Study Notes: Vitamin D3 Synthesis

1. Basics

• Type: Fat-soluble vitamin (accumulates in body fat & liver).

• Family: Multiple compounds, but most important is Vitamin D3 (cholecalciferol).

• Other fat-soluble vitamins: A, D, E, K.

2. Sources of Vitamin D

• Skin (primary source):

  • 7-dehydrocholesterol (epidermis) + UV light → Vitamin D3.

• Dietary sources: Fatty fish, fish oils, fortified dairy/orange juice, supplements.

• Limitations: Few foods naturally rich in vitamin D.

3. Activation Process (Multi-Step)

1. Skin/Diet → Liver

   • Vitamin D3 → 25-hydroxycholecalciferol (calcidiol).

   • Liver regulates levels with feedback inhibition.

   • Excess D3 can be stored in liver.

2. Liver → Kidney

   • 25-hydroxycholecalciferol → 1,25-dihydroxycholecalciferol (calcitriol, active form).

   • Conversion happens in renal cortex (proximal tubules).

   • Controlled by parathyroid hormone (PTH).

4. Major Functions of Active Vitamin D (Calcitriol)

• Calcium absorption (main role):

  • Stimulates small intestinal cells to produce calcium-binding proteins.

  • Increases calcium absorption → maintains blood calcium.

• Bone health:

  • Indirectly protects bones by ensuring calcium stays in bloodstream without needing to pull too much from bone.

  • Prevents bone weakening.

• Other possible roles:

  • Modulates immune function (immune cells have vitamin D receptors).

  • May reduce inflammation.

  • Laboratory studies suggest possible role in inhibiting cancer cell growth (still under research).

5. Deficiency

• Causes:

  • Low sun exposure (especially winter/northern latitudes).

  • Darker skin pigmentation (needs more sun for conversion).

  • Aging (reduced conversion efficiency).

  • Sunscreen (blocks UV → less vitamin D synthesis).

• Symptoms:

  • Mild/moderate deficiency → often asymptomatic (must test blood levels).

  • Severe/prolonged deficiency →

    • Osteomalacia (softening of bone tissue in adults).

    • Rickets (childhood version of osteomalacia).

    • Bone pain, fractures, muscle weakness.

6. Key Hormonal Link

• Parathyroid glands (near thyroid in neck) → release PTH.

• PTH signals kidneys to convert vitamin D into active calcitriol.

• Goal: Raise blood calcium levels (through intestinal absorption, renal retention, and bone mobilization).

Summary Flow:
Skin (UV) / Diet → Vitamin D3 → [Liver] → 25-hydroxy-D → [Kidney, PTH] → 1,25-dihydroxy-D (active) → ↑ calcium absorption in intestines → bone & neuromuscular health.