Understanding Carbohydrates: Composition & Classification

• Carbohydrates are present in high amounts in the body, specifically more than 0.005% of body weight.
• Carbohydrates provide energy, maintain structure, and ensure functional integrity of the body.

Carbohydrates Composition

• Carbohydrates consist of carbon, hydrogen, and oxygen, represented as (CHO).
• Carbohydrates referred to as "carbo" indicates carbon, and "hydrate" implies water.
• Carbohydrates arranged as either monosaccharides or multiple monosaccharides.
• Plants create carbohydrates through photosynthesis by combining CO2, H2O, and the sun's energy to produce glucose.

Ratio of 1C

• The ratio of 1C is 1H20 (CH2O).
• The molar ratio C:H:O is 1:2:1.
• Monosaccharides have the chemical formula C6H12O6.

Classification According to Complexity

• Monosaccharide (Simple): A single sugar unit
• Disaccharide: Contains 2 sugar units
• Oligosaccharide: Contains 3-10 sugar units
• Polysaccharide (Complex): Contains more than 10 sugar units

Classification According to Digestibility

• Digestible carbohydrates include sugars, starches, dextrin, and glycogen.
• Partially digestible carbohydrates include galactogens, mannosan, inulin, and pentosans.
• Indigestible carbohydrates include cellulose and hemicellulose.

Simple Carbohydrates

• Include glucose, fructose, and galactose.
• They possess the same number and kind of atoms but differ in arrangement.
• Simple carbohydrates vary in sweetness.

Glucose

• Also known as physiologic sugar or blood sugar.
• It is the primary product of hydrolysis from starch and sugars.
• It gets oxidized in cells to release energy.

Fructose

• Recognized as fruit sugar or levulose.
• Fructose is one of the two sugars found in every disaccharide and forms the exclusive unit of all polysaccharides.
• It is the sweetest carbohydrate.
• The arrangement of atoms in fructose stimulates taste buds on the tongue, producing a sweet sensation.
• It's hydrolyzed from sucrose and inulin.
• Present in honey, ripe fruits, and vegetables.
• Found in soft drinks, cereals, and desserts sweetened with High Fructose Corn Syrup (HFCS).

Galactose

• Does not occur in foods on its own, but only as part of lactose.
• It results from the hydrolysis of lactose (milk sugar).
• Converted to glucose in human metabolism.
• Functions as a component of cerebrosides in brain and nerve tissues.

Condensation

• Links a pair of monosaccharides.
• A hydroxyl (OH) group from one monosaccharide combines with a hydrogen atom (H) from another to create a water molecule.
• Two separate monosaccharides then link via a single oxygen atom.

Hydrolysis

• Breaks a disaccharide into two monosaccharides.

Sucrose

• Formed from Gluc + Fruc.
• Found in cane, beet, and table sugar.
• Abundant in molasses, sorghum, maple syrup, and certain fruits.

Lactose

• Formed from Gluc + Galact.
• Known as milk sugar.
• Least sweet among the disaccharides.
• Slowly digested.

Maltose

• Formed from Gluc + Gluc.
• Produced during starch breakdown.
• Involved in CHO digestion and fermentation.

Hydrolyzed

• Hydrolyzed from cereal grains by enzymes in the saliva and intestines.
• Ptyalin or salivary amylase is an enzyme involved in saliva.
• It combines with dextrin for infant milk formulas.

Reducing Sugar

• Contains a free carbonyl group that can react with an oxidizing agent.
• Maillard reaction: Reducing sugar + oxidizing amino acids.
• LEORA, GEORA
• Tollens reagent indicates a silver reaction when there is reducing sugar.
• Benedict's solution is a color reaction test.

Polysaccharides

• Made of many monosaccharides linked together.

Glycogen

• Also known as animal starch.
• It is the storage form of carbohydrate in the body.
• This is a highly branched chain that allows for rapid hydrolysis.
• Found in the liver and muscles.
• Can be converted to lactic acid.

Starch

• Serves as a storage form of carbohydrates in plants.
• Found in grains, seeds, tubers, roots, and unripe fruits.
• Complete hydrolysis of starch yields glucose.
• Partial hydrolysis of starch yields glucose.
• It's a complex dietary carbohydrate.

Two components

• Amylose straight (15 - 20%)
• A-14 glycocidic bond
• Amylopectin branched (80 - 85%)
• At 24-30 glucose residues joined by glycosidic linkages with 1-6 linkage

Fiber

• Non-starch polysaccharide found in plant foods.
• Non-digestible because we lack the necessary enzymes.
• Includes cellulose, hemicellulose, pectins, gums, mucilages, and non-polysaccharides like lignins, cutins, and tannins.
• Some fibers broken down by bacteria in the GI tract.
• 20-30 g per day recommended

Soluble (Viscous) Fiber

• Includes gums, pectins, hemicellulose, and mucilages.
• Found in fruits (apples and citrus), carrots, strawberries, oats, barley, and legumes.
• They form a gel and delays the GI transit time (slowing digestion).
• Delays glucose absorption.
• Lowers serum cholesterol (binding).

Insoluble Fiber

• Includes cellulose, many hemicelluloses, and lignin.
• Found in wheat bran, corn bran, whole grain breads and cereals, fruits, edible seeds, and mature vegetables.
• It accelerates GI transit.
• Increases fecal volume/weight.
• Delays glucose absorption.
• Increases water-holding capacity.

Benefits of Dietary Fiber

• Acts as immunomodulators.
• Inhibits growth of harmful microorganisms.
• Reduces food intolerances and allergies.
• Facilitates the growth of a healthy intestinal flora.
• Produces nutrients and digestive enzymes.
• Increases fecal mass and stimulates peristalsis.
• Fiber plays a role in energy balance and body weight control.
• It is also important in constipation, colorectal cancer, CHD, and type II DM.

Carbohydrate Digestion (STARCH)

• Mouth and salivary glands
  • Salivary glands secrete saliva that listen the food
  • Amylase begins digestion, converting starch into small polysaccharides and maltose.
• Stomach
  • Acids inactivates salivary enzymes, halting digestion.
• Small intestine and pancreas
  • Starch → amylase → small polysaccharide, maltose
    • Disaccharide enzymes on the surface of the small intestinal cells hydrolyze the disaccharide into monosaccharides
    • Breakdown of maltose(maltase) into glucose + glucose
    • Sucrose (sucrase) produces glucose + fructose
    • Lactose (lactase) yields glucose + galactose

Carbohydrate Digestion (Fiber)

• Mouth
  • The mechanical action of fiber helps to moisten it for swallowing.
• Stomach
  • Fiber is not digested in the stomach.
  • It delays gastric emptying.
• Small Intestine
  • Fiber is not digested here.
  • It delays the absorption of other nutrients.
• Large Intestine
  • Most fiber passes intact through the digestive tract to large intestine.
  • Bacterial enzyme digests fiber
  • Some fiber bacterial enzyme produces short chained FA, gas - Fiber hold water, regulates bowel, bind substances (bile, cholesterol, and some minerals) carrying the, out of the body

Resistant Starch

• Escapes digestion and absorption in small intestine
• Depends on individual's efficiency in digesting starches and food's physical properties
• Whole legumes, raw potatoes, unripe bananas
• The heat of cooking gelatinizes the starch granules, increasing their susceptibility to enzymatic (a-amylase) breakdown.

Types of Resistant Starch

• starch: partially milled grains and seeds
• ungelatinized crystallite granules of the B-type x-ray pattern (found in bananas and potatoes)
• retrograded amylose (formed during the cooling of starch gelatinized by moist heating)

Carbohydrate Absorption

• Glucose and Galactose traverse the cell lining the small intestine by ACTIVE TRANSPORT.
• Fructose gets absorbed by FACILITATED DIFFUSION
  • which slows its entry and produces a slow rise in blood sugar
  • As the blood from the intestines circulates through the liver, cells take up fructose and galactose and convert them to glucose

Simple diffusion

• Some nutrients, water, and small lipids cross into intestinal cells freely

Active transport

• Glucose and amino acids
• Move against a concentration gradient which requires energy

Liver

• stores 1/3 of the body's total glycogen and releases glucose as needed
• stores last for hours: 12 to 18 hours of fasting

2/3 glycogen

• stored in the muscles and used as fuel during exercise
• Well eaten = rise blood glucose
  • liver cells link the excess glucose molecules into long branched chains of glycogen

Blood glucose falls

• glycogen is broken down into glucose and released into the bloodstream

Glucose Homeostasis

• After eating - blood glucose rise
• High blood glucose stimulate pancreas to release insulin
• Insulin action
  • stimulate the uptake of glucose into cells and storage as glycogen in liver and muscle
  • stimulates the conversion of excess glucose into fat for storage
• Blood glucose declines
• Low blood glucose stimulate the release of glucagon
• Glucagon stimulate liver cells to break down glycogen and release glucose into the blood
• Blood glucose begin to rise

Glycogenesis

• Formation of glycogen from glucose for storage

Glycogenolysis

• Breakdown of glycogen into glucose for energy.

Glycolysis

• Breakdown of glucose into pyruvate to produce energy.

Gluconeogenesis

• Creation of glucose from non-carbohydrate sources.
• Helps maintain energy production when oxygen is scarce.

Glucose Transporters

• GLUT 1
  • Erythroid - Brain Carrier
  • widely distributed in many tissues
    • heart, kidney, adipose cells, fibroblasts, placenta, retina, and brain
  • little is expressed in muscle or liver.
  • Part of blood-brain barrier
  • Asymmetry allows the transporter to be effective when:
    • extracellular glucose is low
    • intracellular demand is high.
• GLUT 2
  • Liver Glucose Transporter
  • has a low affinity for glucose and shows symmetric transport
  • Transport galactose, mannose, and fructose
    • Fructose transport is for GLUT 2 and 5 only
  • predominate expression in ẞ-cells of pancreas, liver, kidney, and small intestine
• GLUT 3
  • Brain Glucose Transporter
  • Present in all tissues
  • Highest in: adult brain (neurons), kidney, and placenta
    • Highly glucose dependent
    • Low levels: adult muscle
  • GLUT 4
  • Insulin-Responsive Glucose Transporter
  • major glucose transporter of:
    • insulin-sensitive tissues,
    • brown and white fat (adipocytes)
    • skeletal and cardiac muscle.
  • Insulin stimulation causes a rapid increase in the number of glucose transporters on the membranes of these cells because the vesicles are translocated toward the membrane and then fuse with it, releasing the molecule. regulation are important components of glucose homeostasis and its role in diabetes
• GLUT 5 -Fructose Transporter
  • primarily expressed in the jejunum (both in the brush border and basolateral membrane)
  • Low levels:
    • human kidney, skeletal muscle, and adipocytes, microglial cells, and the blood brain barrier.
  • Poor glucose transporter.

Blood Glucose

• Normal adult range
  • :3.9 to 5.8 mmol/l
  • 70-105 mg/100 mL
• Energy Utilization
  • A healthy 70-kg man stores approximately:
  • 70 g of liver glycogen,
  • 200 g of muscle glycogen
  • 30 g of glucose in body fluids,
  • Total: 1200 kcal can meet energy needs for only 12 to 18 hours
• Fasting Blood Glucose
  • Normal: 70 - 90 mg/dL
  • Prediabetes: 100 - 125 mg/dL
  • Diabetes: >= 126 mg / dL
• Postprandial (After Eating)
  • 80 mg / dL after an overnight fast
  • 120 mg / dL after meal
• Increase Blood Glucose
  • Insulin = fuel storage (muscle and adipose)
  • Glycogen in liver and muscle
• Decrease
  • Proteolysis and lipolysis
• Post Absorptive
  • Increase
    • Glucagon levels
    • Hepatic glycogenolysis, gluconeogenesis
  • Decrease
    • Insulin = decrease peripheral glucose uptake
• Hormones Involved in Metabolism
  • 51 amino acid peptide
  • Insulin
    • Acts to lower blood glucose levels by facilitating its entrance into insulin-sensitive tissues and the liver by increasing the level of transporters in tissues such as muscle.
    • B- cells of pancreas
    • Binds to specific receptors on the target cell surface where it facilitates glucose transport into cells
      • Suppress endogenous glucose productions,
      • stimulates glycogen synthesis
      • Stimulates secretion of lipoprotein lipase= lipogenesis
      • Stimulates amino acid uptake and CHON synthesis
    • Hypoglycemia (Low Blood Sugar Levels)
      • Prompts the alpha cell of the pancreas to produce glucagon
    • Hyperglycemia (High Blood Sugar Levels)
      • Beta cells of the pancreas produce insulin
  • 29 AA residues
    • Glucagon
    • A-cell of pancreas
    • Stimulate hepatic cells in the liver to cause glycogenolysis

Glycogenolysis

• breakdown of glycogen by enzyme PHOSPHORYLASE
• Enhances gluconeogenesis
• Enhances gluconeogenesis formation of glucose from AA and lactate
• Increase glycogenolysis, gluconeogenesis, ketogenesis
• Decrease fatty acids synthesis

Function of Carbohydrates

• As a primary energy source.
  • 4 kcal/g
  • Serves as the only source for high-intensity anaerobic exercise.
  • E-source for cells of the Central NS, including Muscle: 300 g, Liver : 90 g and Body fluids: 30 g (for physical performance). Become more important as the intensity of exercise increases
• Prevents protein catabolism
  • Spares muscle CHON (protein sparing effect Decrease gluconeogenesis)
• Prevents ketosis
  • Ketosis- process that occurs when your body uses fat as its main fuel source
  • body burns fat for energy
  • ketones as a byproduct
• Normal and healthy function of GIT
• Lowers cholesterol
  • Soluble Fiber (Viscous)
  • Lower LDL - bad cholesterol
• Hunger Satiety and delays
  • Fiber - slows down digestion

Complex - long-lasting

• Weight management - reduce food intake

Lactose

• Aids Metabolism in the brush border
• Promotes growth of beneficial bacteria
• Aids Absorption of Ca
• Stimulates Synthesis of certain B- vitamins

Galactose

• Located Present in brain and nervous tissues

Ribose and Deoxyribose

• Genetic material

Sorbitol, xylitol, and mannitol

• Sub for sucrose
• sugar alcohols
• Inhibits growth of bad bacteria
• Complex and highly regulated
• metabolic pathway
• Converts excess carbs → fatty acids →
• esterified to storage TGs
• TGs could later provide energy via
• B oxidation
• High CHO: excess CHO are metabolized leading to reduced fat oxidation (indirect fat accumulation)

Fiber fermentation in the colon produces SCFAS:

• Propionate inhibits the activity of enzyme hydroxymethylglutaryl reductase needed for cholesterol synthesis
• acids: apoptosis of CA cells in the colon,
• affects differentiation and cancer
  • apoptosis of CA cells in the colon,
  • affects differentiation and cancer
• Apoptosis in cell death

Carbohydrate and Health : Microorganism

• FOS, GOS, inulin, RS stimulate the production of bifidobacteria (health promoting microorganisms) -FOS: fructo-oligosaccharides -GOS: galacto-oligosaccharides - types of prebiotics, non- digestible carbohydrates
• Inulin: prebiotic that is not digested or absorbed in the stomach.
• Types of Starch
  • resistant starch -Prebiotic : non-digestible fibers -Probiotic : live -Synbiotic : both

Liver and adipose tissue