Carbohydrates, Lipids, and Proteins: Core Concepts for Nutrition Students (Chapters 4, 5, 6)

Carbohydrates

Definitions and Key Terms

• Carbohydrate: Organic compounds made of carbon, hydrogen, and oxygen; primary energy source for the body. Examples: glucose, starch, cellulose.

• Sugar: Simple carbohydrates; includes monosaccharides and disaccharides. Example: table sugar (sucrose).

• Monosaccharides: Single sugar units. Examples: glucose (found in fruits), fructose (honey), galactose (part of milk sugar).

• Disaccharide: Two monosaccharides linked. Examples: sucrose (glucose + fructose), lactose (glucose + galactose), maltose (glucose + glucose).

• Polysaccharide: Many monosaccharides linked. Examples: starch (plants), glycogen (animals), cellulose (fiber in plants).

• Condensation: Chemical reaction joining two molecules with the removal of water.

• Hydrolysis: Chemical reaction breaking bonds by adding water.

• Glycogen: Storage form of glucose in animals, mainly in liver and muscles.

• Starch: Storage form of glucose in plants; found in grains, legumes, tubers.

• Dietary fiber: Non-digestible carbohydrates from plants; promotes digestive health.

• Soluble fiber: Dissolves in water; forms gels. Example: oats, apples.

• Insoluble fiber: Does not dissolve in water; adds bulk. Example: whole wheat, vegetables.

• Viscous fiber: Forms thick gels in water; slows digestion.

• Fermentable fiber: Can be broken down by gut bacteria.

• Functional fiber: Isolated fibers added to foods for health benefits.

• Amylase: Enzyme that breaks down starch; found in saliva and pancreas.

• Resistant starch: Starch not digested in the small intestine; acts like fiber.

• Lactose intolerance: Inability to digest lactose due to low lactase enzyme.

• Gluconeogenesis: Formation of glucose from non-carbohydrate sources (e.g., amino acids).

• Ketone body: Compounds produced during fat breakdown when glucose is scarce.

• Glucose homeostasis: Maintenance of stable blood glucose levels.

• Insulin: Hormone from pancreas; lowers blood glucose by promoting uptake into cells.

• Glucagon: Hormone from pancreas; raises blood glucose by stimulating glycogen breakdown.

• Epinephrine: "Fight or flight" hormone; raises blood glucose.

• Diabetes: Chronic disease with impaired glucose regulation.

• Glycemic response: Effect of food on blood glucose levels.

• Glycemic index: Ranking of foods by their effect on blood glucose.

• Hypoglycemia: Abnormally low blood glucose.

• Dental caries: Tooth decay caused by bacteria metabolizing sugars.

• Added sugar: Sugars added during processing/preparation.

• Sugar alcohol: Sweeteners like xylitol, sorbitol; lower calorie than sugar.

• Nutritive sweeteners: Provide calories (e.g., sucrose, honey).

• Non-nutritive sweeteners: Little/no calories; much sweeter than sugar (e.g., aspartame, sucralose).

Categories of Carbohydrates and Food Sources

• Monosaccharides: Glucose (fruits, vegetables), fructose (honey, fruit), galactose (milk).

• Disaccharides: Sucrose (table sugar: glucose + fructose), lactose (milk sugar: glucose + galactose), maltose (malt sugar: glucose + glucose).

• Polysaccharides: Starch (grains, potatoes), glycogen (animal muscle/liver), cellulose (plant cell walls).

Digestion and Absorption of Carbohydrates

• Digestion begins in the mouth with salivary amylase.

• Continues in the small intestine with pancreatic amylase and enzymes (maltase, sucrase, lactase) from the intestinal lining.

• Enzymes break polysaccharides into monosaccharides via hydrolysis.

• Monosaccharides are absorbed by enterocytes via active transport (glucose, galactose) or facilitated diffusion (fructose).

• Absorbed monosaccharides travel to the liver via the portal vein.

Lactose Intolerance

• Symptoms: Bloating, gas, diarrhea after consuming dairy.

• Causes: Low lactase enzyme activity.

• Prevalence: Higher in certain ethnic groups (e.g., Asian, African descent).

• Management: Lactose-free products, lactase supplements, fermented dairy (yogurt, cheese).

Functions and Metabolism of Carbohydrates

• Primary energy source for brain and muscles.

• Stored as glycogen in liver and muscle.

• Excess intake converted to fat.

Glucose Regulation

• Insulin lowers blood glucose; glucagon and epinephrine raise it.

• Homeostasis maintained by hormonal feedback.

Glycemic Response and Index

• Low glycemic foods (whole grains, legumes) cause gradual rise in blood glucose.

• High glycemic foods (white bread, sweets) cause rapid spikes.

• Combining fiber, protein, and fat with carbohydrates lowers glycemic response.

Health Effects of Added Sugars

• Increased risk of obesity, type 2 diabetes, dental caries.

• May contribute to heart disease.

• Recommended maximum: Less than 10% of total daily calories from added sugars (per Dietary Guidelines for Americans).

• Strategies to reduce intake: Limit sugary drinks, choose unsweetened foods, read labels.

Non-Nutritive Sweeteners

• Used to reduce calorie intake and manage blood sugar.

• Generally recognized as safe (GRAS) by FDA when consumed within acceptable daily intake.

Dietary Fiber and Health

• Promotes bowel regularity, lowers cholesterol, helps control blood glucose.

• Recommended intake: 25g/day for women, 38g/day for men (adults).

Lipids

Definitions and Key Terms

• Lipids: Diverse group of hydrophobic molecules; includes fats, oils, phospholipids, sterols.

• Fatty acids: Chains of carbon and hydrogen with a carboxyl group.

• Short-chain fatty acid: Fewer than 6 carbons.

• Medium-chain fatty acid: 6–12 carbons.

• Long-chain fatty acid: 14 or more carbons.

• Unsaturated fatty acid: Contains one or more double bonds; liquid at room temperature.

• Saturated fatty acid: No double bonds; solid at room temperature.

• Monounsaturated fatty acid: One double bond (e.g., olive oil).

• Polyunsaturated fatty acid: Two or more double bonds (e.g., corn oil).

• Omega-3 fatty acid: Double bond at third carbon from methyl end (e.g., alpha-linolenic acid).

• Linolenic acid: Essential omega-3 fatty acid.

• Linoleic acid: Essential omega-6 fatty acid.

• Essential fatty acid: Must be obtained from diet (linoleic and linolenic acids).

• Triglycerides: Main form of fat in food and body; three fatty acids + glycerol.

• Glycerol: Three-carbon backbone of triglycerides.

• Oxidation: Loss of electrons; in nutrition, breakdown of fatty acids for energy.

• Dehydrogenation: Removal of hydrogen atoms.

• Trans fatty acids: Unsaturated fats with trans double bonds; produced by hydrogenation; associated with negative health effects.

• Phospholipids: Lipids with phosphate group; major component of cell membranes (e.g., lecithin).

• Lecithin: Common phospholipid in egg yolks, soybeans; used as emulsifier.

• Emulsifier: Allows fat and water to mix (e.g., bile, lecithin).

• Chylomicron: Lipoprotein that transports dietary fat from intestine to body.

• Micelle: Small transport carrier for lipids in the intestine.

• VLDL: Very-low-density lipoprotein; transports triglycerides from liver.

• LDL: Low-density lipoprotein; delivers cholesterol to cells; "bad cholesterol".

• HDL: High-density lipoprotein; removes cholesterol from cells; "good cholesterol".

• Sterol: Lipid with ring structure; cholesterol is most common.

• Lipase: Enzyme that digests fats.

• Adipose tissue: Body fat storage tissue.

• Adipokine: Hormones produced by adipose tissue.

• Fat replacers: Substances used to mimic fat with fewer calories.

• Mediterranean diet: Dietary pattern rich in olive oil, nuts, fruits, vegetables, whole grains, and fish.

Functions of Lipids in the Body

• Triglycerides: Energy storage, insulation, protection.

• Phospholipids: Structural component of cell membranes, emulsification of fats.

• Cholesterol (Sterols): Precursor for steroid hormones, vitamin D, bile acids; component of cell membranes.

Saturated vs. Unsaturated Fats

• Chemical structure: Saturated fats have no double bonds; unsaturated have one or more.

• Firmness: Saturated fats are solid at room temperature (e.g., butter); unsaturated are liquid (e.g., olive oil).

• Health effects: Saturated fats raise LDL cholesterol; unsaturated fats (especially polyunsaturated and monounsaturated) are associated with better heart health.

Most Prevalent Lipid

• Triglycerides are the most common lipid in both food and the human body.

Lipid Digestion and Absorption

• Begins minimally in the mouth and stomach (lingual and gastric lipase).

• Most digestion occurs in the small intestine with pancreatic lipase.

• Bile (released from gallbladder, signaled by hormone cholecystokinin (CCK)) emulsifies fats.

• Fats are absorbed as fatty acids and monoglycerides, assembled into chylomicrons in enterocytes, and enter the lymphatic system.

Lipoproteins: Structure and Function

Lipoprotein	Where Produced	Where Acts	Action
Chylomicrons	Intestinal cells	Bloodstream	Transport dietary triglycerides to tissues
VLDL	Liver	Bloodstream	Transport endogenous triglycerides to tissues
LDL	From VLDL in blood	Bloodstream/cells	Deliver cholesterol to cells; high levels linked to heart disease
HDL	Liver, intestine	Bloodstream	Remove cholesterol from cells, return to liver; protective against heart disease

Dietary Patterns and Health

• Mediterranean diet and diets high in unsaturated fats (olive oil, nuts, fish) are associated with better health outcomes.

• Limiting saturated and trans fats reduces risk of cardiovascular disease.

Proteins

Definitions and Key Terms

• Amino acid: Building block of protein; contains amino group, carboxyl group, hydrogen, and unique side chain (R group).

• Essential amino acid: Cannot be synthesized by the body; must be obtained from diet.

• Non-essential amino acid: Can be synthesized by the body.

• Conditionally essential amino acid: Normally non-essential, but required in diet under certain conditions (e.g., illness).

• Complete protein (High Quality Protein): Contains all essential amino acids in adequate amounts (e.g., animal proteins, soy).

• Peptide bond: Covalent bond linking amino acids in proteins.

• Primary structure: Sequence of amino acids in a polypeptide chain.

• Secondary structure: Local folding (alpha-helix, beta-sheet) due to hydrogen bonding.

• Tertiary structure: Overall 3D shape of a polypeptide.

• Quaternary structure: Arrangement of multiple polypeptide subunits.

• Denaturation: Loss of protein structure (and function) due to heat, acid, or agitation.

• Protease: Enzyme that digests proteins.

• Enzyme: Protein catalyst that speeds up reactions.

• Amino acid pool: Collection of amino acids available for protein synthesis in the body.

• Nitrogen balance: Difference between nitrogen intake and loss; indicator of protein status.

• Protein turnover: Continuous breakdown and synthesis of proteins in the body.

• Transamination: Transfer of amino group to form new amino acids.

• Deamination: Removal of amino group from amino acid.

• Reference protein: Protein standard used to measure quality of other proteins.

• Lower quality protein: Lacks one or more essential amino acids.

• Complementary protein: Two or more proteins that together provide all essential amino acids (e.g., rice and beans).

• Protein digestibility: Measure of how well protein is absorbed.

• Limiting amino acid: Essential amino acid in shortest supply in a food protein.

• Protein deficiency: Inadequate protein intake; can cause kwashiorkor, marasmus.

• Buffer: Protein that helps maintain pH balance.

• Side chain (R group): Unique group that determines amino acid properties.

• Transcription: DNA to mRNA synthesis in nucleus.

• Translation: mRNA to protein synthesis at ribosome.

Unique Element in Proteins

• Nitrogen is present in proteins but not in carbohydrates or fats.

Role of R Groups (Side Chains)

• Determine the chemical properties and function of each amino acid.

Protein Structure and Denaturation

• Proteins have four levels of structure: primary, secondary, tertiary, quaternary.

• Shape determines function; denaturation disrupts shape and function (e.g., cooking an egg).

Protein Digestion and Absorption

• Begins in stomach (pepsin, HCl), continues in small intestine (pancreatic proteases, peptidases).

• Proteins broken into amino acids and small peptides, absorbed via active transport into enterocytes.

Protein Synthesis

• DNA transcribed to mRNA (transcription), mRNA translated to protein at ribosome (translation).

Roles of Proteins in the Body

• Structural (muscle, skin, hair), enzymes, hormones, immune function, transport, fluid balance, acid-base balance, energy (if needed).

Amino Acid Metabolism

• Used for protein synthesis, energy, or converted to glucose/fat.

• Excess amino acids are deaminated; nitrogen excreted as urea.

Protein Quality

• Determined by amino acid composition and digestibility.

• High-quality proteins: animal sources (meat, eggs, dairy), soy.

• Complementary proteins: combining plant foods to provide all essential amino acids (e.g., legumes + grains).

• Complementary proteins do not need to be eaten at the same meal, but should be consumed within the same day.

Health Consequences of Protein Intake

• Too little: Muscle wasting, weakened immunity, edema.

• Too much: May stress kidneys, increase risk of dehydration, and (if from animal sources) may increase risk of chronic diseases.

Protein Needs

• Recommended Dietary Allowance (RDA) for adults: 0.8 g protein per kg body weight per day.