Chapter 3 – The Remarkable Body

Focus of Chapter 3 – "The Remarkable Body"

• Centrepiece: how nutrients move through and are transformed by the body – “How Does a Sandwich Become Me?” (p. 88)
• Examines: cellular needs ➜ fluid transport ➜ regulation ➜ digestion & absorption ➜ storage ➜ special topic – alcohol

The Body’s Cells, Tissues, Organs, Systems

• All cells require: energy (ATP), oxygen, water, nutrients
• Genes dictate each cell’s work ➜ influence nutrient handling & specialised functions
• Biological hierarchy: Cells ➜ Tissues ➜ Organs ➜ Body Systems
• Significance: nutritional status can affect gene expression; conversely, genetics affects nutrient utilisation

Transport of Nutrients – Body Fluids

Blood (cardiovascular)

• Carries water-soluble nutrients
• Route: small intestine ➜ portal vein ➜ liver ➜ heart ➜ systemic circulation (see Fig 3-3)
• Arteries deliver O₂ & nutrients; capillaries perform two-way exchange with extracellular fluid
• Veins return blood ➜ lungs (CO₂ out, O₂ in) ➜ kidneys (nephrons filter wastes ➜ urine)

Lymph (lymphatic)

• Carries fat-soluble nutrients (lipids, fat-soluble vitamins)
• Path: intestinal lacteals ➜ lymph vessels ➜ thoracic duct ➜ bloodstream near heart (“mixing oil with water” metaphor)

Regulation & Communication

• Hormones: chemical messengers from glands
– Insulin ↓ blood glucose; Glucagon ↑ blood glucose
– Epinephrine triggers “fight-or-flight” ➜ mobilises glucose & fat, raises heart rate, etc.
• Nervous system: senses hunger, regulates gastric motility & secretion
• Integrated hormone-nerve response allows rapid adaptation to threats & feeding state

Digestive System – Overview (Fig 3-8 & Table 3-1)

• Purpose: break macronutrients into absorbable units
– Carbohydrate ➜ monosaccharides
– Protein ➜ amino acids
– Fat ➜ fatty acids + monoglycerides/glycerol

Digestion Step-by-Step

Mouth

• Taste buds assess acceptability (start of food choice & safety)
• Mechanical: chewing + saliva coats food
• Chemical: salivary amylase ⇒ begins starch → maltose

Esophagus

• Propels bolus by peristalsis (wave-like muscle squeezing)
• Cardiac (lower esophageal) sphincter guards entry to stomach; malfunction ➜ heartburn

Stomach

• Mechanical: 3 muscle layers churn food ➜ chyme
• Chemical: gastric juice (≈ pH 2) contains
– HCl → denatures proteins, kills microbes
– Pepsin → initiates protein hydrolysis
• Valves: cardiac sphincter (entry), pyloric valve (exit)
• Issues: heartburn, GERD; antacids give temporary relief

Small Intestine – Major Site (≈ 20 ft)

• Pancreatic bicarbonate neutralises acid
• Enzymes:
– Pancreatic amylase (CHO)
– Pancreatic lipase (fat)
– Trypsin & chymotrypsin (protein)
• Mechanical segmentation mixes chyme
• Hormones from pancreas: insulin & glucagon regulate post-meal metabolism
• Bile (liver-made, gall-bladder-stored) enters via bile duct ➜ emulsifies fat → ↑ surface area for lipase

Microbiota

• Trillions of bacteria reside mainly in large intestine but influence entire tract
• Roles: ferment fibre, produce vitamins, modulate immunity, affect body weight/metabolism

Absorption (Fig 3-12)

• Nutrients cross intestinal epithelium into:
– Blood (water-soluble)
– Lymph (fat-soluble)
• Massive surface: folds ➜ villi ➜ microvilli (brush border)
• Unabsorbed residues (e.g., fibre) proceed to colon

Large Intestine & Elimination

• Reabsorbs water & minerals
• Passes fibre, bacteria, some water ➜ rectum
• Rectum stores feces before defecation

Common Digestive Concerns (Consumer Guide)

• Gas & hiccups – normal fermentation / diaphragm spasm
• Heartburn / GERD – backflow of acid; chronic ➜ esophageal damage
• Ulcer – erosion by HCl + infection (H. pylori)
• Choking / dysphagia – food enters airway (see Fig 3-13)
• Diarrhea – frequent watery stools; risk of dehydration
• Constipation – infrequent, hard stools; often low fibre/fluids or inactivity; remedy slowly ↑ fibre + water + activity
• IBS (irritable) & IBD (inflammatory) – functional vs. autoimmune disorders
• Foods that raise intestinal gas (Table 3-2): beans, cabbage, carbonated drinks, etc.

Body Storage Systems

• Excess energy-yielding nutrients stored as:
– Fat in adipose tissue (largest capacity)
– Glycogen in liver & muscle (limited)
• Other nutrient reserves: bone stores Ca, P, Mg, etc.
• Importance: maintain blood nutrient levels during fasting; over-storage ➜ obesity

Controversy 3 – Alcohol (Ethanol)

Chemical Nature & Energy

• Lipid-solvent; crosses membranes easily; denatures proteins
• Provides energy but not a nutrient
– Energy density: 7\,\text{cal g}^{-1}

Standard Servings & kcal (Fig C3-1)

• 12 oz beer ≈ 150 kcal (light ≈ 100)
• 5 oz wine ≈ 125 kcal (dessert wine ≈ 210)
• 1.5 oz 86-proof liquor ≈ 105 kcal
• 8 oz soft drink (for comparison) ≈ 100 kcal

Absorption & Diffusion (Fig C3-2)

• Some alcohol diffuses through stomach ➜ blood; food slows this
• Most absorbed rapidly in small intestine irrespective of food
• High doses trigger vomiting – protective reflex

Immediate Physical Effects

• Diuretic: ↑ urine output ➜ mineral losses
• Rate matters:
– Slow sipping ➜ liver handles most
– Rapid intake ➜ spill-over to brain & other tissues

Alcohol & the Brain (Fig C3-3)

• Central-nervous-system depressant
• Progressive sedation sequence:

1. Judgment/reasoning (frontal cortex)

2. Voluntary muscle control

3. Respiration & heart (brain stem) – reaching this stage can be fatal
   • “Passing out” (loss of consciousness) limits further intake

Metabolism – The Liver

• Detoxification pathways:
– \sim80\% by alcohol dehydrogenase (ADH)
– \sim10\% by MEOS (microsomal ethanol-oxidising system); inducible ➜ tolerance
– \sim10\% excreted via breath & urine (basis for breathalyser)
• Females possess less gastric ADH ➜ higher blood alcohol for same intake (body composition also differs)

Liver Damage Progression

• Acute & chronic alcohol stimulate hepatic fat synthesis
• Continuum: fatty liver ➜ fibrosis (scar) ➜ cirrhosis (irreversible, functional loss)

Whole-Body Consequences

• ≈ 1.5 h required to metabolise one standard drink – cannot be hastened by coffee, exercise, etc. (Table C3-7)
• Preferential oxidation of ethanol ➜ dietary fat stored, especially visceral/abdominal
• Hangover mechanisms:
– Congener toxicity
– Brain dehydration/re-hydration
– Formaldehyde accumulation (metabolic intermediate)

Long-Term Health Risks

• Impaired amino-acid & lipid metabolism ➜ fatty blood profile
• Myopathy (muscle toxicity)
• Hypertension
• Neurological: ↑ dementia risk
• Cancers: mouth, pharynx, larynx, esophagus, liver, breast
• Young adults: ↑ overall mortality
• Table C3-3 lists populations who should not drink (pregnant, meds, liver disease, etc.)

Alcohol & Nutrition

• Displaces food ➜ inadequate intake
• Interferes with absorption/metabolism of many nutrients
• Classic deficiency: thiamin ➜ Wernicke-Korsakoff syndrome (confusion, ataxia, eye problems)

Moderate Drinking – Benefits?

• Recent evidence: minimal cardiovascular benefit may be outweighed by cancer risk even at low intakes – consensus shifting towards "less is better".

Appetite Effects

• Generally suppresses appetite
• In some older adults, a small glass of wine may stimulate appetite, improving energy intake