Digestive system, nutrients and balanced diet
The tract and its secretions
Mouth: teeth grind food; salivary amylase (ptyalin) converts starch to maltose at about pH 6.8. Oesophagus: peristalsis pushes food down. Stomach: HCl gives pH 1.5–3.5, kills microbes and activates pepsinogen to pepsin for protein; rennin clots milk in infants; intrinsic factor formed here is needed to absorb vitamin B12. Liver: bile, which has no enzyme, emulsifies fat and is stored in the gall bladder. Pancreas: amylase, trypsin, lipase plus bicarbonate. Small intestine: maltase, sucrase, lactase and peptidases; villi and microvilli absorb glucose and amino acids into blood and fatty acids into lacteals - most absorption occurs in the jejunum and ileum. Large intestine absorbs water and salts.
Nutrients and energy
Carbohydrate and protein give 4 kcal per gram, fat 9 kcal per gram. A balanced diet combines cereals, pulses, milk, eggs, vegetables, fruit and some fat in amounts matching age, sex and activity. BMR is the energy used at complete rest.
Deficiency diseases
Vitamin A - night blindness; B1 - beriberi; C - scurvy; D - rickets; iron - anaemia; iodine - goitre; protein-energy - kwashiorkor (with oedema) and marasmus (wasting without oedema).
Worked example
An 18-month-old with a swollen belly, puffy legs, thin hair and peeling skin lives mainly on rice water. Oedema with very low protein intake points to kwashiorkor; severe wasting without oedema would be marasmus. The plan is protein- and energy-dense food while continuing breastfeeds.
Marks-losing traps
Bile has no enzyme; fat digestion begins mainly in the small intestine while protein begins in the stomach and starch in the mouth; vitamins A, D, E and K are fat-soluble, so a low-fat diet can cause their deficiency.