seizures, coma/unconsciousness, death 2. Ketosis occurs after prolonged fasting or deprivation of CHO a. The by-products of incomplete breakdown of fat are called ketone bodies and go directly into the bloodstream b. Ketone bodies are acids that can decrease the pH of the blood c. While glucose is the preferred energy source for the brain, ketones can be utilized 3. Hyperglycemia: high blood glucose a. Hyperglycemia is fasting blood glucose >126 mg/dl or 2h blood glucose >200 mg/dl b. Symptoms usually do not occur until blood glucose >200mg/dl i. Symptoms generally develop slowly (over weeks or months) ii. Early symptoms: frequent urination, increased thirst, blurred vision, fatigue, headache iii. Later symptoms: fruity-smelling breath (ketoacidosis), nausea and vomiting, shortness of breath, dry mouth, weakness, confusion, coma, abdominal pain c. High blood glucose levels damage vital organs i. Nerve damage, numbness, poor circulation 1. Infections, leg and foot amputations ii. Eye damage, blindness iii. Tooth and gum problems iv. Kidney damage v. Increased risk of heart disease Summary: ● Blood glucose is normally maintained within a tight range (60-110 mg/dl) ● Low blood glucose is called hypoglycemia. It is defined as blood glucose 126 mg/dl or postabsorptive 2-h glucose of >200 mg/dl. It occurs more slowly, and chronic hyperglycemia causes organ and tissue damage ● Ketosis can occur because of hypo- or hyperglycemia

8...