Vitamin D3
Cholecalciferol (Vitamin D3)
Vitamin D3 (cholecalciferol) is a fat-soluble secosteroid hormone precursor synthesized in the skin upon UVB exposure or obtained through dietary supplementation. It is primarily used to maintain bone mineral density, support immune function, and correct deficiency states, which are highly prevalent in populations with limited sun exposure. Research also supports roles in mood regulation, cardiovascular health, muscle function, and reduction of all-cause mortality risk.
Mechanism of Action
After ingestion, cholecalciferol is hydroxylated in the liver to 25-hydroxyvitamin D (calcidiol), then further hydroxylated in the kidneys to the active hormone 1,25-dihydroxyvitamin D (calcitriol) by CYP27B1. Calcitriol binds to the nuclear vitamin D receptor (VDR), which is expressed in virtually every tissue, modulating transcription of hundreds of genes involved in calcium absorption, immune modulation, cell differentiation, and inflammation suppression. It upregulates intestinal calcium and phosphate absorption and is essential for parathyroid hormone regulation and skeletal mineralization.
Evidence by Health Goal(18 goals)
Dosing Protocols
With a meal containing dietary fat to maximize absorption
General maintenance dose for adults; deficient individuals (25-OH-D below 20 ng/mL) may require 5000-10000 IU daily for 8-12 weeks under medical supervision to replete stores. Always pair with vitamin K2 (MK-7, 100-200 mcg/day) at doses above 4000 IU to direct calcium appropriately and reduce soft tissue calcification risk. Serum 25-OH-D testing is recommended to guide dosing and target a level of 40-60 ng/mL.
Safety & Side Effects
Vitamin D3 is extremely safe at doses up to 4000 IU/day for most adults, with toxicity being rare below 10000 IU/day in the absence of predisposing conditions; however, individuals with granulomatous diseases (sarcoidosis, tuberculosis), primary hyperparathyroidism, or certain lymphomas face increased risk of hypercalcemia and should only supplement under medical supervision. Serum monitoring of 25-OH-D and calcium is advisable when using doses above 5000 IU/day long-term.
Possible Side Effects
- !Hypercalcemia at excessive doses (>10000 IU/day chronically) causing nausea, vomiting, and weakness
- !Hypercalciuria (elevated urinary calcium) increasing kidney stone risk at high doses
- !Fatigue and malaise associated with vitamin D toxicity
- !Soft tissue and vascular calcification with prolonged supraphysiological dosing without K2 co-administration
- !Anorexia and weight loss in cases of frank toxicity
- !Polydipsia and polyuria due to hypercalcemia-induced nephrogenic diabetes insipidus at toxic levels
- !Headache reported by some individuals, particularly during initial high-dose repletion
Interactions
- -Thiazide diuretics (e.g., hydrochlorothiazide) reduce urinary calcium excretion, significantly increasing hypercalcemia risk when combined with high-dose D3
- -Corticosteroids impair vitamin D metabolism and may reduce efficacy, requiring higher supplemental doses
- -Cholestyramine and orlistat reduce fat-soluble vitamin absorption, decreasing D3 bioavailability by up to 50%
- -Anticonvulsants (phenytoin, phenobarbital) accelerate hepatic catabolism of vitamin D metabolites, increasing requirements
- -Vitamin K2 (MK-7) is a beneficial co-supplement that reduces hypercalcemia and vascular calcification risk at higher D3 doses
Cost & Where to Buy
Generic vitamin D3 softgels (2000-5000 IU) are inexpensive and widely available; cost increases with premium brands, third-party tested products, or combination D3+K2 formulations.
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