What Is Vitamin D Supplement?

Vitamin D is a fat-soluble steroid hormone that regulates calcium and phosphate homeostasis, supports immune function, and influences cell proliferation and differentiation. It exists as D2 (ergocalciferol, plant-derived) and D3 (cholecalciferol, animal-derived and endogenously synthesized).

How Vitamin D Works
Vitamin D undergoes hepatic 25-hydroxylation to form 25(OH)D (calcifediol), the major circulating form used to assess vitamin D status. The kidneys convert calcifediol to 1,25(OH)2D (calcitriol), the active hormonal form that regulates intestinal calcium absorption and bone mineralization.

Indications

Vitamin D deficiency (25(OH)D <20 ng/mL)
Vitamin D insufficiency (20-29 ng/mL) with risk factors
Osteoporosis or osteopenia (adjunctive to calcium)
Malabsorption disorders (celiac, Crohn's disease)
Chronic kidney disease (with renal dosing)
Hypoparathyroidism
Seasonal affective disorder (adjunctive)

Dosage and Administration

Maintenance (Adequate Intake):

Ages 1-70: 600-1000 IU daily
Ages >70: 800-1000 IU daily (higher doses often recommended)
Pregnant/lactating women: 600 IU daily minimum

Deficiency Repletion:

Moderate deficiency: 1000-4000 IU daily until 25(OH)D >30 ng/mL
Severe deficiency: 50,000 IU weekly for 6-12 weeks, then maintenance
Alternative: 4,000-10,000 IU daily for 12 weeks

Target 25(OH)D Levels:

Sufficient: >30 ng/mL (>75 nmol/L)
Optimal for bone health: 30-50 ng/mL
Recheck 8-12 weeks after initiation

Clinical Note
D3 (cholecalciferol) is preferred over D2; achieves higher 25(OH)D levels at equivalent doses. Fat-soluble vitamin; take with meals containing fat for optimal absorption. Some conditions (chronic kidney disease, hypoparathyroidism) require monitoring of serum calcium and phosphate.

Side Effects

Common: Generally well-tolerated at recommended doses; minimal side effects Toxicity (with excessive supplementation): Hypercalcemia, hypercalciuria, kidney stones, nephrocalcinosis, bone pain, weakness, constipation

Contraindications

Hypercalcemia
Vitamin D toxicity
Granulomatous diseases with uncontrolled hypercalcemia (sarcoidosis, TB, histoplasmosis)
Primary hyperparathyroidism

Drug Interactions

Thiazide diuretics: increase serum calcium; monitor for hypercalcemia
Glucocorticoids: reduce vitamin D activation; may require higher supplementation
Anticonvulsants (phenytoin, carbamazepine): increase vitamin D metabolism; monitor 25(OH)D
Antifungals (ketoconazole, fluconazole): reduce vitamin D activation
Orlistat: reduces fat-soluble vitamin absorption; separate dosing

Key Clinical Evidence

D-Health trial (2023) and VITAL trial (NEJM 2022) show mixed evidence on vitamin D supplementation for bone health and cancer prevention. Current consensus supports supplementation to achieve sufficient levels (>30 ng/mL) but questions higher targets. Benefits clear for fracture prevention when combined with calcium.

Monitoring Parameters

Baseline assessment before initiating therapy
Regular clinical follow-up per indication
Laboratory parameters as specified in sections above
Drug interaction screening at each visit

Special Populations

Dosing adjustments may be necessary in:

Renal impairment
Hepatic impairment
Elderly patients
Pregnancy and lactation
Pediatric patients

Patient Counseling

Take exactly as directed; do not modify dose without consulting healthcare provider
Report any unusual symptoms or adverse effects
Maintain regular follow-up appointments
Store at room temperature away from moisture and light
Keep out of reach of children

Disclaimer: This information is for educational purposes only and should not substitute for professional medical advice. Always consult with a qualified healthcare provider before starting, stopping, or modifying any medication.

Vitamin D Supplement: Uses, Dosage & Side Effects

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