25-Hydroxyvitamin D3 (Cholecalciferol)
Other marker
Vitamin D3
25-Hydroxyvitamin D3 (Cholecalciferol)
The form of vitamin D produced in skin from sunlight and found in most supplements, measured as its 25-hydroxy metabolite. In practice it makes up almost all of total vitamin D in people not taking ergocalciferol.
PED Notes
Relevant to athletes beyond bone health: vitamin D status is associated with muscle function, testosterone production, and immune competence, and deficiency is common in anyone training indoors or covering up outdoors. Vitamin D is fat-soluble, so lean, low-body-fat competitors can show different kinetics from the general population, and heavy users of AAS often have suppressed immune function that low vitamin D compounds. If a laboratory reports D2 and D3 separately, add them for the total before judging sufficiency.
When high
High vitamin D3:
- Almost always from over-supplementation. Toxicity is rare below 250 nmol/L but the risk is hypercalcaemia, not the vitamin D itself
- Check serum calcium and PTH immediately -- a raised calcium with high vitamin D requires prompt medical review
- Stop supplementation and recheck in 8-12 weeks; the fat-soluble pool clears slowly
- Review all sources -- multivitamins, fortified foods, cod liver oil, and standalone D3 stack up faster than most people expect
- Maintain hydration and avoid additional calcium supplements until levels normalise
When low
Low vitamin D3:
- Cholecalciferol (D3) -- 1000-2000 IU/day for maintenance; 3000-5000 IU/day to correct a documented deficiency, with rechecking after 8-12 weeks
- Take with dietary fat -- absorption is markedly better with a fat-containing meal
- Sunlight -- 10-30 minutes of midday exposure to arms and legs several times weekly, adjusted for skin tone and local UV index
- Vitamin K2 (MK-7) -- 100-200mcg/day; commonly co-supplemented so that mobilised calcium is directed to bone rather than arterial wall
- Magnesium -- 300-400mg/day; magnesium is a required cofactor for vitamin D activation, and repletion sometimes fails without it
If levels stay low despite adequate dosing:
- Check for malabsorption -- coeliac disease, inflammatory bowel disease, and prior bariatric surgery all impair uptake
- Check body composition -- vitamin D is sequestered in adipose tissue, so higher body fat requires higher doses
- Check adherence and product quality -- degraded or underdosed supplements are common
- Check PTH and calcium -- a raised PTH with low vitamin D confirms physiologically meaningful deficiency
Physician oversight is needed for high-dose loading regimens and for anyone with sarcoidosis, hyperparathyroidism, or kidney stones.
Interpretation (total 25(OH)D, D2 plus D3):
- <30 nmol/L -- deficient
- 30-50 nmol/L -- insufficient
- 50-125 nmol/L -- sufficient, with 75-125 nmol/L a common athletic target
- >250 nmol/L -- potentially toxic; check calcium
History Chart
Reading History
Frequently Asked Questions
Reference Ranges
Standard Range
VitalMetrics Range