☀️ Vitamin D: Metabolism, Functions, Deficiency, and Clinical Approach
- Ann Augustin
- Jun 28
- 3 min read
Updated: Jun 29
✨ Introduction
Vitamin D plays a crucial role in maintaining bone health, calcium homeostasis, and muscle function. Deficiency is common, especially in populations with limited sunlight exposure, dietary insufficiency, or malabsorption disorders.
🔬 Vitamin D Metabolism
Sources
Vitamin D3 (cholecalciferol): Skin synthesis (UVB exposure), oily fish, egg yolk.
Vitamin D2 (ergocalciferol): Plant-based and fortified foods.
Activation Steps
Skin: UVB converts 7-dehydrocholesterol → Vitamin D3
Liver: 25-hydroxylase converts D3 → 25(OH)D (calcidiol)
Kidney: 1-alpha hydroxylase converts 25(OH)D → 1,25(OH)₂D (calcitriol, active form)
Regulation
PTH stimulates kidney activation when calcium is low.
Phosphate levels also influence activation (low phosphate increases calcitriol).
🦴 Functions of Vitamin D
✅ Increases intestinal absorption of calcium and phosphate.
✅ Maintains serum calcium and phosphate for bone mineralisation.
✅ Promotes bone mineralisation by facilitating hydroxyapatite formation.
✅ Regulates bone remodelling via osteoclast activation if dietary calcium is insufficient.
✅ Enhances muscle strength, reducing falls.
✅ Modulates immune system and cell proliferation.
⚠️ Causes of Vitamin D Deficiency
Reduced sunlight exposure: Elderly, institutionalised, covered clothing, sunscreen use, high latitudes.
Dietary insufficiency: Vegan diets, limited intake of oily fish or fortified foods.
Malabsorption disorders: Coeliac disease, Crohn’s disease, chronic pancreatitis, cystic fibrosis, bariatric surgery.
Liver or kidney disease: Impaired activation steps.
Increased requirements: Pregnancy, lactation, growth.
Medications: Anticonvulsants, steroids, rifampicin.
Genetic causes (rare): Vitamin D-dependent rickets, resistance syndromes.
📝 Clinical Presentation
💬 Data Gathering – Key Questions with Reasoning
Fatigue or muscle weakness? → Assess general symptoms of deficiency.
Difficulty rising from a chair or climbing stairs? → Proximal myopathy.
Bone pain? Where? → Osteomalacia (hips, lower back, ribs).
Muscle cramps or spasms? → Hypocalcaemia symptoms.
Dietary intake? Fish, eggs, fortified foods? → Dietary deficiency.
Vegan or restricted diet? → Lack of natural Vitamin D sources.
Sunlight exposure? Covered clothing or sunscreen use? → Reduced synthesis.
History of bowel problems (Crohn’s, coeliac)? → Malabsorption.
Weight loss surgery? → Bariatric malabsorption.
Medications (phenytoin, steroids)? → Increased catabolism.
Any known liver or kidney disease? → Impaired activation.
Pregnant or breastfeeding? → Increased requirements.
🔬 Test Interpretation Example
💡 Explanation to Patient (Sample)
“Your thyroid and blood counts are normal. Your Vitamin B12 is fine. However, your Vitamin D level is low, which can cause tiredness, bone pain, or muscle weakness. Your calcium is also slightly low, likely due to the Vitamin D deficiency, as Vitamin D helps your body absorb calcium.”
💊 Treatment Plan
✅ Loading dose:
Vitamin D 50,000 units once weekly for 6 weeks to replenish stores.
✅ Maintenance dose:
1,000 units daily thereafter to maintain normal levels.
✅ Calcium supplementation:
Increase dietary intake (dairy, leafy greens, fortified foods).
If needed, 500–1000 mg elemental calcium daily.
✅ Follow-up:
Recheck Vitamin D and calcium levels in 3–6 months.
✅ Referral:
“If your levels don’t improve, we may need specialist input to do further investigations to rule out any gut issues affecting absorption.”
🍽 Dietary Sources
🔑 Key Takeaway
Vitamin D is essential for calcium absorption and bone health. Deficiency can lead to osteomalacia in adults and rickets in children. Early recognition, appropriate supplementation, and addressing underlying causes are crucial for optimal patient care.
📚 References:
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