3.1-3.2

Overview and Dietary Role of Carbohydrates

• Energy Supply and Brain Function:

  • Carbohydrate is the preferred energy source for many of the body's functions.
  • As long as carbohydrate is available, the human brain depends exclusively on it as an energy source.
  • Athletes consume a high-carbohydrate diet to store as much muscle fuel as possible.
  • Dietary recommendations urge people to eat carbohydrate-rich foods for better overall health.

• Misconceptions and Weight Management:

  • Carbohydrate-rich foods are sometimes mistakenly viewed as fattening and avoided during weight-loss efforts.
  • Avoiding carbohydrates can be helpful if the sources are concentrated sugars (such as soft drinks, candies, and cookies).
  • Avoiding carbohydrates is counterproductive when they are sourced from whole grains, vegetables, legumes, and fruit. These plant foods provide carbohydrate and fiber with little or no fat.
  • Milk is the only animal-derived food that contains significant amounts of carbohydrate.
  • To lose fat, maintain lean tissue, and stay healthy, individuals should remain physically active, control portion sizes, and design a dietary pattern supplying fiber-rich carbohydrate in balance with other energy nutrients.

• Context within Energy-Yielding Nutrients:

  • Carbohydrates represent the first of three chapters on energy-yielding nutrients.
  • Fats are covered in Chapter 4, and protein is featured in Chapter 5.
  • Alcohol as a contributor of dietary energy is addressed in Nutrition in Practice 19 (noting its well-known undesirable side effects when consumed in excess).

Chemical Structure and Classification of Carbohydrates

• Etymology and General Structure:

  • Carbohydrates are compounds composed of carbon, oxygen, and hydrogen arranged as monosaccharides or multiples of monosaccharides.
  • The term carbohydrate reflects its chemical components: "carbo" means carbon, and "hydrate" means water.

• Chemist's View and Categories:

  • Simple Carbohydrates (Sugars):
  • Monosaccharides: Single sugars ($\text{mono} = 1$, $\text{saccharide} = \text{sugar}$).
  • Disaccharides: Double sugars / pairs of sugar units bonded together ($\text{di} = 2$).
  • Complex Carbohydrates (Polysaccharides):
  • Polysaccharides: Long chains of monosaccharide units arranged as starch, glycogen, or fiber ($\text{poly} = \text{many}$).
  • All carbohydrates are built from the single sugar glucose and compounds closely resembling glucose in composition and structure.

• Chemical Structure of Glucose:

  • On paper, the structure of glucose is drawn flat.
  • In nature, the five carbons and oxygen atom are arranged roughly in a single plane, with $\text{H}$, $\text{OH}$, and $\text{CH}_2\text{OH}$ groups extending above and below the plane.

Monosaccharides (Single Sugars)

• General Characteristics:

  • Three monosaccharides are important in human nutrition: glucose, fructose, and galactose.
  • All three monosaccharides possess identical numbers and kinds of atoms ($\text{C}_6\text{H}_{12}\text{O}_6$), but these atoms are organized in different structural arrangements.

• Glucose:

  • Also known as blood sugar or dextrose.
  • The universal single sugar common to all disaccharides and polysaccharides.
  • Most body cells depend on glucose for fuel to some extent.
  • Cells of the brain and nervous system depend almost exclusively on glucose for energy.
  • To function optimally, the body must maintain blood glucose levels within tight homeostatic limits to allow cells to nourish themselves.

• Fructose:

  • Also known as fruit sugar ($\text{fruct} = \text{fruit}$).
  • The sweetest of all sugars.
  • Occurs naturally in fruits, honey, and as part of sucrose (table sugar).
  • Most dietary fructose is consumed in sweet beverages (soft drinks), ready-to-eat cereals, and products sweetened with high-fructose corn syrup or added sugars.
  • Glucose and fructose are the most common monosaccharides found in nature.

• Galactose:

  • Occurs mostly as part of lactose (milk sugar).
  • Freed as a single sugar during carbohydrate digestion.

Disaccharides (Double Sugars)

• General Characteristics:

  • Disaccharides consist of pairs of single sugars linked together.
  • All three major disaccharides contain glucose as one of their monosaccharide components.

• Sucrose:

  • Composition: $\text{Glucose} + \text{Fructose}$.
  • Commonly known as table sugar, white sugar, beet sugar, or cane sugar ($\text{sucro} = \text{sugar}$).
  • Produced commercially by refining juice from sugar beets or sugarcane to yield brown, white, and powdered sugars.
  • Occurs naturally in many fruits and vegetables.
  • Digestive enzymes split sucrose into its glucose and fructose components.

• Lactose:

  • Composition: $\text{Glucose} + \text{Galactose}$.
  • Commonly known as milk sugar ($\text{lact} = \text{milk}$).
  • The principal carbohydrate of milk.
  • Most human infants are born with the digestive enzymes required to split lactose into glucose and galactose for absorption.
  • Human breast milk and standard cow's milk infant formulas provide an easily digested carbohydrate to meet infant energy needs.
  • Some people lose the ability to digest lactose after infancy, leading to lactose intolerance (addressed in Chapter 18).

• Maltose:

  • Composition: $\text{Glucose} + \text{Glucose}$.
  • Commonly known as malt sugar.
  • A plant sugar produced whenever starch breaks down—such as when plants break down stored starch reserves during sprouting, and in humans during starch digestion.

Polysaccharides: Glycogen and Starch

• General Characteristics:

  • Polysaccharides are long chains of monosaccharide units (chiefly glucose).
  • Three types are important in nutrition: glycogen, starch, and fibers.
  • Glycogen and starch are built entirely of glucose units; fibers contain a variety of monosaccharides and other carbohydrate derivatives.

• Glycogen:

  • Composed of glucose chains that are far more highly branched than those of starch.
  • Serves as the storage form of energy and glucose in humans and animals, synthesized and stored in liver and muscle tissues.
  • Found in meats only to a limited extent (due to rapid breakdown in animal muscle post-slaughter) and absent in plants.
  • Glycogen is not a significant food source of carbohydrate and is not counted as a food polysaccharide.

• Starch:

  • Composed of long, straight (unbranched) or occasionally branched chains of hundreds or thousands of glucose units linked together.
  • Serves as the storage form of energy in plants, packed side by side in plant tissues.
  • Human enzymes split starch into glucose units for cellular energy. Intermediate short glucose chains resulting from starch breakdown are called dextrins (often used as thickening agents on food labels).
  • All starchy foods derive from plants. Grains are the richest food source of starch.
  • Global Staple Grains:
  • Rice: Primary staple in Asia.
  • Wheat: Primary staple in Canada, the United States, and Europe.
  • Corn: Primary staple in Central and South America.
  • Millet, Rye, Barley, and Oats: Primary staples in other global regions.
  • Legumes (Bean and Pea Family): Peanuts and dry beans (butter beans, kidney beans, baked beans, black-eyed peas/cowpeas, chickpeas/garbanzo beans, and soybeans).
  • Root Vegetables / Tubers: Potatoes, cassava, and yams (primary starch sources in many non-Western societies).
  • Grains, legumes, and tubers supply rich starch alongside dietary fiber, protein, and essential nutrients.

Dietary Fibers: Types, Properties, and Functions

• Structural Characteristics:

  • Structural components of plants found in all plant-derived foods (vegetables, fruit, whole grains, and legumes).
  • Most fibers are polysaccharides, but their sugar units are linked by chemical bonds that human digestive enzymes cannot break.
  • Consequently, fibers pass through the small intestine providing little or no direct energy.

• Fiber Components:

  • Cellulose: Main constituent of plant cell walls; present in all vegetables, fruits, and legumes.
  • Hemicellulose: Main constituent of cereal fibers.
  • Pectins: Abundant in fruits (citrus fruits and apples) and vegetables; used commercially to gel jellies and prevent salad dressing separation.
  • Gums and Mucilages: Structurally similar to pectins; used as food additives and stabilizers.
  • Lignins: Non-polysaccharide tough, woody plant components; consumed in negligible amounts.

• Resistant Starches:

  • Starches that escape digestion and absorption in the human small intestine.
  • Occur due to individual digestive efficiency or the physical properties of food.
  • Sources: Whole or partially milled grains, legumes, raw potatoes, intact seeds and kernels, unripe bananas, and chilled cooked potatoes, pasta, and rice.
  • Support colon health similarly to soluble fibers.

• Fermentation and Energy Value:

  • Bacteria in the human large intestine ferment certain fibers anaerobically.
  • Bacterial fermentation generates absorbable products (short-chain fatty acids) yielding $1.5\text{ to }2.5\,\text{kcalories/g}$.

• Classification Categories:

  • Soluble Fibers:
  • Dissolve in water, form viscous gels, and are easily fermented by large intestine bacteria.
  • Food function: Add gel-like consistency (e.g., pectin in jelly, gums in dressings).
  • Body function: Associated with reduced risks of chronic diseases.
  • Sources: Barley, legumes, fruit, oats, and vegetables.
  • Isolated Preparations: Used as medications or additives (e.g., psyllium or methylcellulose pills for constipation; inulin added to boost fiber and reduce kcalories).
  • Insoluble Fibers:
  • Do not dissolve in water, non-viscous, non-gel-forming, and less readily fermented.
  • Retain structure and rough texture after long cooking.
  • Body function: Aid the digestive system by easing elimination.
  • Sources: Outer layers of whole grains (bran), celery strings, seed hulls, and corn kernel skins.

• Dietary Reference Intakes (DRI) Definitions:

  • Dietary Fibers: Naturally occurring fibers in intact foods.
  • Functional Fibers: Added fibers that have demonstrated health benefits.
  • Total Fiber: The sum of dietary fibers and functional fibers ($\text{Total Fiber} = \text{Dietary Fiber} + \text{Functional Fiber}$).

Carbohydrate Digestion and Absorption

• Ultimate Goal:

  • Break down starches and disaccharides into monosaccharides—chiefly glucose—that the body can absorb and utilize.
  • Requirements by class: Starches require extensive breakdown; disaccharides require a single cleavage step; monosaccharides require no digestion.

• Step-by-Step Digestive Process:

  • Mouth and Salivary Glands:
  • Mechanical action crushes and tears fiber, mixing food with saliva for swallowing.
  • Salivary glands secretes salivary amylase to initiate starch breakdown:       $\text{Starch} \xrightarrow{\text{Salivary Amylase}} \text{Small Polysaccharides} + \text{Maltose}$
  • Stomach:
  • Stomach acid inactivates salivary amylase, halting starch digestion.
  • Fiber is not digested; it delays gastric emptying, promoting satiety.
  • Small Intestine and Pancreas:
  • The pancreas releases pancreatic amylase through the pancreatic duct into the small intestine:       $\text{Starch} \xrightarrow{\text{Pancreatic Amylase}} \text{Small Polysaccharides} + \text{Maltose}$
  • Disaccharidase enzymes on the surface of small intestinal cells hydrolyze disaccharides into monosaccharides:       $\text{Maltose} \xrightarrow{\text{Maltase}} \text{Glucose} + \text{Glucose}$$\text{Sucrose} \xrightarrow{\text{Sucrase}} \text{Glucose} + \text{Fructose}$       \text{Lactose} \xrightarrow{\text{Lactase}} \t\text{Glucose} + \text{Galactose}
  • Intestinal cells absorb these monosaccharides into the bloodstream.
  • Fiber passes intact to the large intestine, delaying absorption of certain nutrients.
  • Large Intestine:
  • Bacterial enzymes ferment soluble fibers, producing short-chain fatty acids and gas:       $\text{Soluble Fibers} \xrightarrow{\text{Bacterial Enzymes}} \text{Short-Chain Fatty Acids} + \text{Gas}$
  • Short-chain fatty acids are preferentially and immediately absorbed by the large intestine.
  • Insoluble fibers remain intact, retain water, and bind substances like bile, cholesterol, and minerals, carrying them out of the body.

Key Terminology Summary

• Carbohydrates: Compounds composed of carbon, oxygen, and hydrogen arranged as monosaccharides or multiples of monosaccharides.
• Monosaccharides: Single sugar units ($\text{mono} = 1$, $\text{saccharide} = \text{sugar}$).
• Disaccharides: Pairs of sugar units bonded together ($\text{di} = 2$).
• Polysaccharides: Long chains of monosaccharides arranged as starch, glycogen, or fiber ($\text{poly} = \text{many}$).
• Glucose: Monosaccharide common to all disaccharides and polysaccharides; also called blood sugar or dextrose.
• Fructose: Monosaccharide known as fruit sugar ($\text{fruct} = \text{fruit}$).
• Galactose: Monosaccharide occurring mostly as part of lactose.
• High-Fructose Corn Syrup: Kcaloric sweetener made by treating cornstarch with enzymes to convert glucose to sweet fructose.
• Added Sugars: Sugars, syrups, and kcaloric sweeteners added to foods during processing, preparation, or at table (excludes natural sugars in fruit and milk).
• Sucrose: Disaccharide composed of glucose and fructose; table sugar, beet sugar, or cane sugar ($\text{sucro} = \text{sugar}$).
• Lactose: Disaccharide composed of glucose and galactose; milk sugar ($\text{lact} = \text{milk}$).
• Maltose: Disaccharide composed of two glucose units; malt sugar.
• Glycogen: Polysaccharide composed of glucose, made and stored in liver and muscle tissues as a glucose storage form.
• Starch: Plant polysaccharide composed of glucose, digestible by human enzymes.
• Dietary Fibers: Indigestible plant food polysaccharides (cellulose, hemicellulose, pectins, gums, mucilages) and nonpolysaccharides (lignins).
• Whole Grains: Grains containing all essential parts and naturally occurring nutrients of the entire grain seed (except inedible husk).
• Resistant Starches: Starches escaping digestion and absorption in the small intestine.
• Fermentation: Anaerobic breakdown of carbohydrates by microorganisms, releasing small organic compounds, $\text{CO}_2$, and energy.
• Soluble Fibers: Indigestible food components dissolving in water to form gels.
• Viscous: Having a gel-like consistency.
• Insoluble Fibers: Tough, fibrous plant structures that do not dissolve in water.