9/9, 9/11, 9/14 Lecture Notes

Diet Planning Tools and Standards

• Serving Size vs. Portion Size:

  • Serving Size: Standardized amount listed on the Nutrition Facts label based on typical consumer amounts.

  • Portion Size: The actual amount chosen to eat or drink.

  • Visual Estimation Guide:

  • Fist $\approx 1\,\text{cup}$

  • Palm $\approx 3\,\text{oz}$ meat

  • Thumb $\approx 1\,\text{tbsp}$ peanut butter

• Daily Values (DVs): General reference standards used on food labels based on reference values like Recommended Dietary Allowances (RDAs).

  • Example (Calcium): Daily Value = $1300\,\text{mg}$. A serving providing $260\,\text{mg}$ contains $260 \div 1300 = 20\%$ DV.

• Food Label Claims:

  • Nutrient Content Claim: Direct statements describing nutrient levels (e.g., "Low Fat").

  • Health Claim: Links a food component to reduced disease risk (e.g., "may reduce the risk of heart disease").

  • Structure/Function Claim: Describes nutrient effects on normal body structures or functions (e.g., "Supports strong bones").

• Diet Evaluation Tools:

  • RDAs: Individual nutrient intake recommendations.

  • Food Label: Tool for evaluating individual food items.

  • MyPlate: Population guide for overall dietary patterns.

• Core Dietary Principles:

  • Variety: Selecting diverse foods across and within food groups.

  • Balance: Proportionally including all major food groups.

  • Moderation: Managing portion size and consumption frequency without completely eliminating foods.

The Human Digestive System

The gastrointestinal (GI) tract processes food sequentially:

• Mouth & Salivary Glands: Functions include mastication (chewing), taste perception, lubrication via mucus, and initial chemical breakdown of starch via salivary amylase.

• Esophagus: Moves food mass to the stomach using peristalsis (wavelike muscular contractions). The lower esophageal sphincter prevents gastric acid reflux (heartburn).

• Stomach: Secretes stomach acid ($\text{HCl}$) and pepsin to digest proteins, dissolves minerals for absorption, kills microorganisms, and regulates the release of acidic chyme into the small intestine via the pyloric sphincter.

• Small Intestine: Spans approximately $10\,\text{ft}$ in length; serves as the primary site of nutrient digestion and absorption using folded inner walls covered in villi.

• Large Intestine: Reabsorbs water and electrolytes, maintaining fluid balance. Houses the gut microbiota ($100\,\text{trillion}$ microorganisms) which ferment undigested materials and synthesize vitamins. Compacts waste into feces. Severe diarrheal disease rapidly depletes water and electrolytes, killing approximately $443,832$ children under $5\,\text{years}$ old annually.

• Rectum & Anus: Temporarily holds feces prior to elimination via coordinated muscle contractions.

Nutrient Absorption Mechanisms and Accessory Organs

• Mechanisms of Absorption:

  • Passive Absorption: Movement of nutrients from high to low concentration.

  • Facilitated Absorption: Transport down a concentration gradient using dedicated carrier proteins.

  • Active Absorption: Energy-dependent transport requiring $\text{ATP}$.

• Accessory Organs:

• Liver: Produces bile to emulsify fats and receives absorbed nutrients first via the portal vein.

• Gallbladder: Stores and concentrates bile prior to its release into the small intestine.

• Pancreas: Secretes sodium bicarbonate (neutralizes stomach acid) and digestive enzymes (amylase, proteases, lipases).

  • Nutrient Destination Post-Absorption:

• Immediate Energy/Synthesis: Glucose $\rightarrow$ $\text{ATP}$ production; Amino acids $\rightarrow$ protein synthesis.

• Storage: Glucose $\rightarrow$ glycogen (liver/muscle); Fat $\rightarrow$ adipose tissue; Calcium $\rightarrow$ bone.

• Limited Reserves/Excretion: Excess amino acid nitrogen is converted to urea and excreted in urine; excess water-soluble vitamins are excreted when needs are exceeded.

Carbohydrates: Structures and Functions

Carbohydrates consist of carbon, hydrogen, and oxygen, providing $4\,\text{kcal/g}$ of energy. Plants synthesize glucose via photosynthesis: $6\text{CO}_2 + 6\text{H}_2\text{O} + \text{Energy (Sun)} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$

• Simple Carbohydrates:

  • Monosaccharides: Glucose (blood sugar), Fructose (fruit sugar/high-fructose corn syrup), Galactose (component of lactose), and Pentoses ($5\text{-carbon}$ sugars: Ribose, Deoxyribose).

  • Disaccharides (joined by condensation reactions):

  • Maltose = Glucose + Glucose

  • Sucrose = Glucose + Fructose

  • Lactose = Galactose + Glucose

• Complex Carbohydrates:

  • Oligosaccharides: $3-10$ sugar units (e.g., Raffinose, Stachyose in beans; indigestible, causing bacterial gas production).

  • Polysaccharides:

  • Starch: Digestible plant storage form of glucose.

  • Glycogen: Animal storage form of glucose. Liver glycogen breaks down to maintain blood glucose; Muscle glycogen supplies local muscle energy during exercise.

  • Fiber (Indigestible carbohydrate):

    • Soluble Fiber (pectin, gums, mucilages): Increases satiety, lowers LDL cholesterol.

    • Insoluble Fiber (cellulose, hemicellulose, lignin): Supports bowel function, reduces constipation risk.

• Alternative Sweeteners:

  • Saccharin: Non-caloric; may taste bitter when heated.

  • Aspartame: Contains phenylalanine (carries a warning for individuals with PKU).

  • Sucralose: Heat-stable substitute for baking.

  • Sugar Alcohols (sorbitol, xylitol, erythritol): Large intakes may trigger gas, bloating, or diarrhea.

• Dietary Recommendations:

  • RDA: $130\,\text{g/day}$ (e.g., one $4\text{-inch}$ bagel $\approx 49\,\text{g}$).

  • AMDR: $45\%-65\%$ of total daily calories.

  • Added Sugars: Limit to $<10\%$ of daily intake.

  • Fiber Adequate Intake (AI): $14\,\text{g} / 1000\,\text{kcal}$ ($25\,\text{g/day}$ for women $<50$; $21\,\text{g/day}$ if $50+$; $38\,\text{g/day}$ for men $<50$; $30\,\text{g/day}$ if $50+$).

Carbohydrate Digestion and Metabolism

• Digestion Steps:

  1. Mouth: Salivary amylase breaks starch into polysaccharides and disaccharides. Cooking softens plant tissues, aiding enzymatic action.

  2. Stomach: Stomach acid inactivates salivary amylase; no carbohydrate digestion occurs.

  3. Pancreas & Small Intestine: Pancreatic amylase/dextrinase break polysaccharides into disaccharides. Small intestinal wall enzymes (maltase, sucrase, lactase) break disaccharides into monosaccharides.

  4. Large Intestine: Soluble fiber ferments into short-chain fatty acids and gases; insoluble fiber is excreted in feces.

• Absorption Mechanisms:

  • Glucose & Galactose: Active transport coupled with sodium ($\text{Na}^+$) requiring energy ($\text{ATP}$).

  • Fructose: Facilitated diffusion using carrier proteins without energy expenditure.

• Glycolysis Net Energy: Splitting glucose requires an initial investment of $2\,\text{ATP}$ to yield $4\,\text{ATP}$, producing a net gain of $2\,\text{ATP}$.

Blood Glucose Regulation and Diabetes

• Hormonal Regulation:

  • Insulin: Secreted by pancreatic beta cells following meals; promotes glucose uptake in muscle/adipose tissues and stimulates glycogen synthesis to lower blood glucose.

  • Glucagon: Secreted by pancreatic alpha cells during fasting; stimulates liver glycogen breakdown and gluconeogenesis to raise blood glucose.

  • Epinephrine: Mobilizes stored glycogen during stress or fight-or-flight conditions.

• Pathologies and Disruption:

  • Hyperglycemia: Abnormally high blood glucose levels.

  • Hypoglycemia: Abnormally low blood glucose levels.

  • Lactose Intolerance: Reduced lactase enzyme activity leads to unabsorbed lactose reaching the colon, causing bacterial fermentation, gas, bloating, and diarrhea.

  • Type 1 Diabetes Mellitus: Autoimmune destruction of pancreatic beta cells leads to absent or low insulin production; requires life-long exogenous insulin therapy.

  • Type 2 Diabetes Mellitus: Characterized by insulin resistance and eventual beta-cell exhaustion; managed with nutrition, physical activity, medications, and insulin.

  • Complications: Severe hyperglycemia causes dehydration and diabetic ketoacidosis (DKA); long-term complications include cardiovascular disease, kidney damage, neuropathy, vision loss, and impaired wound healing.