Comprehensive Study Notes: The Nature and Function of Carbohydrates

Course and Textbook Reference

• These lecture notes are for HUN1201 Nutrition.
• The material is based on the course textbook: Williams' Essentials of Nutrition & Diet Therapy, 13th edition.
• Exam and quiz questions are derived directly from this book.

The Nature of Carbohydrates (CHO)

• Carbohydrates occur naturally in plant foods in two primary forms: starches and sugars.
• Photosynthesis is the process by which plants transform the sun's energy into the stored fuel form of carbohydrates.
• Necessary ingredients for photosynthesis include:   - $CO_2$   - $H_2O$   - Chlorophyll   - Sunlight
• Yield of photosynthesis: Carbon ($C$), Hydrogen ($H$), and Oxygen ($O$).
• The overall chemical reaction for photosynthesis is: $6H_2O + 6CO_2 \rightarrow C_6H_{12}O_6 + 6O_2$.
• Starch stored by plants for their own energy needs serves as a source of fuel for humans who consume those plants.
• Carbohydrates are often referred to as “quick energy” foods and represent our primary source of energy.
• Chemical composition: Carbohydrates consist of carbon, hydrogen, and oxygen, with a hydrogen-to-oxygen ratio usually equivalent to that of water ($CH_2O$).

Dietary Importance of Carbohydrates

• In many countries, carbohydrate-containing foods such as fruits, vegetables, grains, and cereals make up the major part of the diet.
• Rice serves as a primary food source for 3 billion people in developing countries.
• In the typical American diet, approximately $\frac{1}{2}$ of total kilocalories come from carbohydrates.
• Carbohydrate foods are advantageous because they can be easily stored for long periods without spoiling.
• Carbohydrates supply $4\,kcal/g$, whereas fat supplies $9\,kcal/g$.

Classification of Carbohydrates

• Carbohydrates are classified based on the number of basic sugar or saccharide units in their structure:   - Simple carbohydrates: Monosaccharides and Disaccharides.   - Complex carbohydrates: Polysaccharides.

Monosaccharides

• Monosaccharides represent the simplest form of sugar. Three are of major importance in human nutrition:
• Glucose:   - A moderately sweet sugar found naturally in only a few foods, such as corn syrup.   - The common body fuel oxidized by cells to provide energy.   - Supplied through the digestion of starch and the conversion of other simple sugars.   - This is the form of sugar circulating in the blood; it is referred to as dextrose in intravenous solutions.
• Fructose:   - The sweetest of the simple sugars.   - Found in fruits and other natural substances like honey.   - Absorbed less efficiently than glucose; intake of $25-50\,g$ can cause gastrointestinal (GI) distress.   - Accounts for $9\%$ of total energy intake for Americans over 2 years of age due to high-fructose corn syrup (HFCS).
• Galactose:   - Not found free in foods.   - Released through the digestion of lactose (milk sugar) and then converted to glucose in the liver.   - The reaction is reversible: During lactation, glucose is reconverted to galactose for use in milk production.

Disaccharides

• Disaccharides are "double sugars" made of two monosaccharides linked together. Three are of physiologic importance:
• Sucrose ($\text{one glucose} + \text{one fructose}$):   - Common "table sugar," made commercially from sugar cane and sugar beets.   - Occurs naturally in some fruits and vegetables.
• Lactose ($\text{one glucose} + \text{one galactose}$):   - The sugar found in milk.   - The least sweet of the disaccharides, measuring only one-sixth as sweet as sucrose.   - Cheese contains very little lactose.
• Maltose ($\text{one glucose} + \text{one glucose}$):   - Result of the breakdown of starch.   - Found in commercial malt products and germinating cereal grains.

Sugar Intake and Guidelines

• Most sugar is added during food preparation or processing.
• Added sugar intake accounts for approximately $17\%$ of daily energy intake in U.S. adults and $14\%$ in children.
• Sweetened beverages are the primary contributors of added sugar.
• Dietary Guidelines for Americans recommend that added sugar contribute less than $10\%$ of total energy intake.
• Specifically, sugar intake should be limited to:   - Women: $< 100\,kcal/d$   - Men: $< 150\,kcal/d$

Sugar Alcohols (Polyols)

• Also known as polyols, these may be listed as "sugar-free" on food labels.
• Energy density: Only $0.2$ to $3.0\,kcal/g$.
• Physiological properties:   - Do not require insulin for metabolism, offering an alternative for individuals with diabetes.   - Do not promote tooth decay because they are not fermented by oral bacteria.   - High intakes can cause abdominal distress or exert a laxative effect.
• Three types occurring in nature and industry: Sorbitol, Mannitol, and Xylitol.

Polysaccharides and Complex Carbohydrates

• Made up of many single glucose (saccharide) units.
• Starch:   - The most important energy-yielding polysaccharide.   - Large, complex substance with coiled and branching chains of glucose.   - Cooking improves flavor and eases digestion.   - Used for thickening and gel-like qualities.
• Resistant Starch:   - Starch that is not digested in the small intestine.   - Fermented by bacteria in the colon, producing short-chain fatty acids (the preferred energy source for cells lining the colon).
• Glycogen:   - The storage form of carbohydrate in animals.   - Synthesized in liver cells and stored in small amounts in the liver and muscles.
• Dextrins:   - Polysaccharide compounds formed as intermediate products during the breakdown of starch.

Oligosaccharides

• Small fragments of partially digested starch ranging from 3 to 10 glucose units.
• Characteristics:   - Formed via digestion or produced commercially by acid hydrolysis.   - Used in sports drinks.   - Polymers containing 2 to 20 sugar molecules.   - Water-soluble and often sweet.
• Types and Sources:   - Stachyose and Raffinose.   - Found in legumes (beans, peas, soybeans).   - Human enzymes cannot break the bonds; they remain undigested until reached by colonic bacteria, which ferment them and produce gas.
• Dietary Context:   - Avoided in the FODMAP diet for Irritable Bowel Syndrome (IBS).   - FODMAP stands for Fermentable Oligosaccharides, Disaccharides, Monosaccharides, and Polyols.

Glycemic Index (GI)

• Measures the relative effect of different carbohydrate-containing foods on blood glucose levels.
• Calculation: Compares the rise in blood glucose over 2 hours after eating a food containing $50\,g$ of CHO with a reference food (usually white bread or a glucose solution).
• Categories:   - Low GI: $< 50$   - Intermediate GI: $50 - 70$   - High GI: $> 70$
• Influencing factors: Type of carbohydrate, cooking method, particle size, fiber content, and the presence of other macronutrients.
• Nutritional status: Evidence on using GI to modify diets and prevent chronic disease is not yet conclusive.

Functions of Carbohydrates in the Body

• Primary Energy Source: Starches and sugars provide energy to cells, particularly brain cells which depend on glucose.
• Brain Dependence: The Central Nervous System (CNS) has very low carbohydrate reserves (only enough for $10 - 15\,minutes$) and depends on a minute-to-minute supply of glucose. Glucose also increases the synthesis of acetylcholine.
• Glycogen Reserves:   - Total storage: Approximately $300 - 350\,g$ in the liver and muscles.   - Blood glucose: Approximately $10\,g$.   - Function: Protects brain cells from depressed metabolic function and supports urgent muscle responses.
• Heart Action: Fatty acids are the preferred fuel for the heart, but cardiac glycogen is an essential emergency source of contractile energy.
• Protein-sparing action: Adequate carbohydrate intake allows protein to be used for tissue building and repair rather than being diverted for energy.
• Antiketogenic effect: Sufficient carbohydrates prevent the excessive oxidation of fat.

Fat Metabolism and Ketosis

• Under extreme conditions (starvation, uncontrolled diabetes, or very-low-carbohydrate diets), fat is oxidized at excessive rates when carbohydrates are inadequate.
• Ketones are intermediate products of fat metabolism.
• When ketones accumulate, the result is ketoacidosis.
• Sufficient dietary carbohydrate prevents this damaging excess.

Recommended Intakes

• Recommended Dietary Allowance (RDA): $130\,g/day$ for all persons over 1 year of age (ensures enough glucose for the brain for 1 day).
• Acceptable Macronutrient Distribution Range (AMDR):   - Carbohydrates: $45 - 65\%$ of total energy intake.   - Fat: $20 - 35\%$   - Protein: $10 - 35\%$   - Sugar: $< 10\%$
• Whole Grains: Choices should emphasize whole grains. Refined grains are enriched with B vitamins, iron, and folate but lack the fiber and trace minerals found in whole grains.

Nonnutritive Sweeteners

• Yield little or no energy; used to indulge a taste for sweets while limiting calories.
• Specifically FDA-approved examples:   - Aspartame: Contains phenylalanine; must be avoided by those with PKU (Phenylketonuria).   - Sucralose (Splenda): $600$ times sweeter than sucrose; provides no energy.   - Stevia extract: Recently approved.

Oral Health and Carbohydrates

• Dental caries are influenced by oral hygiene, diet, and specific nutrients.
• Fluoride strengthens enamel, while whole-grain cereals are protective.
• Risk factors: Continuous sipping of sugar-sweetened drinks.
• Synergistic Relationship: Malnutrition leads to the deterioration of teeth and gums, which further compromises nutrient intake. Periodontal disease can lead to systemic infections and worsen blood glucose control in diabetics.
• Factors related to tooth decay: Age-related osteoporosis (tooth loss) and Xerostomia (dry mouth).

Imbalances in Carbohydrate Intake

• High-CHO diets: Defined as $65 - 75\%$.   - Can lead to a rise in plasma triglycerides and a drop in HDL cholesterol.   - Puts heavy demands on pancreatic beta cells for insulin.
• Low-Carbohydrate Diets: Defined as $< 20\%$ or $< 130\,g$.   - Often restrict CHO to $< 40\,g$ or $10\%$ of total kcal.   - Risks: High-fat focus increases cardiovascular risk; high-protein focus burdens kidneys; often lacks recommended servings of fruits, vegetables, and whole grains.   - Potential benefits for specific conditions: Diabetes, weight loss, pediatric epilepsy, nonalcoholic fatty liver disease, certain cancers, and neurodegenerative conditions (Alzheimer's and Parkinson's).   - Concerns: Deficiencies in water, calcium, and potassium.
• Hyperglycemia: Elevated blood glucose.
• Hypoglycemia: Blood glucose below the normal range.

Fiber: Nondigestible Carbohydrates

• Fiber consists of polysaccharides that cannot be broken down by human digestive enzymes.
• Dietary Fiber: Nondigestible CHO and lignin intact in plant foods.   - Soluble: Pectin, gums, beta-glucans. Sources: Oats, beans, legumes, nuts, fruits.   - Insoluble: Cellulose, hemicellulose, lignin. Sources: Wheat bran, whole grains, seeds, root vegetables, cabbage family.
• Functional Fiber: Nondigestible polysaccharides isolated from plants or commercially produced and added to foods (e.g., flaxseed, psyllium).
• Health Benefits:   - Increases fecal mass.   - Binds bile acids and cholesterol.   - Positive effect on colonic microflora, blood glucose, and insulin levels.   - Decreased risk of Type 2 Diabetes (T2DM), Cardiovascular Disease (CVD), and metabolic syndrome.
• Dietary Reference Intake (DRI) for Fiber:   - Men < 51: $38\,g/day$   - Women < 51: $25\,g/day$   - Men > 51: $30\,g/day$   - Women > 51: $21\,g/day$
• U.S. Intake: Median intake is only $16-18\,g/day$ for men and $12-14\,g/day$ for women.
• Phytochemicals: Benefit-rich plant chemicals found in CHO-rich whole foods.

Carbohydrate Digestion and Absorption

• Mouth:   - Mechanical chewing mixes food with salivary secretions.   - Salivary amylase (ptyalin) from the parotid gland breaks starch into dextrins and maltose ($20-30\%$ of starch breakdown occurs here).
• Stomach:   - Peristalsis continues mechanical breakdown.   - Gastric action halts salivary amylase; no further CHO digestion occurs in the stomach.   - Food becomes creamy chyme and passes through the pyloric valve to the duodenum.
• Small Intestine:   - Pancreatic amylase breaks starch into maltose.   - Intestinal secretions (sucrase, lactase, maltase) complete digestion.
• Absorption:   - Surfaces enhanced by mucosal folds, villi, and microvilli.   - Glucose is absorbed via an active transport pumping system requiring energy and sodium as a carrier.   - Absorption ratio: $80\%$ glucose, $20\%$ galactose and fructose.   - Monosaccharides enter portal circulation via capillaries in the villi to reach the liver.

Carbohydrate Metabolism

• Liver Function: Converts fructose and galactose into glucose.
• Homeostasis: Maintenance of blood glucose at $70 - 100\,mg/dL$.
• Key Processes:   - Glycogenolysis: Breakdown of glycogen into glucose.   - Gluconeogenesis: Formation of glucose from non-carbohydrate sources.
• Hormonal Regulation:   - Insulin (Pancreas): Regulates blood glucose uptake.   - Glucagon (Pancreas): Stimulates liver to release glycogen.   - Somatostatin (Pancreas/Hypothalamus): Inhibits both insulin and glucagon.