Lecture 3 - Carbohydrates PDF

LECTURE 3: CARBOHYDRATE S KIN 150 – Fall 2024 Sophia Williams, MS, RD  Carbohydrates are a class of organic molecules consisting of a carbon (C) backbone with attached oxygen (O) and hydrogen (H) atoms. Carbo means “carbon” WHAT IS...

LECTURE 3: CARBOHYDRATE S KIN 150 – Fall 2024 Sophia Williams, MS, RD  Carbohydrates are a class of organic molecules consisting of a carbon (C) backbone with attached oxygen (O) and hydrogen (H) atoms. Carbo means “carbon” WHAT IS A Hydrate means “water,” or H2O, thus giving a hint CARBOHYDRATE as to how these molecules are formed. ? STRUCTURE The chemical formula for these simple sugars is CnH2nOn, where n equals a number from 3 to 7. Example: the most important simple sugar for the human body is glucose. It has 6 carbons in its chemical structure Thus, its formula is C6H12O6. HOW ARE THEY FORMED? Glucose and most of the other types of CHOs that exist in nature are synthesized by plants in a process, known as photosynthesis The energy required to construct a CHO comes from the sun. The sun’s light energy is captured by plants and used to combine carbon dioxide (CO2) from the air and water (H2O) from the soil to create simple sugars.  Carbohydrates are composed of simple sugars known as monosaccharides (most basic building block of CHOs). Disaccharides: 2 linked simple sugars Oligosaccharides: 3 to 10 linked simple sugars (complex) Polysaccharides: 11 or more linked simple sugars (complex) WHAT IS A CARB? SIMPLE CARBOHYDRATES Monosaccharides (mono: one, saccharide: sugar) Glucose Most important and most common It rarely exists as a monosaccharide in food but is joined with other sugars to form disaccharides and other complex CHOs. Main source of energy for the body (circulates in the blood) Fructose Abundant in fruit, honey, and vegetables The sweetest sugar Galactose Sugar in milk Found in dairy products, fruits, and vegetables SIMPLE CARBOHYDRATES Disaccharides (di: two, saccharide: sugar) Maltose = glucose + glucose Sweet potatoes, pears, honey Wheat, barley, cornmeal Sucrose = glucose + fructose Naturally in fruits, vegetables, nuts In a lot of fast foods and processed packaged foods Lactose = glucose + galactose Milk and dairy products Butter COMPLEX CARBOHYDRATES Polysaccharides (poly: many, saccharide: sugar) Starch: storage form of energy in plants Contains 2 polymers composed of glucose units: Amylose: straight-chain polymer Amylopectin: highly-branched polymer Glycogen: storage form of energy in animals Structure similar to Amylopectin Easily broken down Cellulose: insoluble, thick fiber that gives structural integrity to plant cell walls, fruits, and vegetables COMPLEX CARBOHYDRATES IN FOOD The complex carbohydrates found in foods are starches and fiber Starches: Grains, rice, pasta, cereals, breads, legumes Some vegetables: potatoes, yams, green peas, corn, squash Fun fact: the CHO composition in bananas changes during the ripening process (starch  simple sugars) which makes it sweeter Fiber: All plant foods contain some fiber of varying types. Most fiber is indigestible by the body, and therefore provides no caloric or carbohydrate value TWO TYPES OF FIBER Classified based on their solubility in water: soluble vs. insoluble Consuming foods that contain both soluble and insoluble fiber can help prevent high cholesterol and diverticular disease, regulate blood glucose levels, and help prevent and/ or treat constipation. Recommended intake for total fiber/day for men vs. women by age category TWO TYPES OF FIBER Soluble Fiber Insoluble Fiber Soluble in water: forms a gel, binds to cholesterol in Not soluble in water: rather, it absorbs fluids and the intestine to remove from body sticks to other materials Functions: Functions: Prevents/reduces diarrhea (by attracting water) Adds bulk to stools  eases and prevents constipation Too much  constipation Too much  diarrhea Decreases rate of gastric emptying Increases rate of gastric emptying Slows digestion & nutrient absorption  satiety Supports insulin sensitivity Blood glucose control and lowers blood cholesterol Colon & digestive health Foods: oats, barley, nuts, legumes (dried beans, peas, Foods: whole grain products (whole wheat, quinoa, lentils), fruits (strawberries, citrus, apples), many brown rice), nuts, seeds, and some vegetables (leafy vegetables. greens), berries. FIBER RICH FOODS WHAT IS THE Complex carbohydrates take DIFFERENCE longer to digest and provide BETWEEN longer-lasting energy SIMPLE AND Simple carbohydrates are quicker COMPLEX to digest and provide immediate energy CARBOHYDRA TES? THEY UTIMATELY END UP AS EXACTLY THE SAME COMPOUND: GLUCOSE WHY DO WE NEED GLUCOSE? CELLULAR RESPIRATION Cellular Respiration: How we derive energy from the food we eat. Why is glucose so important? Why do we need glucose? To initiate glycolysis Why is glycolysis important? To make ATP (Adenosine Triphosphate) Adenosine Triphosphate Why is ATP important? Almost all cellular processes need ATP (proved energy to energy-requiring reactions) Including: cell division synthesis of proteins from amino acids active transport muscle cell contraction transmission of nerve impulses Note: ATP is too unstable to store; therefore, cells store energy in the form of glucose  The monosaccharides glucose and galactose are absorbed across the intestinal lining via active transport (i.e., requiring transporter proteins) TRANSPORT OF SGLUT1: the transporter protein found in the GLUCOSE intestines SGLT1 = Sodium Glucose Cotransporter 1 For SGLT1 to transport these simple sugars through the intestinal cell membrane, it must first bind to a sodium ion (Na+) TRANSPORT OF GLUCOSE, CONT. Once the simple sugar molecules cross the intestinal cell membranes and enter the blood, they are transported to the liver via the hepatic portal system STORAGE OF CARBOHYDRATES Once the simple sugars reach the cells of the liver, those that are not in the form of glucose (e.g., fructose, galactose) are converted to glucose. The glucose can then be stored as glycogen, released into the bloodstream (to be used as energy), or converted to fat (stored in adipose tissue) The two major sites of glycogen storage: liver skeletal muscle The concentration of glycogen is higher in the liver than in muscle (10% versus 2% by weight), but more glycogen is stored in skeletal muscle overall because of its much greater mass. STORAGE OF CARBOHYDRATES CHOs are stored in the body (liver and muscle) as glycogen The body can store 400 – 600 grams of CHOs in the liver and muscle = 1,600 – 2,400 kcal of stored energy, depending on body size, time of day, dietary intake Only ~400 – 500 kcal are directly available to be used for maintaining BG levels The remaining 1,200 – 1,900 kcal from glycogen are found in muscle cells Muscle cells cannot directly release stored glucose directly into the bloodstream, whereas liver cells can release stored glucose back into the bloodstream to maintain BG levels between meals Once liver glycogen is depleted, BG levels begin to decrease Compared to fats (the major source of energy in the body), very little glycogen is stored Fat cells (adipocytes) can store ~90,000 kcals of energy and can share this energy with the rest of the body, but it is less efficient GLYCOGEN DEPLETION During muscular activity (exercise), glycogen  lactate  blood glucose One of the reasons that fatigue during exercise occurs is the depletion of glycogen stores in active muscles After exercise, the rate of glycogen synthesis is increased  helps to replenish glycogen stores INSULIN AND GLUCAGON Both glucagon and insulin are hormones produced in the pancreas Glucagon: promotes the breakdown of glycogen to glucose in the liver Raises BG levels Made and released by alpha cells Insulin: regulates the amount of glucose in the blood Lowers BG levels Made and released by beta cells T1 Diabetes: destruction of insulin-producing beta cells DIABETES Diabetes results when either: The beta cells cannot produce enough insulin to lower blood glucose levels (T1DM) The beta cells produce insulin to which the body’s tissues do not respond normally (T2DM) Can cause abnormally high BG levels (sometimes >2-4x the normal level) or low BG levels GLYCOGEN STORES FULL Lipogenesis: synthesis of lipids - fatty acids and triglycerides – from various precursors. Broad term. De Novo Lipogenesis: synthesis of fatty acids from non-lipid precursors (carbohydrates, protein, alcohol, etc.) which bind to glycerol molecules to form fat GLYCEMIC INDEX (GI) The glycemic index (GI) indicates how much a certain food raises blood glucose (BG) levels when consumed in isolation. The index is calculated by measuring the incremental area under the BG curve following ingestion of a test food that provides 50 grams of CHOs (minus the fiber) over a 2- hour period, compared with the area under the curve following an equal carbohydrate intake from a reference food. GI testing occurs after an overnight fast Glucose and white bread are most often used as the food standard GI value of 100 A GI of 70 indicates that consuming 50 grams of the food in question provides an increase of blood glucose 70% as great as that for ingesting 50 grams of pure glucose. GLYCEMIC INDEX (GI) In testing, 50g of CHO are ingested 50g of CHO in one sitting may be reasonable for some foods, but not others 1 cup rice = 53 g CHO White rice GI = 72 Brown rice GI = 50 1 cup beets = 13 g CHO 4 cups = 52 g CHO, GI = 64 Low GI food does NOT mean it is ‘healthier’ than a higher GI food What affects Glycemic Index? Timing of Meal Food Combinations Time since last meal can affect GI Combining CHO food with other foods can impact Form of the Food GI Liquid sources of CHO tend to have higher GI Foods cooked with fat and protein can slow CHO absorption  lower GI Amount Consumed Quantity of CHO (more  higher GI) Type of Carbohydrate Cannot be determined based on classification Fiber Content (mono-, di, polysaccharides). Higher fiber typically lowers GI of a food Too simplistic to instruct people to eat more Fiber Type complex than simple CHOs to keep glycemic Soluble fiber tends to have a lower GI than response low insoluble Protein & Fat Content Breaking down the soluble fiber lowers the GI of Higher protein content of a CHO food slows CHO other foods when they are digested, while absorption  lower GI insoluble fiber cannot be broken down Higher fat content of a CHO food slows CHO absorption  lower GI TERMS Genesis = build up Lysis = break down Glycogen: the storage form of carbohydrates in animal cells Glycogen consists of intricately branched chains of linked glucose molecules. Gluconeogenesis: formation of glucose from noncarbohydrate sources such as proteins Glycogenesis: formation of glycogen from glucose Glycogenolysis: breakdown of glycogen into glucose Provides immediate energy Helps maintain blood glucose levels during fasting Glycolysis: the breakdown of glucose by enzymes Releases energy that is required for cellular metabolism ADDED SUGAR RECOMMENDATIONS The American Heart Association suggests an added-sugar limit of no more than: Females: 24 g of sugar (6 teaspoons) Males: 36 g of sugar (9 teaspoons) The Dietary Guidelines for Americans recommends limiting added sugars to