Nutritional Sciences: Carbohydrates

Classification and Properties of Simple Carbohydrates

  • Conceptual Overview of Carbohydrates: Carbohydrates are organic compounds composed of one or more sugar molecules. They are abundant in a wide variety of foods and are categorized based on their structural complexity.
  • General Classification:     * Simple Carbohydrates: Consist of monosaccharides (one sugar unit) and disaccharides (two sugar units).     * Complex Carbohydrates: Consist of oligosaccharides (33 to 1010 sugar units) and polysaccharides (more than 1010 sugar units).
  • Detailed Taxonomic Hierarchy:     * Monosaccharides: Glucose, Fructose, Galactose.     * Disaccharides: Lactose, Sucrose, Maltose.     * Oligosaccharides: Raffinose, Stachyose.     * Polysaccharides: Glycogen, Starch, and Dietary Fiber.
  • Monosaccharide Chemical Composition:     * Composed of Carbon (CC), Hydrogen (HH), and Oxygen (OO).     * The atomic ratio is 1C:2H:1O1C:2H:1O.     * Common forms in food are hexose sugars, meaning they contain 66 carbon atoms.     * The general formula for hexose sugars is C6H12O6C_{6}H_{12}O_{6}.
  • Structural Differences:     * Glucose and Galactose: These possess a 66-sided ring structure. They have nearly identical chemical structures, with the primary distinction being the direction of the hydroxyl groups (−OH-OH), which face in opposite directions.     * Fructose: This possesses a 55-sided ring structure.

Profiles of Specific Monosaccharides

  • Glucose:     * It is the most abundant form of sugar in the body and blood.     * It is produced in plants via photosynthesis.     * Function: Provides cells with a source of energy. It is the preferred or sole energy source for critical systems, specifically the nervous system and red blood cells.     * Metabolic Pathways: It can be converted into amino acids or fat, or stored as glycogen.
  • Photosynthesis Mechanism:     * Plants capture light energy from the sun and take in carbon dioxide and water.     * Formula: 6(CO2)+6(H2O)→1(C6H12O6)+6(O2)6(CO_{2}) + 6(H_{2}O) \rightarrow 1(C_{6}H_{12}O_{6}) + 6(O_{2}).     * The energy from the sun is transferred to the chemical bonds of glucose.
  • Fructose:     * It is the most abundant sugar found in fruits and vegetables.     * High-Fructose Corn Syrup (HFCS): A widely used commercial sweetener. Consumption trends show a sharp increase in HFCS use from the 1970s1970s peaking through the late 1990s1990s, paralleling discussions regarding the obesity epidemic.
  • Galactose:     * Few foods contain galactose in its free form.     * It is primarily derived from the dairy product disaccharide, lactose.     * In-Body Roles: It can be converted to glucose, used to build cell membranes, or used to synthesize lactose in breast milk.

Disaccharides and Glycosidic Bonds

  • Common Disaccharides:     * Lactose (Milk Sugar): Composed of Galactose and Glucose. Found in milk and most dairy products.     * Maltose: Composed of Glucose and Glucose. Result of starch breakdown; very few foods contain it naturally.     * Sucrose (Table Sugar): Composed of Fructose and Glucose. Often added to foods for sweetness.
  • Glycosidic Bonds: The chemical bond that joins two monosaccharides.     * Formation: The hydroxyl group (−OH-OH) on one glucose molecule bonds with a hydrogen atom (HH) from another molecule, releasing water (H2OH_{2}O).     * Alpha ($\alpha$) Bond: Faces down. Found in maltose (specifically an $\alpha−1,4-1,4 glycosidic bond).     * Beta ($\beta$) Bond: Faces up. Found in lactose.

Sugars and Sweeteners in the Diet

  • Naturally Occurring vs. Added Sugars: Food labels currently group both under the term "sugar." Naturally occurring sugars are generally superior for nutrient density compared to added sugars.
  • Consumption Statistics: The average intake of added sugars is approximately 89 grams/day89\,\text{grams/day}, which equates to 355 kcal355\,\text{kcal}.
  • Nonsugar Sweeteners: These are low-calorie alternatives with low nutritional value that do not necessarily facilitate weight loss.     * Common forms: Saccharin, Aspartame, Acesulfame K, Sucralose, Sugar alcohols, and Stevia.

Complex Carbohydrates: Oligosaccharides and Polysaccharides

  • Oligosaccharides:     * Contain 33 to 1010 monosaccharides.     * Sources: Beans, soybeans, peas.     * Types: Raffinose and stachyose.     * Biological Role: Components of the cell membrane for interaction and recognition.     * Digestibility: Humans lack the enzymes to digest them. Consequently, they pass into the large intestine where they can cause abdominal discomfort, bloating, and flatulence (gas).
  • Polysaccharides:     * Consist of more than 1010 monosaccharides. The most common types are starch, glycogen, and dietary fiber.

Detailed Structure of Starch and Glycogen

  • Starch: The storage form of glucose in plants.     * Amylose: A linear (unbranched) glucose chain bonded by $\alpha−1,4-1,4 glycosidic bonds.     * Amylopectin: A highly branched arrangement. It contains both $\alpha−1,4-1,4 bonds and $\alpha−1,6-1,6 glycosidic bonds at the branch points.     * Food Use: Common sources include grains and potatoes. Cornstarch is used in cooking to provide texture and stability to sauces and gravies.
  • Glycogen: The animal storage form of glucose.     * Location: Found in liver and skeletal muscles.     * Structure: Highly branched (containing $\alpha−1,4-1,4 and $\alpha−1,6-1,6 bonds) which allows enzymes to hydrolyze many glucose molecules simultaneously for quick release.     * Liver Glycogen: Can be broken down and released into the blood as glucose.     * Muscle Glycogen: Used locally; it does not release glucose into the blood.     * Carbohydrate Loading: A technique used to increase muscle glycogen stores to enhance physical activity performance.

Dietary Fiber: Classification and Physiological Impact

  • Definition: Carbohydrate components that cannot be digested by the human small intestine. They promote the growth of beneficial intestinal bacteria.
  • Classification:     * Dietary Fiber: Naturally occurring in plants (Soluble and Insoluble).     * Functional Fiber: Isolated fiber added to foods for health benefits.     * Total Fiber: The sum of Dietary Fiber and Functional Fiber.
  • Chemical Difference from Starch: While starch uses $\alpha−1,4-1,4 bonds, fiber frequently contains $\beta−1,4-1,4 glycosidic bonds that human enzymes cannot break.
  • Health Benefits:     * Cardiovascular Disease: Soluble, viscous fiber can lower blood cholesterol.     * Obesity and Type 2 Diabetes: Helps manage weight and blood glucose response.     * Digestive Health: Insoluble fiber absorbs water to prevent constipation and diverticular disease (weakening of the colon wall).
  • Diverticular Disease: The formation of small, protruding pouches called diverticula along the large intestine wall.
  • Common Dietary Fibers and Sources:     * Cellulose: Insoluble; main structural component of plant cell walls (Whole grains, bran, broccoli).     * Hemicellulose: Insoluble; variety of monosaccharides (Bran, cereals, legumes).     * Pectin: Soluble; found in ripening fruit skins; used for jams (Apples, citrus, strawberries).     * $\beta$-Glucan: Branched glucose chains (Mushrooms, barley, oats). Lowers cholesterol and reduces glycemic response.     * Gums: Highly soluble and viscous; used as thickeners (Oats, legumes, berries).     * Psyllium: Insoluble non-polysaccharide unit (Wheat). May decrease lipid absorption.

Anatomy of Whole Grains

  • Bran: The outer layer containing fiber.     * Germ: The core containing vitamins and minerals.     * Endosperm: The large middle section containing starch.
  • FDA Definition: Whole-grain foods must provide components in the proportions found in nature.

Digestion of Carbohydrates

  • Starch Digestion Process:     * Mouth: Salivary glands release salivary $\alpha-amylase$, which hydrolyzes $\alpha-1,4 glycosidic bonds in amylose and amylopectin to form **dextrins**.\n    * **Stomach**: No digestion occurs. Acidity of gastric juice destroys the enzymatic activity of salivary $\alpha-amylase$.     * Small Intestine (Pancreatic Phase): The pancreas releases pancreatic $\alpha-amylase$. This breaks dextrins down into **maltose** and **limit dextrins**.\n    * **Small Intestine (Brush Border)**: Enzymes on the surface of enterocytes complete digestion.\n        * **Maltase**: Hydrolyzes maltose into free glucose.\n        * **$\alpha$-dextrinase**: Hydrolyzes $\alpha-1,6 bonds in limit dextrins to form free glucose.\n* **Disaccharide Digestion**: Occurs on the brush border surface via specific disaccharidases:\n    * **Sucrase**: Sucrose $\rightarrow$ Glucose + Fructose.\n    * **Lactase**: Lactose $\rightarrow$ Glucose + Galactose.\n    * **Maltase**: Maltose $\rightarrow$ Glucose + Glucose.\n* **Lactose Intolerance**: Result of insufficient lactase. Symptoms include abdominal cramping, bloating, flatulence, and diarrhea. Risk factors include certain ethnic groups, diseases, medications, or surgeries.\n\n# Absorption and Circulation\n\n* **Mechanisms of Absorption**:\n    * **Carrier-Mediated Active Transport**: Used for **Glucose and Galactose**. This process requires energy (ATP) and moves molecules from lower to higher concentration with transport proteins.\n    * **Facilitated Diffusion**: Used for **Fructose**. Moves molecules from higher to lower concentration with the assistance of transport proteins.\n* **Circulation**: Absorbed monosaccharides enter the blood and travel directly to the liver via the **hepatic portal system**.\n\n# Glycemic Response: Index and Load\n\n* **Glycemic Response**: The change in blood glucose levels following the ingestion of food.\n* **Glycemic Index (GI)**:\n    * A scale of 0toto100.\n    * Pure glucose is the reference standard (100).\n    * Based on a standard amount (50\,g) of carbohydrate.\n    * **Values**: High ($\ge 70$), Medium (56-69), Low ($\le 55$).\n* **Glycemic Load (GL)**:\n    * Considers the actual portion of food eaten.\n    * Formula: GL = \frac{GI}{100} \times \text{grams of carbohydrate per serving}.\n    * **Values**: High ($\ge 20$), Medium (11-19), Low ($\le 10$).\n\n# Hormonal Regulation of Blood Glucose\n\n* **The Pancreas (Islets of Langerhans)**:\n    * **Beta ($\beta$) Cells**: Release **Insulin** in response to elevated blood glucose.\n    * **Alpha ($\alpha$) Cells**: Release **Glucagon** in response to low blood glucose.\n* **Action of Insulin**:\n    * Stimulates storage of glucose as glycogen (Glycogenesis) in the liver.\n    * Enables **insulin-responsive glucose transporters** to take up glucose from blood into skeletal muscle and adipose tissue.\n    * Promotes protein synthesis in muscle and fat synthesis in adipose tissue.\n    * Lowers blood glucose.\n* **Action of Glucagon**:\n    * Stimulates the breakdown of liver glycogen (Glycogenolysis).\n    * Promotes **Gluconeogenesis**: Synthesis of glucose from noncarbohydrate sources (amino acids, glycerol, lactate).\n    * Increases blood glucose.\n* **Adrenal Hormones (Fight-or-Flight)**:\n    * **Epinephrine and Cortisol**: Released for immediate energy. They stimulate glycogenolysis and gluconeogenesis.\n\n# Metabolic Pathways and States\n\n* **Energy Storage Conditions**:\n    * **Glycogenesis**: Conversion of glucose to glycogen in liver and muscle. In the liver, this is reversible. In adipose tissue, storage as fat is irreversible.\n* **Hypoglycemia**: Low blood glucose.\n    * **Reactive**: Occurs shortly after a meal.\n    * **Fasting**: Often caused by excessive insulin (e.g., from a pancreatic tumor).\n* **Ketogenesis and Ketosis**:\n    * Breakdown of fatty acids in the relative absence of glucose produces **ketones**.\n    * Occurs in the liver; stimulated by glucagon.\n    * **Ketosis**: Occurs when the production of ketones exceeds their usage.\n\n# Dietary Requirements\n\n* **Dietary Reference Intakes (DRIs)**:\n    * **RDA**: Minimum of 130\,g/dayforadults.Forpregnancy/lactation,itincreasestofor adults. For pregnancy/lactation, it increases to>210\,g/day.\n    * **AMDR**: 45toto65\%oftotaldailycalories(approximatelyof total daily calories (approximately300\,g/dayforafor a2,000\,kcal diet).\n    * **Added Sugar Limit**: Less than 25\% of total calories.\n    * **Fiber Adequate Intakes (AI)**:\n        * Target: 21toto38\,g/day.\n        * Average American intake: only 15\,g/day.\n\n# Diabetes Mellitus\n\n* **General Definition**: A metabolic disorder characterized by elevated blood glucose levels.\n* **Classification**:\n    * **Type 1 Diabetes**: Autoimmune disorder. Antibodies destroy pancreatic $\beta$-cells, leading to a lack of insulin production. Requires daily insulin injections. Primarily develops in childhood/adolescence. Can lead to **diabetic ketoacidosis** (coma or death).\n    * **Type 2 Diabetes**: Most common form (effective for 1outofout of4 people in the U.S.). Characterized by **insulin resistance** in skeletal muscle and adipose tissue. Transport of glucose across membranes is impaired.\n        * **Risk Factors**: Ethnicity (African, Hispanic, Native American, Pacific Island descent), Genetics, Obesity (BMI $\ge 25\,kg/m^{2}$), Sedentary lifestyle, high BP ($\ge 140/90\,mmHg$).\n        * **Management**: Lifestyle changes (diet, activity), weight loss, and medications.\n    * **Gestational Diabetes**: Temporary insulin resistance brought on by hormonal changes during pregnancy. Affects 4toto7\%$$ of pregnant women. Increases risk for later Type 2 diabetes.     * Secondary Diabetes: Result of other diseases, medical conditions, or specific medications.
  • Symptoms Comparison:     * Both Types: Frequent urination, excessive thirst, fatigue.     * Type 1 Specific: Unusual weight loss, extreme hunger, ketosis.     * Type 2 Specific: Frequent infections, blurred vision, slow-healing cuts/bruises, tingling in extremities (neuropathy).