Overview
Vitamin D3 is tracked as a supplement in Milligram's compound library. Fat-soluble secosteroid.
With a half-life of ~15 days (360 hours), Vitamin D3 supports less frequent dosing schedules. Taken orally, it has a bioavailability of approximately 95% via its primary route.
Half-life approximately 15 days for the 25-hydroxyvitamin D metabolite. Absorption enhanced with dietary fat. One-compartment model.
Mechanism of Action
Vitamin D3's pharmacological activity involves the following key pathways:
Primary Mechanism
Vitamin D3 exerts its effects through its established pharmacological pathway. Its mechanism has been characterized in published research.
Bioavailability Profile
Vitamin D3 has high oral bioavailability, which influences dosing strategy and administration timing.
Dosing Protocols
The following protocols represent commonly observed dosing patterns. These are observational summaries, not recommendations.
Dose
2000 IU/day
Route
Oral
Frequency
Every 10-14 days
Duration
Ongoing
Dose
5000 IU/day
Route
Oral
Frequency
Every 10-14 days
Duration
Ongoing
Frequently Asked Questions
What is Vitamin D3 and how does it work?
Vitamin D3 (Cholecalciferol) is a fat-soluble secosteroid that functions as a prohormone. After ingestion, the liver converts it to 25-hydroxyvitamin D (calcidiol), the primary circulating form used to assess vitamin D status. The kidneys then convert calcidiol to 1,25-dihydroxyvitamin D (calcitriol), the biologically active hormone that regulates calcium absorption, bone mineralization, immune modulation, and gene expression across over 200 genes.
How long does Vitamin D3 take to reach steady state?
Vitamin D3 has one of the longest saturation timelines of any supplement. The circulating form 25(OH)D has a half-life of approximately 15 days, meaning plasma steady state requires approximately 2–3 months. However, research indicates that true equilibrium may take 4–5 months due to vitamin D's lipophilic nature and distribution into fat tissue. Follow-up blood testing is most informative when performed at least 3 months after starting or changing dose.
What Vitamin D3 dose is needed to correct deficiency?
Doses of 2000–5000 IU/day are commonly observed for daily maintenance supplementation. For correcting documented deficiency (serum 25(OH)D below 20 ng/mL), higher initial doses or loading protocols are frequently used under practitioner guidance. A commonly cited estimate is that each 1000 IU/day of D3 raises serum 25(OH)D by approximately 10 ng/mL over 2–3 months, though individual response varies with body weight, baseline levels, and genetics.
Should Vitamin D3 be taken with Vitamin K2?
Co-supplementation with Vitamin K2 (specifically MK-7) is commonly observed, based on their complementary roles in calcium metabolism. Vitamin D3 increases calcium absorption from the gut, while K2 activates proteins (osteocalcin and matrix GLA protein) that direct calcium into bones rather than soft tissues and arteries. The synergy is most relevant at higher D3 doses above 4000 IU/day and in long-term supplementation.
Does Vitamin D3 affect testosterone levels?
Vitamin D receptors are present in male reproductive tissue, and observational studies have found a positive correlation between serum 25(OH)D levels and testosterone. A randomized controlled trial in vitamin D-deficient men observed a significant increase in total testosterone after 12 months of supplementation with 3332 IU/day. The testosterone-supporting effect appears most relevant in individuals with deficient baseline vitamin D status rather than those already in the sufficient range.
When is the best time to take Vitamin D3?
Vitamin D3 is fat-soluble and absorption is significantly improved when taken alongside a meal containing dietary fat. Morning or midday dosing is more commonly observed than evening dosing, as some individuals report that late-night vitamin D supplementation interferes with melatonin production and sleep quality. Given its extremely long half-life, exact timing within the day is far less important than consistent daily intake.