Vitamin D
Vitamin D
Snapshot
Vitamin D is a fat‑soluble prohormone essential for calcium and phosphorus homeostasis, bone health, immune modulation, and muscle function.
What is Vitamin D?
Vitamin D refers to a group of secosteroids—primarily D₃ (cholecalciferol) and D₂ (ergocalciferol)—that undergo hepatic and renal hydroxylation to form the active hormone calcitriol, which regulates gene expression via the vitamin D receptor.
Where It Comes From
Vitamin D₃ is synthesized in the skin from 7‑dehydrocholesterol upon ultraviolet B exposure. Dietary sources include fatty fish, egg yolks, and fortified foods. Supplements provide cholecalciferol or ergocalciferol in oil, tablet, or liquid form.
Key Nutrients & Compounds
Provides vitamin D₃ or D₂, which are converted to 25‑hydroxyvitamin D (25(OH)D) in the liver and then to active 1,25‑dihydroxyvitamin D (calcitriol) in the kidneys, modulating calcium‑binding proteins and parathyroid hormone activity.
Health Benefits
Vitamin D enhances intestinal absorption of calcium and phosphorus; supports bone mineralization and reduces fracture risk; modulates innate and adaptive immunity; promotes muscle strength and function; and may protect against chronic diseases such as osteoporosis, certain cancers, and autoimmune disorders.
Recommended Dosage
The RDA is 600 IU (15 µg) per day for adults up to age 70 and 800 IU (20 µg) per day for those over 70. To achieve serum 25(OH)D levels above 30 ng/mL, supplementation of 1,000–2,000 IU daily is often used, with higher therapeutic doses (5,000–10,000 IU) under medical supervision.
How to Use It
Take vitamin D supplements with a meal containing fat to enhance absorption. For weekly dosing protocols, divide the total weekly dose evenly. Monitor serum 25(OH)D concentrations periodically to adjust dosing and avoid toxicity.
Who Should Use It?
Individuals with limited sun exposure, older adults, those with malabsorption syndromes, darker skin pigmentation, obesity, or diets low in vitamin D may require supplementation to maintain optimal status.
Possible Interactions or Cautions
Excessive vitamin D can cause hypercalcemia, hypercalciuria, and soft tissue calcification. Use cautiously with thiazide diuretics or calcium supplements. Monitor serum calcium and 25(OH)D levels during high‑dose therapy.
Final Thoughts
Vitamin D is crucial for skeletal integrity, immune health, and muscle performance. Personalized supplementation based on baseline levels and risk factors ensures sufficiency while minimizing adverse effects.
Scientific Studies
Holick MF. Vitamin D deficiency. N Engl J Med. 2007;357(3):266–281. https://doi.org/10.1056/NEJMra070553
Bischoff‑Ferrari HA, Heaney RP, Dawson‑Hughes B, et al. Optimal serum 25‑hydroxyvitamin D levels for multiple health outcomes. Adv Nutr. 2014;5(5): 658–679. https://doi.org/10.3945/an.114.006476
Manson JE, Cook NR, Lee IM, et al. Vitamin D supplements and prevention of cancer and cardiovascular disease. N Engl J Med. 2019;380(1):33–44. https://doi.org/10.1056/NEJMoa1809944.