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BI8.1-6 | Vitamins and Nutrition — Part 1
CLINICAL SCENARIO
Three patients arrive at a district hospital outpatient in Odisha on the same morning:
Patient A — 4-year-old child: night blindness (cannot see in dim light), Bitot's spots on the conjunctiva (foamy white patches)
Patient B — 18-year-old pregnant woman from a hill district: no sunlight exposure (stays indoors; fully covered when outdoors), bone pain, waddling gait, generalised muscle weakness
Patient C — 8-month-old exclusively breastfed infant: excessive crying, blood-stained spots on the gums, bruising on the thighs; mother has been on a diet with no green leafy vegetables or citrus
Each patient has a vitamin deficiency. Can you identify which vitamin — and explain the biochemistry behind each clinical feature?
WHY THIS MATTERS
Vitamin deficiencies are not historical curiosities — they remain a significant public health burden in India:
- Vitamin A deficiency is the leading cause of preventable blindness in children in India
- Vitamin D deficiency affects 70–90% of Indians due to limited sun exposure and dietary factors
- Iron-deficiency anaemia (often associated with B12 and folate deficiency) affects ~50% of Indian women of reproductive age
- Pellagra (niacin deficiency) still occurs in sorghum-belt areas of Maharashtra and Karnataka
The biochemical mechanisms you learn here directly explain these clinical syndromes — and will inform your prescribing throughout your career.
RECALL
Before beginning, recall:
- Fat-soluble vitamins: A, D, E, K — absorbed with dietary fat, stored in liver/adipose, can be toxic in excess
- Water-soluble vitamins: B-complex (8 vitamins) + C — excreted in urine when in excess, not stored (except B12)
- Coenzyme concept: many B vitamins function as coenzymes (NAD⁺, FAD, CoA, pyridoxal phosphate, TPP) in metabolic pathways you've already studied
- From Anatomy: the retina structure, rod vs cone cells (relevant to Vitamin A)
- From Physiology: calcium and phosphate homeostasis (relevant to Vitamin D)
Fat-Soluble Vitamins — Overview
Fat-Soluble vs Water-Soluble Vitamins — Key Differences
| Feature | Fat-Soluble (A, D, E, K) | Water-Soluble (B-complex, C) |
|---|---|---|
| Absorption | With dietary fat via micelles; require bile salts | Directly absorbed in small intestine; no bile needed |
| Transport | Via lipoproteins (chylomicrons, then other lipoproteins) | Free in plasma or bound to specific carriers |
| Storage | Stored in liver and adipose tissue; significant body reserves | Minimal storage (except B12 stored in liver for 3-5 years) |
| Excretion | Not readily excreted; accumulate with excess intake | Readily excreted in urine; excess rapidly cleared |
| Toxicity risk | High (especially A and D) — hypervitaminosis possible | Low (excess excreted); rare exceptions (B6 neuropathy) |
| Deficiency onset | Slow (weeks-months due to body stores) | Faster (days-weeks, except B12 which takes years) |
| Conditions causing deficiency | Fat malabsorption (coeliac, cystic fibrosis, obstructive jaundice) | Dietary deficiency, alcoholism, increased requirements (pregnancy) |
Fat-soluble vitamins (A, D, E, K) share key properties: they dissolve in lipids, are absorbed with dietary fat via micelles (aggregates of bile salts + lipids), require bile for absorption, are transported by lipoproteins, and can be stored in the liver and adipose tissue.
Figure: Fat-Soluble Vitamins — Overview
Unlike water-soluble vitamins, they are NOT readily excreted — so toxicity (hypervitaminosis) is possible with excessive supplementation. This is especially important for Vitamins A and D.
Absorption pathway: Dietary fat + fat-soluble vitamins → emulsified by bile salts → micelles in jejunum → absorbed into enterocytes → incorporated into chylomicrons → transported via lymphatics → systemic circulation.
Vitamin A — Vision, Immunity, and Cell Differentiation
Vitamin A exists in three active forms:
- Retinol (alcohol form — transport in blood)
- Retinal (aldehyde — visual cycle; chromophore)
- Retinoic acid (acid — gene regulation via nuclear receptors)
Figure: Vitamin A — Vision, Immunity, and Cell Differentiation
Sources: Preformed retinol from animal foods (liver, eggs, dairy); provitamin beta-carotene from orange/yellow vegetables (carrots, papaya, mango) and dark green leafy vegetables (palak, drumstick leaves). Beta-carotene is cleaved by beta-carotene 15,15'-dioxygenase in the intestinal mucosa → 2 retinal molecules.
Functions:
1. Visual cycle — retinal is the chromophore in rhodopsin (rod cells, dim light) and iodopsin (cone cells, colour vision). 11-cis-retinal + opsin → rhodopsin → light isomerises to all-trans-retinal → nerve impulse → phototransduction
2. Cell differentiation — retinoic acid binds RAR/RXR nuclear receptors → regulates genes maintaining epithelial integrity
3. Immunity — maintains mucosal barriers; supports T-cell function
4. Reproduction and embryonic development — retinoic acid is teratogenic in excess
Deficiency (India context):
- Night blindness (nyctalopia) — earliest sign; rhodopsin regeneration impaired
- Xerophthalmia — dry eyes; corneal dryness
- Bitot's spots — foamy white patches on conjunctiva (squamous metaplasia)
- Keratomalacia — corneal ulceration → perforation → blindness (irreversible)
- Increased susceptibility to infections (measles, diarrhoea)
National Vitamin A Programme: India gives children 9 months–5 years 200,000 IU every 6 months. This is safe because the liver stores it — but chronic daily megadosing causes hepatotoxicity.
Vitamin D — The Hormone of Calcium Homeostasis
Vitamin D is more accurately a prohormone than a vitamin — the body synthesises it from cholesterol using sunlight.
Figure: Vitamin D — The Hormone of Calcium Homeostasis
Two-step activation:
1. In skin: 7-dehydrocholesterol + UV-B light → cholecalciferol (Vitamin D₃)
2. In liver: D₃ + hydroxylase → 25-hydroxyvitamin D (storage form; measured in blood)
3. In kidney: 25-OH-D₃ + 1α-hydroxylase → 1,25-dihydroxyvitamin D₃ = calcitriol (active hormone)
PTH stimulates 1α-hydroxylase in the kidney → increases calcitriol production when calcium falls.
Functions of calcitriol (1,25-(OH)₂D₃):
- Intestine: increases absorption of calcium and phosphate
- Bone: promotes bone mineralisation (with adequate Ca/P); also mobilises Ca from bone in excess
- Kidney: promotes renal calcium reabsorption
- Immune modulation, muscle function, cell differentiation (via nuclear receptors — VDR)
Deficiency:
- Children → Rickets: inadequate bone mineralisation → soft bones → bowing of legs (genu varum), swelling at costochondral junctions (rachitic rosary), frontal bossing, delayed dentition
- Adults → Osteomalacia: bone pain, muscle weakness, pseudofractures on X-ray (Looser's zones)
- Severe hypocalcaemia: tetany, carpopedal spasm, positive Chvostek/Trousseau sign
India context: 70–90% of Indians are vitamin D deficient due to: limited outdoor activity, indoor work, full clothing coverage, high melanin skin (reduces UV absorption), and phytate-rich vegetarian diets (phytates bind Ca in gut). Even in sunny India, rickets remains prevalent.