Chapter 4: Carbohydrates

Definition and Classification of Carbohydrates

• Carbohydrates are organic compounds produced by plants during photosynthesis and serve as a primary energy source for humans.

• They are classified by the number of sugar units:

  • Monosaccharides: Single sugar units (e.g., glucose, fructose, galactose).

  • Disaccharides: Two sugar units joined by condensation (e.g., sucrose, lactose, maltose).

  • Polysaccharides: Long chains of glucose units (e.g., starch, glycogen, fiber).

• Condensation forms disaccharides; hydrolysis breaks them down.

Types of Carbohydrates and Food Sources

• Monosaccharides: Glucose (fruits, honey), fructose (fruits, honey), galactose (rare, part of lactose in dairy).

• Disaccharides:

  • Sucrose (glucose + fructose): Table sugar, sugar cane, sugar beets.

  • Lactose (glucose + galactose): Milk, dairy products.

  • Maltose (glucose + glucose): Malted grains, beer.

• Polysaccharides:

  • Starch (amylose, amylopectin): Potatoes, grains, legumes.

  • Glycogen: Stored in animal liver and muscle (not present in foods).

  • Fiber (cellulose): Whole grains, fruits, vegetables.

Carbohydrate Digestion and Absorption

• Digestion begins in the mouth (salivary amylase), continues in the stomach (minimal), and is completed in the small intestine (pancreatic amylase).

• Enzymes involved: Amylase (salivary, pancreatic), maltase, sucrase, lactase (intestinal brush border).

• Carbohydrates are absorbed as monosaccharides (glucose, galactose, fructose) via active and facilitated transport into enterocytes, then transported to the liver via the portal vein.

Lactose Intolerance

• Caused by insufficient lactase enzyme, leading to maldigestion of lactose.

• Symptoms: Nausea, cramps, bloating, flatulence, diarrhea.

• Management: Limit dairy, use lactose-free products, or consume lactase supplements.

Functions of Carbohydrates

• Main energy source (4 kcal/g) for cells, especially the brain and red blood cells.

• Spare protein from being used for energy (protein-sparing effect).

• Support digestive health via dietary fiber.

Carbohydrate Metabolism and Storage

• Dietary carbohydrates are metabolized to glucose for energy.

• Excess glucose is stored as glycogen in liver and muscle; further excess is converted to fat.

Blood Glucose Regulation

• Maintained by hormones:

  • Insulin (from pancreatic beta cells): Lowers blood glucose by promoting cellular uptake and glycogenesis.

  • Glucagon (from pancreatic alpha cells): Raises blood glucose by stimulating glycogenolysis and gluconeogenesis.

  • Epinephrine, cortisol, growth hormone: Counter-regulatory, raise blood glucose during stress or fasting.

  • Somatostatin: Modulates insulin and glucagon release.

Glycemic Response and Index

• Glycemic response: How quickly blood glucose rises after eating carbohydrates.

• Glycemic index (GI): Ranks foods by their effect on blood glucose.

• Low-GI foods (lentils, vegetables) cause gradual rises; high-GI foods (white bread, sugary snacks) cause rapid spikes.

• To reduce glycemic response: Eat fiber-rich foods, combine carbs with protein/fat, include acidic foods.

Health Effects of Added Sugars

• Excess added sugars can lead to:

  • Increased risk of obesity and type 2 diabetes.

  • Dental caries (tooth decay).

  • Elevated triglycerides and cardiovascular disease risk.

• Recommended limit: <10% of daily calories (about 50g for a 2,000 kcal diet); AHA suggests even lower limits.

• Strategies to reduce intake: Choose whole foods, read labels, reduce sugary drinks, use fruit for sweetness.

Non-Nutritive Sweeteners

• Provide intense sweetness with little/no calories or carbohydrates.

• Used for calorie reduction, blood glucose management, and dental health.

• Considered safe by FDA in moderation, though long-term effects are still studied.

Dietary Fiber

• Soluble fiber: Dissolves in water, forms gels, fermented by gut bacteria (e.g., oats, legumes).

• Insoluble fiber: Adds bulk, not fermented (e.g., wheat bran, vegetables).

• Health effects: Promotes regularity, lowers cholesterol, stabilizes blood sugar, supports weight management.

• Recommended intake: 25–38g/day for adults.

Chapter 5: Lipids

Definition and Types of Lipids

• Lipids are hydrophobic compounds including triglycerides, phospholipids, and sterols.

• Functions: Energy storage, insulation, cell membrane structure, hormone synthesis, fat-soluble nutrient transport.

Fatty Acids: Structure and Classification

• Fatty acids: Carbon chains (2–80 carbons) with a carboxyl group.

• Classified by chain length:

  • Short-chain (<8 C), medium-chain (8–12 C), long-chain (>12 C; most common in foods).

• By saturation:

  • Saturated: No double bonds, solid at room temp (e.g., butter, lard).

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

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

• Trans fatty acids: Produced by hydrogenation, associated with increased heart disease risk.

Essential Fatty Acids

• Linoleic acid (omega-6) and alpha-linolenic acid (omega-3) are essential; must be obtained from diet.

• Omega-3: First double bond at third carbon from omega end (e.g., flaxseed, fish).

• Omega-6: First double bond at sixth carbon (e.g., vegetable oils).

Triglycerides, Phospholipids, and Sterols

• Triglycerides: Three fatty acids + glycerol; main storage and dietary fat.

• Phospholipids: Two fatty acids + phosphate + glycerol; form cell membranes, act as emulsifiers (e.g., lecithin).

• Sterols: Four-ring structure (e.g., cholesterol); component of membranes, precursor for hormones and vitamin D.

Lipid Digestion and Absorption

• Begins in mouth (lingual lipase), continues in stomach (gastric lipase), mainly occurs in small intestine.

• Cholecystokinin (CCK) signals gallbladder to release bile for emulsification.

• Pancreatic lipase breaks triglycerides into monoglycerides and free fatty acids, which form micelles for absorption.

• Absorbed as fatty acids and monoglycerides into enterocytes.

Lipid Transport: Lipoproteins

Health Effects and Dietary Recommendations

• Reduce saturated and trans fats; increase unsaturated fats (olive oil, nuts, fatty fish).

• Monounsaturated and polyunsaturated fats are associated with better health outcomes.

• Mediterranean diet: Emphasizes plant-based foods, healthy fats, and moderate fish/poultry.

Chapter 6: Proteins

Amino Acids and Protein Structure

• Amino acids: Building blocks of proteins; contain carbon, hydrogen, oxygen, and nitrogen.

• Types:

  • Essential (9): Must be obtained from diet.

  • Nonessential (11): Synthesized by the body.

  • Conditionally essential: Normally nonessential, but required in certain conditions.

• R groups (side chains) determine each amino acid's properties (size, charge, hydrophobicity).

Levels of Protein Structure

• Primary: Linear sequence of amino acids (peptide bonds).

• Secondary: Alpha-helices and beta-sheets (hydrogen bonding).

• Tertiary: 3D folding due to side chain interactions (disulfide bonds, hydrophobic interactions).

• Quaternary: Multiple polypeptide chains forming a functional protein.

• Denaturation: Loss of structure (and function) due to heat, pH, or chemicals; primary structure remains intact.

Protein Digestion and Absorption

• Begins in stomach (pepsin, HCl), continues in small intestine (pancreatic proteases, brush border enzymes).

• Proteins are broken down to amino acids, dipeptides, and tripeptides, which are absorbed into enterocytes and then into the bloodstream.

Protein Synthesis

• Transcription: DNA sequence copied to mRNA in the nucleus.

• Translation: mRNA decoded at the ribosome to assemble amino acids into a polypeptide chain.

Functions of Proteins

• Structural: Build and repair tissues, muscles, bones.

• Enzymatic: Catalyze biochemical reactions.

• Transport: Carry substances (e.g., hemoglobin).

• Regulatory: Hormones, immune function, fluid balance, acid-base balance (buffers).

Protein Quality and Dietary Needs

• Quality determined by essential amino acid profile and digestibility (PDCAAS score).

• Complete proteins: Contain all essential amino acids (e.g., animal proteins, soy).

• Complementary proteins: Combining plant foods to provide all essential amino acids (do not need to be eaten at the same meal).

• Recommended intake: 10–35% of daily calories from protein.

Nitrogen Balance and Protein Turnover

• Nitrogen balance: Difference between nitrogen intake and excretion.

  • Positive: Growth, pregnancy, recovery.

  • Negative: Illness, injury, inadequate intake.

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

Health Effects of Protein Intake

• Deficiency: Muscle loss, weakness, impaired immunity.

• Excess: May stress kidneys, increase risk of chronic disease if from poor-quality sources.

Special Features of Amino Acids

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

• Deamination: Removal of amino group for energy production or excretion (produces ammonia).

Additional info: Protein digestibility is measured by the Protein Digestibility Corrected Amino Acid Score (PDCAAS). Reference proteins are those with an ideal balance of essential amino acids, such as egg or milk protein.