Nutrition & Digestion

**1. Introduction & Core Concept**

You just finished a plate of biryani — rice, meat, oil, and spices. Your body now has to break down those complex molecules into forms it can actually use. That process is digestion. And the reason your body needs different food types — carbohydrates, proteins, fats, vitamins, minerals — is nutrition.

Malnutrition is one of Pakistan's major public health challenges. According to UNICEF, nearly 40% of Pakistani children under 5 suffer from stunted growth due to nutrient deficiency. Understanding nutrition is not just textbook knowledge — it is directly relevant to Pakistan's health crisis.

**2. Core Theory**

2.1 — Nutrients and Their Functions

| Nutrient | Source | Function |

|---------|--------|---------|

| Carbohydrates | Rice, roti, sugar | Energy (glucose for respiration) |

| Proteins | Meat, dal, eggs | Growth, repair, enzymes, antibodies |

| Fats | Ghee, oil, nuts | Energy storage, insulation, cell membranes |

| Vitamins | Fruits, vegetables | Specific metabolic functions (see below) |

| Minerals | Dairy, green veg | Specific structural/metabolic roles |

| Fibre | Vegetables, whole grain | Moves food through gut, prevents constipation |

| Water | Drinks, food | Transport, reactions, temperature regulation |

Key vitamins:

• Vitamin C: prevents scurvy (bleeding gums). Source: citrus fruits.
• Vitamin D: needed for calcium absorption; prevents rickets. Source: sunlight, fish.
• Vitamin A: needed for vision (especially night vision). Source: carrots, liver.

Key minerals:

• Iron: makes haemoglobin (red blood cells). Deficiency → anaemia.
• Calcium: strong bones and teeth; muscle contraction.
• Iodine: makes thyroid hormone; deficiency → goitre.

2.2 — The Digestive System

Food travels: Mouth → Oesophagus → Stomach → Small Intestine → Large Intestine → Rectum → Anus

| Organ | Function |

|-------|---------|

| Mouth | Mechanical digestion (teeth); salivary amylase breaks starch to maltose |

| Stomach | HCl (kills bacteria, activates pepsinogen); pepsin digests protein |

| Small intestine | Pancreatic enzymes (amylase, protease, lipase); bile emulsifies fat; absorption |

| Large intestine | Water absorption; faeces formation |

2.3 — Enzymes in Digestion

| Enzyme | Substrate | Product | Where made |

|--------|---------|--------|-----------|

| Amylase | Starch | Maltose | Salivary glands, pancreas |

| Protease (pepsin, trypsin) | Protein | Amino acids | Stomach, pancreas |

| Lipase | Fats (lipids) | Fatty acids + glycerol | Pancreas |

| Maltase | Maltose | Glucose | Small intestine |

Bile: produced by liver, stored in gall bladder. Emulsifies (breaks fat into droplets) — increases surface area for lipase.

2.4 — Absorption in Small Intestine

Villi increase surface area. Features of villi:

• Large surface area (finger-like projections with microvilli)
• One cell thick (short diffusion distance)
• Good blood supply (capillaries) for quick absorption
• Lacteals for fat absorption (lymph vessels)

Glucose and amino acids → absorbed into capillaries → portal vein → liver

Fatty acids + glycerol → absorbed into lacteals → lymphatic system → bloodstream

**3. Worked Examples**

Example 1: Why does the stomach produce hydrochloric acid?

*Answer:* (1) Creates acidic pH (pH 2) optimal for pepsin enzyme. (2) Kills harmful bacteria in food.

Example 2: Explain how the small intestine is adapted for efficient absorption.

*Answer:* The small intestine has a folded inner lining covered with millions of villi. Each villus is one cell thick and has a dense capillary network and a lacteal. This gives a huge surface area, short diffusion distance, and rapid removal of absorbed nutrients into the blood.

Example 3: A child in rural Sindh has bowed legs and soft bones. Suggest the most likely deficiency.

*Answer:* Vitamin D deficiency (rickets). Vitamin D is needed for calcium absorption — without it, bones cannot harden properly.

**4. Pakistan Angle**

Pakistan's National Nutrition Survey (2018) revealed alarming statistics: 40% of children are stunted, 29% are underweight, and 54% are anaemic. Iron-deficiency anaemia is the most common nutritional disorder — directly due to insufficient iron in diets dominated by roti and rice without sufficient meat, dal, or green vegetables. Understanding haemoglobin synthesis (iron + protein) is biology with a direct national health impact.

DRAP (Drug Regulatory Authority of Pakistan) regulates food fortification — flour in Pakistan is now fortified with iron and folic acid by law, directly addressing anaemia. This policy is rooted in the nutritional biology you study in 4BI1.

**5. Exam Strategy**

• Enzyme questions: always link enzyme to its substrate and product. State the organ where it is produced.
• Bile is NOT an enzyme — it emulsifies fat but does not chemically digest it. This is a very common exam error.
• Villi adaptations: give at least three features with a reason for each (surface area, thickness, blood supply).
• Deficiency diseases: know: Vitamin C → scurvy; Vitamin D → rickets; Iron → anaemia; Iodine → goitre.
• Enzyme temperature/pH questions: enzymes denature above optimum temperature (shape changes, active site disrupted). Stomach enzymes work at pH 2; intestinal enzymes at pH 7-8.