3.1 Classes of Food
Human health depends on consuming a balanced diet containing all seven classes of food in correct proportions.
The Seven Classes of Food
- Carbohydrates: Supply the main source of energy. Composed of carbon, hydrogen, and oxygen (e.g., starch, glycogen, cellulose). Examples: Rice, bread, potatoes, sugar.
- Proteins: Essential for growth, repair of damaged tissues, and synthesis of enzymes and hormones. Composed of carbon, hydrogen, oxygen, and nitrogen. Examples: Fish, meat, eggs, milk, beans.
- Fats (Lipids): Act as a concentrated high-density energy store, thermal insulator, and solvent for fat-soluble vitamins (A, D, E, K). Examples: Butter, oil, nuts.
- Vitamins: Organic chemical compounds required in small quantities to maintain metabolic health and immunity.
- Water-soluble: Vitamins B and C.
- Fat-soluble: Vitamins A, D, E, and K.
- Minerals: Inorganic nutrients needed in small amounts to regulate bodily functions (e.g., Calcium, Iron, Iodine, Sodium, Potassium).
- Fibre (Roughage): Indigestible cellulose material that stimulates peristalsis in intestines to prevent constipation. Examples: Vegetables, fruits, whole grains.
- Water: Regulates body temperature, acts as a solvent for chemical reactions, and transports digested nutrients and waste products.
Vitamin Deficiency Diseases
| Vitamin |
Deficiency Disease / Symptoms |
| Vitamin A |
Night blindness, dry skin |
| Vitamin B |
Beri-beri, anaemia |
| Vitamin C |
Scurvy (bleeding gums) |
| Vitamin D |
Rickets (soft bones in children) |
| Vitamin E |
Sterility, nerve damage |
| Vitamin K |
Slow blood clotting / prolonged bleeding |
Mineral Deficiency Diseases
- Calcium & Phosphorus: Rickets, osteoporosis (brittle bones).
- Iron: Anaemia (reduced hemoglobin and red blood cells).
- Iodine: Goitre (swelled thyroid gland).
- Sodium & Potassium: Muscle cramps, dehydration.
3.2 Importance of a Balanced Diet
A balanced diet contains all seven food classes in the right quantities to meet the daily energy and physiological requirements of the body.
Factors Affecting Caloric Needs
- Age: Children and teenagers require more energy for rapid physical growth compared to elderly adults.
- Gender: Males generally require higher caloric intake than females due to higher muscle mass and metabolic rate.
- Body Size: Larger body frames require more energy to maintain bodily functions.
- Physical Activity: Manual laborers require significantly higher energy than sedentary desk workers.
- Climate: People living in cold climates need more energy to maintain constant body temperature.
- Health Condition: Pregnant and lactating women require extra energy and nutrients (calcium, iron, protein).
Calorific Value of Food
The calorific value (energy value) is the amount of heat energy released when 1 gram of food is completely oxidized. Measured in Joules (J) or Calories (cal).
1 calorie (cal) = 4.2 Joules (J)
3.3 Human Digestive System
Digestion is the process of breaking down large, complex food molecules into small, soluble molecules that can be absorbed by the body.
Digestive Tract Pathway & Functions
- Mouth: Physical digestion via teeth chewing; Chemical digestion via salivary amylase converting starch into maltose.
- Oesophagus: Food bolus moves down to the stomach via rhythmic muscular contractions called peristalsis.
- Stomach: Secretes gastric juice containing hydrochloric acid (kills bacteria and provides acidic pH) and protease (pepsin) which breaks proteins into polypeptides.
- Duodenum: Receives bile from the liver (emulsifies fats) and pancreatic juice from the pancreas containing:
- Pancreatic amylase: Starch → Maltose
- Protease: Polypeptides → Peptides
- Lipase: Emulsified fats → Fatty acids + Glycerol
- Small Intestine (Ileum): Secretes intestinal enzymes:
- Maltase: Maltose → Glucose
- Protease: Peptides → Amino acids
- Large Intestine (Colon): Reabsorbs water and mineral salts from indigestible food residues.
- Rectum & Anus: Stores feces temporarily and excretes them through defaecation.
3.4 Process of Absorption and Assimilation of Digested Food
Adaptations of the Small Intestine for Absorption
- Villi: Millions of tiny finger-like projections that dramatically increase the total surface area for absorption.
- Microvilli: Microscopic folds on the epithelial cells of villi providing additional surface area.
- Thin Walls: Epithelial wall is only one cell thick for rapid nutrient diffusion.
- Dense Blood Capillary Network: Transports glucose, amino acids, water-soluble vitamins, and minerals directly to the liver via the hepatic portal vein.
- Lacteal: A central lymphatic vessel inside each villus that absorbs fatty acids, glycerol, and fat-soluble vitamins (A, D, E, K).
Assimilation
Assimilation is the process where absorbed nutrients are utilized by body cells to build structural components or produce energy:
- Glucose: Oxidized during cellular respiration to release energy. Excess glucose is stored in the liver and muscles as glycogen.
- Amino Acids: Used to synthesize new proteins, repair tissues, and form enzymes.
- Fatty Acids & Glycerol: Used to form cell membranes and stored beneath the skin as adipose tissue for thermal insulation.