Vitamin Deficiency Diseases: Mechanism and Symptoms

Night Blindness (Vitamin A):

Pathway: Retinol → Retinal → Rhodopsin (retinal + opsin)

Rhodopsin absorbs light: 11-cis retinal --[light]--> all-trans retinal + nerve impulse

Regeneration: all-trans retinal → 11-cis retinal requires retinol supply

Without vitamin A: Rhodopsin cannot regenerate → failure of rod cell function → night blindness

Advanced deficiency: Vitamin A maintains epithelial cell differentiation; deficiency → squamous metaplasia of corneal epithelium → xerophthalmia → keratomalacia → blindness

Rickets (Vitamin D):

$\text{7-Dehydrocholesterol} \xrightarrow{\text{UV-B (skin)}} \text{Vit D}_3 \xrightarrow{\text{liver (25-OH)}} \xrightarrow{\text{kidney (1,25-OH)}} \text{Calcitriol (active)}$

Calcitriol → stimulates intestinal $Ca^{2+}$ and $PO_{4}^{3-}$ absorption

Without calcitriol: low plasma $Ca^{2+}$/$PO_{4}^{3-}$ → defective bone mineralisation → rickets (child) / osteomalacia (adult)

Scurvy (Vitamin C):

Prolyl hydroxylase: Pro + $O_{2}$ → Hydroxyproline (requires $Fe^{2+}$ and ascorbate as cofactor)

Without ascorbate: $Fe^{2+}$ → $Fe^{3+}$ cannot be recycled → enzyme inactive → no hydroxyproline → unstable collagen → connective tissue failure → scurvy

Symptoms: bleeding gums, loose teeth, perifollicular haemorrhages, poor wound healing

Pellagra (Vitamin B3/Niacin):

Niacin → N$AD^{+}$/NA$DP^{+}$ (coenzymes for >400 redox reactions)

Without niacin: impaired carbohydrate, fat, amino acid metabolism

Classic "4 Ds": Dermatitis (sun-exposed skin), Diarrhoea, Dementia, Death (severe)

Beriberi (Vitamin B1/Thiamine):

TPP → pyruvate dehydrogenase complex + alpha-ketoglutarate dehydrogenase

Without TPP: pyruvate accumulates (cannot enter Krebs cycle) → impaired energy metabolism in cells with high demand (nerves, heart)

Dry beriberi: peripheral neuropathy; Wet beriberi: cardiac involvement + oedema