Week 2 – Digestion & Absorption

Nutrient Reference Values

  • Estimated Average Requirement (EAR): intake that meets needs of 50\% of group

  • Recommended Dietary Intake (RDI): meets 97{-}98\% of group

  • Adequate Intake (AI): observed/experimental intake used when RDI absent

  • Estimated Energy Requirement (EER): intake predicted to maintain energy balance (age, sex, wt, ht, activity)

  • Upper Level (UL): highest average intake with no adverse effects

  • Acceptable Macronutrient Distribution Range (AMDR): % of energy from each macronutrient compatible with health

AMDR Ranges

  • Carbohydrate: 45{-}65\% energy

  • Protein: 10{-}35\% energy

  • Fat: 20{-}35\% energy

  • Example: 8000\,\text{kJ/day} → fat within AMDR \rightarrow 43{-}76\,\text{g} (1\,\text{g fat}=37\,\text{kJ})

Digestion vs Absorption

  • Digestion: mechanical & chemical breakdown of food → absorbable units

  • Absorption: transfer of nutrients across intestinal cells into blood or lymph

GI Tract Overview

  • Continuous muscular tube mouth → anus

  • Functions: nutrient extraction & waste elimination

  • Motility: peristalsis (propulsion) & segmentation (mixing)

Mouth

  • Mastication + saliva → bolus; average saliva ≈ 1\,\text{L/day}

  • Salivary amylase: starts starch hydrolysis

  • Lingual lipase: minor fat hydrolysis

  • Epiglottis directs bolus away from trachea

Oesophagus

  • Peristaltic transit to stomach; lower oesophageal sphincter prevents reflux (heartburn if weakened)

Stomach

  • Churns bolus + gastric juice → chyme; pyloric sphincter meters release (≈3 contractions/min)

  • Gastric juice: \text{HCl} (denatures protein, activates pepsin), gastric lipase, mucus

  • Digestion: carbs continue briefly; proteins begin; lipids ≈20\% hydrolysed

Small Intestine

  • Sections: duodenum 30\,\text{cm}, jejunum 2.5\,\text{m}, ileum 4\,\text{m}

  • Surface amplification: folds, villi, microvilli → area ≈ tennis court

  • Secretions
    • Pancreatic juice: \text{HCO}_3^- + amylase, lipase, proteases
    • Bile (liver, stored gallbladder): emulsifies fat
    • Intestinal enzymes complete hydrolysis

  • Slightly alkaline pH; most nutrient absorption occurs here

Absorption Mechanisms

  • Simple diffusion: water, small lipids

  • Facilitated transport: water-soluble vitamins

  • Active transport: glucose, amino acids

  • Transport pathways
    • Bloodstream: water-soluble nutrients, short-chain FAs
    • Lymph (lacteals): large fats + fat-soluble vitamins → chylomicrons → thoracic duct → blood

Large Intestine (Colon)

  • Ileocaecal valve → rectum; reabsorbs water/electrolytes; forms faeces

  • Microbiota ferment fibre, synthesize Vitamin K; metabolites may be reabsorbed

Gut Microbiota & Health

  • Diverse microbiota impede pathogens, modulate immunity, influence brain (gut–brain axis)

  • Probiotics: live microbes providing health benefit (e.g., diarrhoea relief, immune support)

  • Prebiotics: substrates selectively used by microbes conferring health benefit (e.g., fermentable fibres)

Hormonal & Neural Regulation

  • Gastrin (stomach): food → \text{HCl} release

  • Secretin (duodenum): acidic chyme → pancreatic \text{HCO}_3^-

  • Cholecystokinin (CCK) (intestinal wall): fat/protein → bile + pancreatic enzymes, slows motility

  • Feedback ensures pH, enzyme mix match chyme composition; rapid diet change may cause discomfort

Circulatory Transport

  • Hepatic portal vein: carries water-soluble nutrients to liver for processing

  • Hepatic artery: oxygenates liver

  • Lymphatic system: one-way, pump-less; delivers chylomicrons to venous blood near heart, bypassing liver initially

Key Digestive Secretions

  • Saliva: lubricates; amylase initiates starch breakdown

  • Gastric juice: \text{HCl} + pepsin begin protein digestion; mucus protects mucosa

  • Pancreatic juice: enzymes + bicarbonate neutraliser

  • Bile: emulsifies fat for lipase action

  • Intestinal juice: enzymes finish carb, fat, protein hydrolysis; mucus protects wall

Study Reminders

  • Know definitions of EAR, RDI, AI, EER, UL, AMDR

  • Trace macronutrient digestion: enzymes & sites

  • Differentiate absorption routes: blood vs lymph

  • Recall hormonal controls (gastrin, secretin, CCK)

  • Balanced diet, adequate fibre & variety support healthy GI tract and microbiota