Human Nutrition
Summary: A balanced diet consists of carbohydrates, proteins, lipids, vitamins, minerals, fibre (roughage), and water in appropriate proportions. Malnutrition results from deficiencies or excesses of specific nutrients. The digestive system breaks down large insoluble food molecules into small soluble molecules through mechanical digestion and enzymatic hydrolysis. Absorption of digested products occurs primarily in the ileum via villi. Tags: igcse biology human-nutrition diet digestive-system enzymes absorption Created: 2026-07-16 Last Updated: 2026-07-16
1. Balanced Diet
A balanced diet provides all necessary nutrients in the correct proportions to maintain health. The seven components of a balanced diet are:
| Nutrient | Function | Sources | Notes |
|---|---|---|---|
| Carbohydrates | Primary energy source (respiration) | Rice, bread, pasta, potatoes, cereals | Starch is broken down to glucose; excess converted to glycogen or fat |
| Proteins | Growth and repair of tissues; synthesis of enzymes, hormones, antibodies | Meat, fish, eggs, dairy, beans, nuts | Made of amino acids; 20 different amino acids, 9 essential (must come from diet) |
| Lipids (fats/oils) | Long-term energy storage; thermal insulation; protection of organs; component of cell membranes | Butter, oils, nuts, fatty meat, dairy | More than twice the energy per gram compared to carbohydrates |
| Vitamin A | Making rhodopsin (visual pigment in retina) — needed for vision in dim light; healthy skin and mucous membranes | Liver, carrots, green leafy vegetables, fish liver oils | Deficiency: night blindness — difficulty seeing in dim light; severe cases lead to total blindness |
| Vitamin C | Synthesis of collagen (connective tissue); antioxidant; aids iron absorption | Citrus fruits, green vegetables, tomatoes | Deficiency: scurvy — bleeding gums, poor wound healing, weakened connective tissue |
| Vitamin D | Promotes calcium absorption for bone formation | Sunlight (UV on skin), oily fish, eggs, fortified milk | Deficiency: rickets (children) — soft, deformed bones; osteomalacia (adults) |
| Calcium (mineral) | Formation of bones and teeth; muscle contraction; blood clotting | Milk, cheese, yogurt, green leafy vegetables | Deficiency: weak bones and teeth, osteoporosis risk |
| Iron (mineral) | Component of haemoglobin (carries oxygen in red blood cells) | Red meat, liver, spinach, beans | Deficiency: anaemia — fatigue, pale skin, shortness of breath (fewer/less effective RBCs) |
| Fibre (roughage) | Adds bulk to food; stimulates peristalsis; prevents constipation | Wholegrains, fruits, vegetables, nuts | NOT digested. Provides bulk to move food through the gut. Lack of fibre linked to bowel cancer |
| Water | Universal solvent for metabolic reactions; transport medium (blood plasma); temperature regulation (sweating); component of cytoplasm | Drinking water, most foods, metabolic water from respiration | ~70% of body mass. Continuous loss through urine, sweat, breath, faeces — must be replaced |
2. Malnutrition and Deficiency Diseases
Malnutrition refers to an unbalanced diet — either too little, too much, or incorrect proportions of nutrients.
| Condition | Cause | Effects |
|---|---|---|
| Starvation / Marasmus | Severe overall energy (calorie) deficiency | Extreme weight loss, muscle wasting, weakened immune system |
| Kwashiorkor | Severe protein deficiency (may have adequate calories) | Swollen abdomen (oedema), stunted growth, muscle wasting, fatty liver |
| Scurvy | Vitamin C deficiency | Bleeding gums, loose teeth, poor wound healing, bruising |
| Rickets | Vitamin D or calcium deficiency | Soft, deformed bones (bowed legs) in children |
| Night blindness | Vitamin A deficiency | Difficulty seeing in dim/dark light |
| Anaemia | Iron deficiency | Fatigue, weakness, pale skin — fewer/less effective red blood cells |
| Obesity | Excess energy intake (calories) over expenditure | Increased risk of heart disease, Type 2 diabetes, joint problems, high blood pressure |
| Constipation | Lack of dietary fibre | Difficulty passing faeces; increased bowel cancer risk |
| Coronary heart disease | Excess saturated fats and cholesterol | Fatty plaques (atheroma) build up in coronary arteries, reducing blood flow to heart muscle |
3. The Digestive System — Anatomy and Functions
The digestive system (alimentary canal) is a tube running from mouth to anus, with associated glands.
| Structure | Function | Key Processes |
|---|---|---|
| Mouth (buccal cavity) | Ingestion of food; mechanical digestion (chewing); begins chemical digestion of starch | Teeth grind food (increase surface area for enzyme action); salivary amylase begins breaking down starch to maltose; tongue shapes food into bolus |
| Oesophagus | Transports food bolus from mouth to stomach | Peristalsis — waves of muscular contraction push food downwards; no digestion occurs here |
| Stomach | Temporary storage; mechanical digestion (churning); chemical digestion of proteins begins | Gastric glands secrete hydrochloric acid (pH ~2) which kills bacteria and provides optimum pH for pepsin (a protease). Pepsin breaks down proteins into shorter polypeptides. Churning action mixes food with gastric juice → semi-liquid chyme |
| Small intestine — duodenum (first part) | Receives chyme from stomach; main site of chemical digestion | Receives bile from liver/gall bladder (emulsifies fats, neutralises stomach acid) and pancreatic juice from pancreas (contains amylase, trypsin/protease, lipase). Digestion of starch, proteins, and lipids is completed here |
| Small intestine — ileum (second part) | Absorption of digested food molecules | Inner surface covered with villi (finger-like projections) and microvilli on villus cells — enormous surface area. Glucose, amino acids, fatty acids, glycerol, vitamins, minerals absorbed into blood (or lymph for some lipids) |
| Large intestine (colon) | Absorption of water and mineral ions | Undigested material passes through; water is reabsorbed; bacteria in colon produce vitamins (B and K). Faeces are formed |
| Rectum | Storage of faeces | Faeces stored until elimination |
| Anus | Egestion of faeces | Faeces (undigested food, bacteria, dead cells) expelled from the body |
Associated glands:
| Gland | Secretion | Contents and Function |
|---|---|---|
| Salivary glands | Saliva | Contains amylase (starch → maltose); water and mucus to lubricate food |
| Gastric glands (stomach lining) | Gastric juice | Contains hydrochloric acid (HCl) and pepsin (a protease active at pH ~2) |
| Pancreas | Pancreatic juice | Contains amylase, trypsin (a protease), lipase, and sodium hydrogencarbonate (alkaline — neutralises stomach acid). Secreted into duodenum via pancreatic duct |
| Liver | Bile | Bile does NOT contain digestive enzymes. Contains bile salts (emulsify fats — break large fat globules into smaller droplets, increasing surface area for lipase); bile pigments (from breakdown of haemoglobin). Bile is alkaline — neutralises stomach acid. Stored in gall bladder |
4. Peristalsis
Definition: Peristalsis is the rhythmic, wave-like contraction and relaxation of the circular and longitudinal muscles in the wall of the alimentary canal, which pushes food along the gut.
Mechanism:
- Circular muscles contract behind the food bolus (narrowing the tube)
- Longitudinal muscles contract ahead of the food bolus (widening the tube)
- This coordinated contraction-relaxation wave pushes food in one direction only — towards the anus
- Peristalsis occurs throughout the gut — oesophagus, stomach, small intestine, large intestine
5. Digestive Enzymes
Digestive enzymes are hydrolases — they break down large molecules by hydrolysis (adding water to break chemical bonds).
| Enzyme | Source | Site of Action | Substrate | Product(s) | Optimum pH |
|---|---|---|---|---|---|
| Amylase | Salivary glands, pancreas | Mouth, duodenum | Starch (polysaccharide) | Maltose (disaccharide) | ~7 |
| Maltase | Small intestine (villus cell membranes) | Ileum | Maltose | Glucose | ~7-8 |
| Protease (pepsin) | Stomach (gastric glands) | Stomach | Proteins | Shorter polypeptides | ~2 |
| Protease (trypsin) | Pancreas | Duodenum | Proteins and polypeptides | Shorter polypeptides and amino acids | ~8 |
| Peptidases | Small intestine | Ileum | Polypeptides | Amino acids | ~7-8 |
| Lipase | Pancreas | Duodenum | Lipids (triglycerides) | Fatty acids + glycerol | ~7-8 |
6. Bile
Bile is produced by the liver, stored in the gall bladder, and released into the duodenum via the bile duct.
Bile has two main functions:
- Emulsification of fats: Bile salts break large fat globules into smaller droplets. This increases the surface area for lipase to act on — speeds up lipid digestion. Bile does NOT chemically digest fats; it is not an enzyme.
- Neutralisation of stomach acid: Bile is alkaline. It neutralises the acidic chyme from the stomach, raising pH to ~7-8 which is the optimum for pancreatic and intestinal enzymes (trypsin, lipase, maltase).
7. Absorption in the Ileum — Villi
The ileum is highly adapted for absorption of digested food molecules.
Structure of a villus (singular: villus; plural: villi):
- A finger-like projection of the inner intestinal wall (~1 mm tall)
- Outer layer: epithelial cells (one cell thick — short diffusion distance)
- Epithelial cells have microvilli on their surface (a “brush border”) — further increases surface area
- Inside the villus: a dense network of blood capillaries (absorb glucose, amino acids, water-soluble vitamins, minerals)
- Inside the villus: a lacteal (lymph vessel) which absorbs fatty acids and glycerol (reformed into lipids, packaged into lipoproteins called chylomicrons)
Adaptations of the ileum for absorption:
- Enormous surface area — villi + microvilli + great length of small intestine (~6 m) + circular folds
- Thin epithelium — one cell thick (short diffusion distance for nutrients)
- Dense capillary network — maintains a steep concentration gradient (nutrients are carried away quickly in blood, so more diffuse in)
- Lacteal — absorbs lipid products into the lymphatic system
8. Assimilation
Definition: Assimilation is the uptake and use of absorbed nutrients by body cells.
| Absorbed Nutrient | Fate in the Body |
|---|---|
| Glucose | Used in respiration to release energy (ATP); excess converted to glycogen (stored in liver and muscles) or fat |
| Amino acids | Used to synthesise new proteins (enzymes, hormones, haemoglobin, antibodies, muscle tissue, structural proteins); excess amino acids are deaminated in the liver — the amino group (NH2) is removed and converted to urea (excreted by kidneys), the remaining part is used for energy or converted to fat |
| Fatty acids and glycerol | Recombined into triglycerides; used for energy, stored in adipose tissue, or incorporated into cell membranes |
| Vitamins and minerals | Used in metabolic pathways (e.g. iron for haemoglobin, calcium for bone, vitamin C for collagen synthesis) |
Sources
- BBC Bitesize GCSE Biology — Nutrition, digestion, and excretion, BBC (free educational resource)
- OpenStax Biology 2e — Ch. 34 Animal Nutrition and the Digestive System, Rice University (free, CC BY 4.0)
- Cambridge IGCSE Biology 0610 — Syllabus 7: Human nutrition, Cambridge Assessment International Education
- CK-12 Biology for High School — Digestive System, CK-12 Foundation (free, CC BY-NC 3.0)
Related Notes
- Biological Molecules — Structure of carbohydrates, proteins, lipids; food tests
- Enzymes — Mechanism of enzyme action, lock-and-key model, digestive enzymes
- Movement Into and Out of Cells — Absorption of digested nutrients by diffusion and active transport
- Transport in Animals — How absorbed nutrients are transported in the blood
- Excretion in Humans — Deamination of amino acids in the liver, urea excretion by kidneys
- IGCSE-Bio-Index — Full IGCSE Biology index
Common Misconceptions
| Misconception | Reality |
|---|---|
| ”Bile contains digestive enzymes” | Bile does NOT contain enzymes. It contains bile salts for emulsification and is alkaline for neutralisation |
| ”The stomach digests all food types” | The stomach mainly digests proteins (pepsin). Carbohydrate digestion (amylase) is halted in the acidic stomach and resumes in the small intestine |
| ”Fibre is digested and provides energy” | Fibre (cellulose) is NOT digested by humans. It provides bulk to stimulate peristalsis and prevent constipation |
| ”Absorption only happens in the small intestine” | Water and mineral ions are absorbed in the large intestine (colon) as well as the small intestine |
| ”All digested food goes directly into the blood” | Most nutrients enter the blood capillaries, but fatty acids and glycerol enter the lacteal (lymph system) before eventually reaching the blood |
| ”The liver only produces bile” | The liver has many functions: bile production, deamination of amino acids, detoxification of alcohol/drugs, storage of glycogen, regulation of blood glucose, breakdown of old red blood cells |