Vitamin D Supplementation Guidelines for Adults with Serum 25‑Hydroxy‑vitamin D ≈ 28 ng/mL

1. Definition of Deficiency and Target Levels

Serum 25‑(OH)D = 28 ng/mL is classified as suboptimal/insufficient by multiple expert panels. 1
Consensus recommendations set the optimal range for bone health and extraskeletal benefits at 30–50 ng/mL, with an upper safety ceiling of 100 ng/mL. 1
The National Comprehensive Cancer Network (NCCN) advises supplementation to raise levels to ≥ 30 ng/mL for bone health. 2

1,000 IU vitamin D₃ daily for adults with 25‑(OH)D between 20–30 ng/mL, with a repeat level in ≥ 3 months. NCCN guideline. 2
A rule‑of‑thumb suggests 1,000 IU/day raises serum 25‑(OH)D by ≈ 10 ng/mL, moving a level of 28 ng/mL to roughly 38 ng/mL. 1
This conservative approach minimizes toxicity risk while achieving target concentrations. NCCN. 2

Ergocalciferol (vitamin D₂) 50,000 IU weekly for 8 weeks, followed by maintenance dosing. Recommended by both NCCN and expert consensus. 2
After the 8‑week loading phase, re‑measure 25‑(OH)D to assess response. NCCN. 2
Maintenance: 800–1,000 IU vitamin D₃ (or equivalent intermittent dosing) daily. Expert consensus. 1
If the level remains < 30 ng/mL after re‑check, increase the maintenance dose. Expert consensus. 1

Vitamin D₃ (cholecalciferol) is preferred over D₂ for intermittent dosing because it sustains serum concentrations longer. Expert consensus. 1
For daily dosing, D₂ and D₃ have comparable efficacy. Expert consensus. 1
The 50,000 IU weekly formulation is typically ergocalciferol (vitamin D₂). NCCN. 2

Re‑check serum 25‑(OH)D at least 3 months after initiating any supplementation regimen. Expert consensus. 1
Individual response varies with baseline level, body weight, and other factors; therefore, follow‑up testing is essential. Expert consensus. 1
Ensure the assay measures both 25‑(OH)D₂ and 25‑(OH)D₃ to capture total vitamin D status. Expert consensus. 1

Recommend total calcium intake of 1,000–1,200 mg/day from diet and supplements. NCCN. 2
Adults < 50 years: ≥ 1,000 mg/day; ≥ 50 years: ≥ 1,200 mg/day. NCCN. 2
Calcium supplements should be divided into doses ≤ 600 mg to optimize absorption. NCCN. 2
Adequate dietary calcium is necessary for optimal response to vitamin D therapy. Expert consensus. 1

Upper safety limit: serum 25‑(OH)D ≤ 100 ng/mL. Expert consensus. 1
Toxicity is rare but may occur with daily doses > 50,000 IU that push levels > 150 ng/mL. NCCN. 2
Long‑term daily dosing up to 10,000 IU has been used without adverse events. Expert consensus. 1
Avoid single annual high‑dose regimens (e.g., 500,000 IU once yearly) due to documented adverse outcomes. Expert consensus. 1
Prefer daily, weekly, or monthly dosing over very infrequent high‑dose schedules. Expert consensus. 1

Do not assume uniform response; individual variability necessitates repeat testing. Expert consensus. 1
Do not neglect calcium assessment; vitamin D alone may not achieve bone health goals. Expert consensus. 1
Do not re‑measure too early; allow at least 3 months for steady‑state levels. Expert consensus. 1
Do not use calcitriol or other activated vitamin D analogs for treating nutritional vitamin D deficiency. American Journal of Kidney Diseases. 8