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Nutrition and Health Part 2

Relationship Between Food, Nutrients & Body Weight

  • Food supplies macronutrients (carbo-, proteins, lipids) + micronutrients (vitamins, minerals, trace elements)
  • Adequate, balanced intake ➔ growth, maintenance, prevention of chronic disease
  • Inadequate intake (too low or too high energy) ➔ mal-nutrition spectrum
    • Underweight, clinical deficiencies
    • Overweight, obesity and associated NCDs (T2DM, CVD, cancers)

Assessing Healthy Weight & Prevalence Data

  • Common anthropometric tools
    • \text{BMI}=\dfrac{\text{weight\,(kg)}}{[\text{height\,(m)}]^2}
      • <18.5 underweight · 18.5–24.9 healthy · 25–29.9 overweight · 30–34.9 obesity class I · 35–39.9 class II · \ge 40 class III
    • Waist circumference & waist-hip ratio for central adiposity
  • Australian data (National Nutrition & Health Surveys)
    • Rising prevalence of overweight/obesity in adults & children
    • Policy translation gap: strong science, but need public-friendly tools

National Dietary Policy & Reference Systems (Australia)

  • 3 key, publicly accessible references
    1. Australian Guide to Healthy Eating (AGHE) – ‘plate’ model
    2. Nutrient Reference Values (NRVs) – EAR, RDI, AI, UL for each nutrient
    3. Australian Dietary Guidelines (2013) – 5 evidence-based guidelines

1 Australian Guide to Healthy Eating (Plate Model)

  • Visual plate/circle divided into 5 food groups
    • Largest sectors: grain (cereal) foods; vegetables + legumes/beans
    • Moderate: fruit; milk, yoghurt, cheese & alternatives; lean meats, poultry, fish, eggs, tofu, nuts, seeds, legumes
  • Surrounding images (oils, spreads, sugary drinks, alcohol) placed outside ➔ discretionary choices
  • Emphasises dietary diversity: wide variety within & across groups

2 Nutrient Reference Values (NRVs)

  • Based on:
    • National intake data, biomarker studies, disease-prevention evidence
    • Uses healthy population intakes to model minimums & safe upper limits
  • Provide population-specific RDIs for sex/age/life-stage (pregnancy, lactation)

3 Australian Dietary Guidelines (ADGs)

  1. Achieve & maintain a healthy weight, be physically active, choose nourishing foods
  2. Enjoy a wide variety of nutritious foods from the 5 groups + 2–2.5\,\text{L} water daily
  3. Limit intake of foods high in added sugars, saturated fat & salt; choose reduced-salt/fat versions; limit alcohol (nil for pregnancy/lactation)
  4. Encourage, support & promote breastfeeding
  5. Care for your food: safe preparation & storage practices

Hydration Guideline

  • General recommendation: 2\text{–}2.5\,\text{L water·day}^{-1} for adults (adjust for climate, activity)

Digestion & Absorption Fundamentals

  • 6 key sequential events = bioavailability cascade
    1. Ingestion & mechanical breakdown (mouth)
    2. Solubilisation to absorbable forms
    3. Absorption across intestinal epithelium
    4. Organ/tissue uptake via receptors
    5. Metabolism (chemical transformation, storage, utilization)
    6. Secretion (enzyme/hormone release) & excretion (urine, faeces, bile)
  • Bioavailability = nutrient quantity that reaches systemic circulation (X − digestive losses − absorptive losses − utilisation losses)

Gastro-intestinal Anatomy Recap

  • Oral cavity → oesophagus → stomach → small intestine (duodenum, jejunum, ileum) → large intestine
  • Accessory organs: pancreas, liver, gallbladder; portal vein to liver (first-pass metabolism)
  • Small-intestinal mucosa covered in villi/microvilli = “brush border” (large surface area for absorption)

Macronutrient Digestion Details

Protein

  • Begins in stomach (acidic pH \sim 1.5–2)
    • Gastric juice: \text{HCl} + pepsin (infants: rennin – milk-coagulating)
  • Small intestine (alkaline pH \sim 7–8): pancreatic proteases/peptidases continue hydrolysis
  • Absorption as free AAs, di- & tri-peptides through brush border; ~10–30\% enter as peptides

Carbohydrate

  • Starts in mouth: salivary amylase ptyalin hydrolyses starch → maltose
  • Activity halts in acidic stomach; resumes in small intestine via pancreatic amylase + disaccharidases (maltase, lactase, sucrase) at brush border → monosaccharides (glucose, galactose, fructose) absorbed

Lipid

  • Minimal oral digestion (possible lingual lipase)
  • Stomach: churning + acid form coarse oil-in-water emulsion
  • Duodenum: bile salts (liver/gallbladder) emulsify further; pancreatic lipase hydrolyses triacylglycerol → 2-monoacylglycerol + free fatty acids
  • Products + bile salts form micelles → diffuse into jejunal mucosa; re-esterified to TAGs, packed into chylomicrons → lymph → blood

Micronutrient Malnutrition

Common Global Deficiencies

  • Iron (most prevalent)
  • Vitamin A
  • Iodine
  • Folate (Vitamin B9)
  • Others: zinc, vitamin D, calcium, selenium, manganese

Iron Deficiency & Anaemia

  • Indicator: low haemoglobin (Hb) cut-offs
    • Children 6 mo–6 yr <11\,\text{g dL}^{-1}
    • Children 6–14 yr <12\,\text{g dL}^{-1}
    • Adult men <13; non-pregnant women <12; pregnant women <13
  • Symptoms: fatigue, pallor (palpebral conjunctiva), spoon-shaped nails (koilonychia), impaired cognition, ↓ immune function, ↓ work capacity; adverse pregnancy outcomes
  • Anaemia type: microcytic hypochromic (small, pale RBCs)

Iodine Deficiency Disorders (IDD)

  • Thyroid hormone (thyroxine T4) synthesis requires iodine (≥150\,\mu g\,\text{day}^{-1} adults)
  • Endemic in mountainous/rain-leached soils ➔ low iodine crops
  • Spectrum: goitre (thyroid enlargement), hypothyroidism, growth retardation, cretinism
  • Prevention: iodised salt (global public-health success)

Folate (Vitamin B9) Deficiency

  • Functions: one-carbon transfers for DNA/RNA synthesis, methylation reactions
  • Increased demand in pregnancy/lactation
  • Deficiency manifestations:
    • Megaloblastic/macrocytic anaemia (large, fragile RBCs)
    • Neural Tube Defects (NTDs) in foetus: spina bifida, anencephaly, encephalocoele
      • Neural tube closes by day \approx23 post-conception ➔ periconceptional folate status critical
  • Intake targets: adults 400\,\mu g\,\text{day}^{-1}; +200 pregnancy; +150 lactation
  • Australian mandatory fortification: folic acid added to bread-making flour → ↓ NTDs by \sim14\%

Vitamin A Deficiency

  • Earliest sign: night blindness (nyctalopia)
  • Progressive ocular changes (WHO stages):
    1. Night blindness
    2. Conjunctival xerosis (dry, inflamed sclera) – reversible
    3. Bitot’s spots (foamy patches)
    4. Corneal xerosis/keratomalacia – irreversible blindness
  • Prevention: adequate dietary retinol/β-carotene; food-based strategies (e.g., biofortified “golden” crops)

Additional Numerical & Reference Values

  • Water intake: 2\text{–}2.5\,\text{L day}^{-1}
  • WHO upper limit for total fat: \le30\% total energy
  • Energy densities: carbohydrates & protein \approx4\,\text{kcal g}^{-1}, fat 9\,\text{kcal g}^{-1}, alcohol 7\,\text{kcal g}^{-1}

Safe Food Handling & Nutrition

  • Guideline 5 emphasises food safety: hygienic prep, correct storage, avoid contamination ➔ nutrition & safety are inseparable

Translating Science to Consumers

  • Need clear, engaging tools: AGHE plate, traffic-light labels, Health Star Rating, mobile apps
  • Example classroom activity: “FoodSwitch” app
    • Barcode scan ➔ reveals nutrient profile, traffic-light colours, Health-Star Rating, ‘switches’ for lower salt/fat/sugar alternatives
    • Specialised modes: FatSwitch, SaltSwitch, SugarSwitch, EnergySwitch, GlutenSwitch (subscription)
    • Encourages real-time healthier choices (e.g., muffins high fat, Oreo double-stuff high added sugar 44.1\,g/serve, Coke Zero shows 0 added sugar but still acidic, recommend water)

Key Enzymes & Their Substrates (Exam Quick-Look)

  • Ptyalin (salivary amylase) – starch
  • Pepsin – protein (stomach)
  • Rennin – casein in milk (infants)
  • Pancreatic amylase – starch/maltodextrins
  • Pancreatic proteases (trypsin, chymotrypsin, carboxypeptidase) – peptides
  • Pancreatic lipase – triacylglycerols
  • Brush-border disaccharidases – maltose, lactose, sucrose

Exam & Study Reminders

  • Always link nutrient to primary role, deficiency sign, vulnerable group & preventive strategy
  • Remember the bioavailability mantra: intake ≠ absorbed ≠ utilized
  • Be able to sketch/label GI tract + accessory organs + first-pass to liver
  • Know Australian Dietary Guidelines wording + 5 food groups
  • Practise calculating BMI, energy contribution of macros, interpreting traffic-light labels