Carbohydrate Digestion and Classification

Carbohydrate Digestion, Absorption, and Metabolism

• Carbohydrates are crucial for energy and health
• Humans eat carbohydrates, which the body converts to glucose for immediate energy and glycogen for storage
• Key elements are carbon, oxygen, and hydrogen. These elements have specific bonding capacities
• Carbohydrates are classified as simple (monosaccharides and disaccharides) and complex (polysaccharides)

Carbohydrate Classification

• Simple Carbohydrates:

  • Monosaccharides: Single sugar units (e.g., glucose, fructose, galactose)
    • Glucose: primary energy source, mildly sweet
    • Fructose: intensely sweet
    • Galactose: hardly sweet
  • Disaccharides: Double sugar units (e.g., sucrose, lactose, maltose)
    • Sucrose (glucose + fructose): table sugar
    • Lactose (glucose + galactose): milk sugar
    • Maltose (glucose + glucose): product of starch digestion

• Complex Carbohydrates:

  • Polysaccharides: Multiple sugar units (e.g., starch, glycogen, dietary fiber)
    • Starch: stored form of glucose in plants, primary source of dietary carbohydrates
    • Glycogen: storage form of glucose in animals
    • Dietary fiber: non-digestible component of plants, promotes bowel health

Monosaccharides

• Most monosaccharides in nutrition are hexoses (6-carbon sugars)
• Monosaccharides are the simplest form of carbohydrate that cannot be broken down into smaller units
• The three important monosaccharides have the same chemical composition but differ in arrangement (C6H12O6)
• They differ in how sweet they taste

Glucose

• The basic single sugar in body metabolism
• Supply the primary fuel for cells
• Not usually found as such in the diet
• Comes mainly from the digestion of starch
• Also known as blood sugar or dextrose

Fructose

• Sweetest of the sugars
• Has the same chemical formula as glucose but a different structure
• Occurs naturally in fruits and honey
• The amount of fructose in fruits varies with ripeness
• Also called fruit sugar

Galactose

• Naturally found as a single sugar in few foods
• Has the same number and kinds of atoms as glucose and fructose, but a different arrangement
• Not typically found as such in the diet, part of lactose
• Milk is the primary source
• Contained mainly from the digestion of milk sugars

Disaccharides

• Pairs of monosaccharides linked together
• Glucose is the second element in all of the three types
• Other elements are fructose, galactose, or another glucose
• Formed through a condensation reaction, whereby water is released.
• Broken down via hydrolysis whereby water is added.

Sucrose

• Also known as table sugar
• Formed naturally in fruits, vegetables, and grains
• Refined from sugar cane and sugar beets
• Available in various forms from brown to white to powdered, dependence in the level of refinement
• Hydrolyzed by enzyme sucrase in the small intestine

Lactose

• Also known as milk sugar
• Primarily derived from plants
• Lactose is the only common sugar not found in plants
• Less soluble and less sweet than sucrose
• Broken down by enzyme lactase in the small intestine .

Maltose

• Composed of two glucose units
• Primarily derived from the body in response to the breakdown of starch
• Also considered a product of starch digestion in the body
• Not generally found in diet

Polysaccharides

• Composed of many single sugar units linked together
• Important polysaccharides include starch, glycogen, and dietary fiber
• Starch: found primarily in grains, legumes, and tubers
• Glycogen: found primarily in liver and muscles
• Dietary fiber: not digested, promotes bowel health

Starch

• Long chains and unbranched chains of glucose units
• Insoluble polysaccharide
• Primary source of dietary carbohydrates
• Broken down into glucose in the body during digestion
• Remains insoluble, swell in water and promote bowel health

Glycogen

• Storage form of glucose in animals
• Found in liver and muscles
• Highly branched chains of glucose molecules
• Allows for rapid hydrolysis
• Maintains blood glucose during fasting

Dietary Fiber

• Structural parts of plants
• Found in plant-derived foods (vegetables, fruits, whole grains, legumes).
• Most dietary fibers are polysaccharides
• Cannot be broken down by human digestive enzymes.
• Contribute no monosaccharides and little energy
• Can be beneficial to gut bacteria

Soluble Fiber

• Dissolves in water
• Forms a gel (viscous)
• Fermented by bacteria in the colon
• Commonly found in oats, barley, legumes, and citrus fruits
• Associated with protecting against heart disease and diabetes

Insoluble Fiber

• Does not dissolve in water
• Does not form gels
• Less readily fermented
• Found mainly in whole grains, bran, and vegetables
• Promotes bowel health

Types of Fibers

• Cellulose: primary constituent of plant cell walls
• Non-cellulose polysaccharides: including pectin, gums, and mucilage
• Lignin: woody parts of vegetables

Total Fibers

• Can be the sum of dietary fibers and functional fibers
• Functional fibers: extracted from plants or manufactured, added to foods or used as supplements

Resistant Starch

• A category of starch classified as dietary fiber
• Resists digestion in the small intestine
• Starch may resist digestion for various reasons, including starch-digesting efficiency, and food physical properties
• High in whole legumes, raw potatoes, unripe bananas

Phytic Acid

• Not classified as fiber
• Often accompanies fiber in foods
• Binds with minerals, preventing absorption
• Risk of mineral deficiencies is minimized with sufficient mineral intake alongside fiber.

High-Fiber Diet

• Diet exceeding 40 grams of dietary fiber daily
• Sources include fruits, vegetables, breads, and cereals
• Dietary fiber can cause several digestive issues like abdominal fullness, increased flatulence and nausea

Carbohydrate Sources

• Table presents sources of carbohydrates based on chemical classes.

Recommended Intakes of Carbohydrates

• Recommendations suggest carbohydrates should provide more than half of energy needs.
• Daily servings of vegetables and fruits, cereals, grains and legumes should be consumed.
• Minimum daily intake of 20-25 grams of dietary fiber is recommended

Functions of Carbohydrates

• Basic fuel supply: provide primary fuel
• Reserve fuel supply: store as glycogen
• Special tissue functions: liver and muscle glycogen maintains energy exchange in the body.
• Protein sparing action: Prevents protein breakdown for energy if adequate carbohydrate intake is available
• Central nervous system function: Brain depends on glucose for energy
• Fiber function: cellulose and other non-cellulose polysaccharides promote proper bowel movements, absorb water, and prevent spastic colon. Assist in the absorption of bile (and other cholesterol based substances)

Digestion, Absorption, and Metabolism of Carbohydrate

• Goal: break down carbohydrates into simple sugars for absorption and energy
• Initial digestion begins in the mouth with mastication
• Saliva contains amylase to start starch digestion.
• Stomach acid stops starch digestion; small amounts continue to be broken down in the stomach
• Further digestion in small intestine involving pancreatic amylase to convert starch to monosaccharides
• Hydrolysis of disaccharides inside the small intestine into monosaccharides
• Monosaccharides are absorbed
• Some substances like fructose and galactose are converted into glucose in the liver for utilization.

Glucose in the Body

• Every body cell depends on glucose to some degree
• Nervous system depends almost exclusively on glucose for energy
• Glucose is continuously supplied via blood to meet energy needs from glycogen breakdowns or through absorption

Maintaining Glucose Homeostasis

• Importance of maintaining blood glucose within a normal range
• Blood glucose homeostasis is maintained by insulin and glucagon
• Insulin: moves glucose into cells
• Glucagon: brings glucose out of storage

Making Glucose from Protein

• Body can convert protein into glucose when carb stores are low, a process called gluconeogenesis
• Adequate carbs spare protein from being used as an energy source

Making Ketone Bodies from Fat Fragments

• An insufficient supply of carbohydrates can shift body energy metabolism
• Fat is broken down into ketone bodies to provide energy
• Excessive ketone production leads to ketosis, disrupting acid-base balance

Using glucose to make fat

• When glucose exceeds the body's needs, the liver converts excess glucose to fat that is stored in adipose tissues

Metabolism of CHO

• Glycogenolysis: breakdown of glycogen for glucose
• Glycogenesis: synthesis of glycogen to store excess glucose

Lactose intolerance

• Inability to digest the milk sugar lactose
• Symptoms include bloating, gas, abdominal discomfort, diarrhea
• Lack of lactase enzyme to break down lactose

Galactosemia

• Genetic disorder affecting galactose metabolism
• Leads to toxic buildup of galactose-1-phosphate, causing serious health issues such as liver damage, kidney problems, and cataracts.

Glycemic Response

• The rate at which a food raises blood glucose levels
• Extent to which a food raises glucose levels and elicits an insulin response
• High glycemic response can cause issues with blood glucose regulation

Glycemic Index

• A measure of how quickly a food raises blood glucose.
• Classifies food according to their potential for raising blood glucose

Sources of Blood Glucose

• Dietary carbohydrates, glycogen stored in the body, and breakdown products from other sources such as lactic and pyruvic acids.

This quiz covers the essential aspects of carbohydrate digestion, absorption, and metabolism. Participants will learn about different types of carbohydrates, including simple and complex forms, as well as their functions in energy production and storage. Test your knowledge on monosaccharides, disaccharides, and polysaccharides!