Nutrients
What the Body Actually Needs🥦 Food Is More Than Fuel
A nutrient is any substance obtained from food that the body uses for energy, growth, repair, or regulation. Nutrients are divided into macronutrients (needed in large amounts: carbohydrates, proteins, fats) and micronutrients (needed in small amounts: vitamins and minerals). Water is also essential. Each nutrient has specific roles — deficiency in any one leads to predictable disease.
Carbohydrates
Primary energy source. Broken down to glucose for cellular respiration.
Proteins
Growth, repair, enzymes, antibodies, hormones. Made of amino acids.
Fats (Lipids)
Energy store, insulation, cell membranes, fat-soluble vitamins.
Vitamins
Regulate metabolic processes. Fat-soluble (A,D,E,K) or water-soluble (B,C).
Minerals
Structural roles (Ca in bones) and regulatory roles (Fe in haemoglobin).
Water
Solvent for reactions, transport medium, temperature regulation.
Macronutrients in Detail
Carbohydrates, Proteins & Fats
The big three — sources, functions, and what they break down into.
| Nutrient | Building blocks | Sources | Functions | Deficiency |
|---|---|---|---|---|
| Carbohydrates | Monosaccharides (glucose, fructose, galactose) | Bread, rice, pasta, fruit, sugar | Primary energy source; glucose used in cellular respiration; excess stored as glycogen (liver/muscle) or fat | Fatigue, muscle breakdown (body catabolises protein for energy) |
| Proteins | Amino acids (20 types; 8 essential — must be obtained from diet) | Meat, fish, eggs, legumes, dairy | Growth and repair of tissues; enzymes; antibodies; hormones (e.g. insulin); haemoglobin | Kwashiorkor (protein deficiency) — muscle wasting, oedema, enlarged belly in children |
| Fats (Lipids) | Fatty acids + glycerol | Oils, butter, nuts, meat fat, avocado | Long-term energy storage; insulation; cell membrane phospholipids; absorbing fat-soluble vitamins (A,D,E,K); myelin sheath | Fat-soluble vitamin deficiencies; essential fatty acid deficiency |
Micronutrients
Key Vitamins & Minerals to Know
Small amounts, big consequences. Know the deficiency diseases by name.
| Nutrient | Function | Source | Deficiency Disease / Effect |
|---|---|---|---|
| Vitamin A | Vision (rhodopsin production); immune function; skin | Liver, carrots, leafy greens | Night blindness; xerophthalmia (dry eyes) |
| Vitamin C | Collagen synthesis; antioxidant; immune function | Citrus fruit, peppers, broccoli | Scurvy — bleeding gums, slow wound healing, joint pain |
| Vitamin D | Calcium absorption from gut; bone mineralisation | Sunlight, fatty fish, fortified dairy | Rickets (children) — soft, bowed bones; osteomalacia (adults) |
| Iron (Fe) | Component of haemoglobin — oxygen transport in red blood cells | Red meat, spinach, legumes | Iron-deficiency anaemia — fatigue, pale skin, breathlessness |
| Calcium (Ca) | Bone and teeth structure; muscle contraction; nerve impulses; blood clotting | Dairy, leafy greens, sardines | Rickets/osteoporosis; muscle cramps; poor clotting |
| Iodine (I) | Component of thyroid hormones (thyroxine) — regulate metabolic rate | Iodised salt, seafood | Goitre (enlarged thyroid); cretinism in foetus if mother deficient |
⚠️ Exam Watch — Kwashiorkor vs Marasmus
Both are forms of malnutrition in children but differ in cause. Kwashiorkor = protein deficiency (may have adequate calories) → oedema (fluid retention causing swollen belly), muscle wasting, skin lesions. Marasmus = total calorie and protein deficiency (starvation) → extreme wasting of all body tissues, no oedema. The swollen belly in kwashiorkor is from low blood protein causing fluid to leak from blood into tissues, not from food.
Ingestion & the Mouth
Where It All Begins🦷 The Loading Bay
Ingestion is the taking in of food. Digestion begins in the mouth — both mechanically (teeth and tongue breaking food into smaller pieces) and chemically (salivary amylase beginning starch digestion). The result is a bolus — a soft, rounded ball of partially digested food — which is swallowed and enters the oesophagus.
Mechanical Digestion
Types of Teeth & Their Functions
Four types of teeth, four different jobs. Know the dental formula and what each type does.
| Tooth Type | Number (adult) | Shape | Function | Position |
|---|---|---|---|---|
| Incisors | 8 (4 top, 4 bottom) | Flat, chisel-shaped | Biting and cutting food into pieces | Front of mouth |
| Canines | 4 (2 top, 2 bottom) | Pointed, conical | Tearing and gripping food (especially meat) | Either side of incisors |
| Premolars | 8 (2 per quadrant) | Broader, 2 cusps | Crushing and grinding food; assist molars | Behind canines |
| Molars | 12 (including wisdom teeth) | Large, flat, multiple cusps | Grinding and crushing food into fine particles | Back of mouth |
📌 Tooth Structure — Know All Layers
From outside in: Enamel — hardest substance in the body; protects crown; no living cells; cannot repair itself. Dentine — bone-like; makes up bulk of tooth; sensitive if exposed. Pulp cavity — living core; contains blood vessels (nourish tooth) and nerves (sensation). Cement — covers root; anchors tooth to jawbone via periodontal ligament. Crown = part above gum. Root = part below gum in socket (alveolus).
Chemical Digestion Begins
Saliva & Salivary Amylase
The mouth's secret weapon — chemical digestion starts before you even swallow.
💧 Composition of Saliva
- ~99% water — dissolves food chemicals for taste
- Salivary amylase (ptyalin) — enzyme that breaks starch → maltose
- Mucus — lubricates food to form bolus; eases swallowing
- Antibacterial agents (lysozyme) — defence against pathogens
- Bicarbonate ions — buffer; maintains pH ~7 optimal for amylase
- Produced by 3 pairs of salivary glands: parotid, sublingual, submandibular
🧪 Salivary Amylase Action
- Substrate: starch (polysaccharide)
- Product: maltose (disaccharide)
- Optimal pH: ~7 (neutral) — works in mouth
- Inactivated in stomach (pH drops to 1.5–3.5) — acid denatures it
- Only acts on starch — not on proteins or fats
- This is why chewing bread for a long time produces a slightly sweet taste — maltose accumulating
From Mouth to Stomach
Swallowing & Peristalsis
The bolus travels 25 cm down the oesophagus in about 8 seconds. No digestion here — just transport.
🔄 Peristalsis
- Wave-like muscular contractions that push food along the gut
- Circular muscle behind bolus contracts (narrows) — pushes forward
- Circular muscle ahead relaxes (widens) — allows food to move in
- Occurs throughout the entire digestive tract
- Works against gravity — why astronauts and even upside-down people can swallow
🛡️ Epiglottis — The Traffic Controller
- Flap of cartilage at the back of the throat
- During swallowing: epiglottis folds DOWN over the trachea (windpipe)
- This directs food into the oesophagus, not the lungs
- Failure of this reflex = choking
- You cannot breathe and swallow simultaneously — epiglottis ensures this
Digestion
The 9-Metre Production Line⚗️ Breaking It All Down
Digestion is the breakdown of large, insoluble food molecules into small, soluble ones that can be absorbed into the blood. It happens in two ways: mechanical digestion (physical breakdown — chewing, churning) increases surface area, and chemical digestion (enzyme action) breaks molecular bonds. Different regions of the gut specialise in digesting different nutrients.
The Human Digestive System — Journey of a Meal
Station 2
The Stomach
A muscular bag that churns food and begins protein digestion. pH drops to 1.5 — nothing survives unchanged.
🏗️ Structure & Mechanical Digestion
- J-shaped muscular bag; capacity ~1.5 litres
- Three layers of smooth muscle — churns food in all directions
- Produces chyme — thick, acidic, semi-liquid mixture
- Rugae (folds) — allow stomach to expand when full
- Pyloric sphincter — controls release of chyme into small intestine
- Cardiac sphincter (lower oesophageal) — prevents acid reflux
🧪 Gastric Juice — What the Stomach Secretes
- Hydrochloric acid (HCl) — pH 1.5–3.5; kills bacteria; denatures proteins; activates pepsinogen → pepsin; stops salivary amylase action
- Pepsinogen — inactive precursor activated by HCl → pepsin
- Pepsin — protease enzyme; breaks proteins → polypeptides
- Mucus — protects stomach lining from self-digestion
- Renin (in infants) — curdles milk protein casein for digestion
⚠️ Exam Watch — Why Pepsin Is Secreted as Pepsinogen
If pepsin were secreted in its active form, it would digest the stomach wall itself. Secreting the inactive precursor (pepsinogen) and only activating it in the acidic stomach lumen (away from the mucus-protected wall) is a safety mechanism. This is also why antacids help with heartburn — they neutralise excess HCl, reducing pepsin activation and acidity.
Station 3 — The Main Event
The Small Intestine
6–7 metres of digestion AND absorption. The duodenum finishes digestion; the ileum absorbs everything.
🏭 Duodenum (first 25 cm)
- Receives chyme from stomach + bile from liver + pancreatic juice from pancreas
- Bile — produced in liver; stored in gall bladder; emulsifies fats (breaks large fat globules into small droplets — increases surface area for lipase). Bile is NOT an enzyme.
- Pancreatic juice — alkaline (neutralises stomach acid); contains pancreatic amylase, pancreatic lipase, trypsin, chymotrypsin
- Most chemical digestion completed here
🔬 Ileum (remaining ~6 m)
- Secretes intestinal juice (succus entericus) — contains maltase, sucrase, lactase, peptidases
- Final digestion of disaccharides → monosaccharides; peptides → amino acids
- Highly adapted for absorption — villi, microvilli (brush border)
- Monosaccharides and amino acids → blood capillaries
- Fatty acids and glycerol → lacteal (lymph vessel) in villi
📌 Bile Is Not an Enzyme — A Common Mistake
Bile does not chemically digest fats. It emulsifies them — breaking large fat droplets into many small ones, massively increasing the surface area available for lipase to act on. Think of bile as dish soap — it makes fat mix with water but doesn't break the chemical bonds. Lipase does the actual chemical digestion.
Station 4
Large Intestine & Egestion
No more digestion — just water recovery and forming faeces. ~1.5 metres of waste management.
💧 Large Intestine Functions
- Reabsorbs water and mineral ions from undigested material
- Contains billions of symbiotic bacteria (microbiome) that produce vitamin K and some B vitamins
- Compacts waste into faeces (indigestible material, dead cells, bacteria)
- Faeces stored in rectum until defecation
- No digestive enzymes secreted here
🚽 Egestion vs Excretion
- Egestion = removal of undigested/unabsorbed material as faeces. This material never entered the blood or body cells — it was never fully part of the body.
- Excretion = removal of metabolic WASTE produced inside body cells (e.g. urea, CO₂, excess water). These were inside the body's metabolism.
- Faeces = egestion. Urine = excretion. CO₂ via lungs = excretion.
Absorption
Getting Into the Bloodstream🩸 From Gut to Blood
Absorption is the movement of digested food molecules from the small intestine into the blood and lymph. The ileum is supremely adapted for this — its surface area is roughly the size of a tennis court, despite fitting into the abdominal cavity. Three levels of folding achieve this: circular folds → villi → microvilli (brush border).
Key Adaptation
Villi — The Absorption Specialists
Finger-like projections that massively increase surface area. Know every structural feature and why it exists.
| Feature of Villus | How It Aids Absorption |
|---|---|
| Finger-like projection (~1mm tall) | Greatly increases surface area for absorption — more membrane exposed to digested food |
| Microvilli (brush border) on epithelial cells | Further multiplies surface area; also contain digestive enzymes (maltase, peptidases) |
| Single layer of epithelial cells | Short diffusion distance — molecules cross quickly into blood or lymph |
| Dense network of blood capillaries | Absorb glucose and amino acids directly; maintain concentration gradient (blood carries absorbed molecules away) |
| Central lacteal (lymph vessel) | Absorbs fatty acids and glycerol (reformed into triglycerides inside epithelial cells, then packaged as chylomicrons) |
| Thin walls | Reduces diffusion distance for all nutrients |
| Good blood supply (portal vein) | Maintains steep concentration gradient; blood flow constantly removes absorbed nutrients |
🍬 What Goes Where
- Glucose → blood capillaries → hepatic portal vein → liver
- Amino acids → blood capillaries → hepatic portal vein → liver
- Fatty acids + glycerol → reassembled into triglycerides in epithelial cell → packaged as chylomicrons → lacteal → lymph → bloodstream
- Water-soluble vitamins (B, C) → blood capillaries
- Fat-soluble vitamins (A, D, E, K) → lacteal (with fats)
🏥 The Hepatic Portal System
- All blood from the small intestine drains into the hepatic portal vein → liver BEFORE entering general circulation
- Liver regulates blood glucose (stores excess as glycogen)
- Liver detoxifies absorbed substances (alcohol, drugs)
- Liver deaminates excess amino acids → urea
- This means the liver "sees" everything absorbed from food first
Digestive Enzymes
The Chemical Breakdown Crew🔬 Enzymes — The Molecular Scissors
Digestive enzymes are biological catalysts that break large food molecules into smaller ones. Each enzyme is specific to its substrate, works best at a particular pH, and is named after its substrate with the suffix -ase. Knowing which enzyme acts where, on what, and at what pH is non-negotiable exam content.
| Enzyme | Where Produced | Where Acts | Substrate | Product | Optimal pH |
|---|---|---|---|---|---|
| Salivary amylase | Salivary glands | Mouth | Starch | Maltose | ~7 (neutral) |
| Pepsin | Stomach (as pepsinogen) | Stomach | Proteins | Polypeptides | ~2 (acidic) |
| Pancreatic amylase | Pancreas | Duodenum | Starch | Maltose | ~7–8 |
| Pancreatic lipase | Pancreas | Duodenum | Emulsified fats | Fatty acids + glycerol | ~7–8 |
| Trypsin | Pancreas (as trypsinogen) | Duodenum | Proteins/polypeptides | Smaller peptides | ~8 (alkaline) |
| Maltase | Intestinal wall (brush border) | Ileum | Maltose | Glucose + Glucose | ~7 |
| Sucrase | Intestinal wall | Ileum | Sucrose | Glucose + Fructose | ~7 |
| Lactase | Intestinal wall | Ileum | Lactose | Glucose + Galactose | ~7 |
| Peptidases (erepsin) | Intestinal wall | Ileum | Peptides | Amino acids | ~7–8 |
⚠️ Exam Watch — Complete Digestion Summary
Carbohydrates: Starch → (amylase) → Maltose → (maltase) → Glucose + Glucose. Sucrose → (sucrase) → Glucose + Fructose. Lactose → (lactase) → Glucose + Galactose.Proteins: Protein → (pepsin) → Polypeptides → (trypsin) → Peptides → (peptidases) → Amino acids.
Fats: Large fat globules → (bile, emulsification) → Small fat droplets → (lipase) → Fatty acids + Glycerol.
Application
Lactose Intolerance — When an Enzyme Is Missing
Why some people cannot drink milk — a real-world application of enzyme biology.
🔬 The Biology
- Lactase is the enzyme that breaks lactose (milk sugar) → glucose + galactose
- Most humans produce lactase as infants — needed for breast milk
- In many adults (especially in African, Asian, and Indigenous populations), lactase production decreases after weaning
- Without lactase: undigested lactose reaches the large intestine
- Gut bacteria ferment the lactose → gas (CO₂, H₂) and lactic acid → bloating, cramping, diarrhoea
💊 Management
- Avoid or reduce dairy products
- Use lactase enzyme supplements before eating dairy
- Consume lactose-free dairy products (pre-treated with lactase)
- Yoghurt and hard cheeses have lower lactose — often better tolerated
- Lactose intolerance is NOT an allergy — it is an enzyme deficiency. Dairy allergy involves an immune response to milk proteins (casein, whey).
🎯 Catering Company Inspection
Eight questions on animal nutrition.
Question 1 of 8
Salivary amylase begins digesting starch in the mouth, but its action stops in the stomach. Why?
Question 2 of 8
Bile is described as "emulsifying" fats. What does emulsification mean, and why is it important for fat digestion?
Question 3 of 8
A child presents with a swollen abdomen, muscle wasting, and oedema (fluid accumulation under the skin), but has been receiving adequate calories from a carbohydrate-based diet. What condition does this describe?
Question 4 of 8
Why is pepsin secreted as the inactive precursor pepsinogen rather than in its active form?
Question 5 of 8
List the correct sequence of events in the complete digestion of a starch molecule from mouth to absorption.
Question 6 of 8
The ileum is highly adapted for absorption. List TWO structural features of a villus and explain how each feature aids absorption.
Question 7 of 8
After absorption, glucose and amino acids travel in the blood to the liver first, via the hepatic portal vein. Why is this important?
Question 8 of 8
What is the difference between egestion and excretion? Give an example of each.