What are the recommended vitamin D3 (cholecalciferol) dosing regimens for infants, children, adults, older adults, and pregnant or lactating women—including loading and maintenance doses—and should calcium and vitamin K2 be co‑prescribed with monitoring guidelines for deficiency and safety?

Vitamin D Supplementation Across the Lifespan: Evidence-Based Dosing, Co-Supplementation, and Monitoring

Age-Specific Dosing Regimens

Infants (<12 months)

For infants, provide 400 IU (10 µg) of vitamin D3 daily starting from birth, with an upper safety limit of 1,000 IU/day. 1

• Breastfed infants require supplementation because breast milk contains insufficient vitamin D 1
• Formula-fed infants consuming <1 liter/day of fortified formula also require supplementation 1

Children and Adolescents (1–18 years)

Treatment of deficiency (<20 ng/mL):

• Severe deficiency (<5 ng/mL): 8,000 IU daily for 4 weeks, then 4,000 IU daily for 2 months 1, 2
• Mild deficiency (5–15 ng/mL): 4,000 IU daily for 12 weeks 1, 2
• Insufficiency (16–30 ng/mL): 2,000 IU daily 1, 2

Alternative loading protocol for adolescents: 50,000 IU of cholecalciferol weekly for 8–12 weeks, followed by maintenance of 600–1,000 IU daily or 50,000 IU monthly 2

Maintenance after repletion: 200–1,000 IU daily depending on age and risk factors 1

Adults (19–70 years)

Treatment of deficiency (<20 ng/mL):

• Standard loading regimen: 50,000 IU cholecalciferol weekly for 8–12 weeks 3, 1
• Severe deficiency (<10 ng/mL): 50,000 IU weekly for 12 weeks, or 8,000 IU daily for 4 weeks followed by 4,000 IU daily for 2 months 3, 1
• Insufficiency (20–30 ng/mL): 4,000 IU daily for 12 weeks or 50,000 IU every other week for 12 weeks 1

Maintenance after repletion: 800–2,000 IU daily or 50,000 IU monthly (equivalent to ~1,600 IU daily) 3, 1

For general prevention in adults without deficiency: 2,000 IU daily is sufficient to maintain serum 25(OH)D >30 ng/mL in >90% of the population and is safe for prolonged use 4

Older Adults (≥65 years)

Older adults require a minimum of 800 IU daily even without baseline measurement, though 700–1,000 IU daily more effectively reduces fall and fracture risk. 3, 1

• Target serum 25(OH)D ≥30 ng/mL for anti-fracture efficacy; ≥24 ng/mL for anti-fall benefits 3
• Doses of 700–1,000 IU daily achieving levels ≥30 ng/mL reduce falls by 19% and non-vertebral fractures by 20% 3

Pregnant and Lactating Women

Pregnant and lactating women have increased vitamin D demands and should receive 600–2,000 IU daily, with higher doses (4,000 IU daily) considered safe during pregnancy. 3

• Pregnancy and lactation increase vitamin D requirements due to fetal skeletal development and milk production 3
• Target serum 25(OH)D ≥30 ng/mL throughout pregnancy 3

Cholecalciferol (D3) vs. Ergocalciferol (D2)

Cholecalciferol (vitamin D3) is strongly preferred over ergocalciferol (vitamin D2) because D3 maintains serum levels longer and has superior bioavailability, particularly with intermittent dosing. 3, 1, 2

• D3 is the physiological form produced by skin synthesis 1
• D3 maintains 25(OH)D concentrations for longer periods with weekly or monthly dosing 3, 2

Calcium Co-Supplementation

Ensure total calcium intake of 1,000–1,500 mg daily from diet plus supplements, as adequate calcium is necessary for vitamin D to exert its full bone-protective effect. 3, 1, 2

• Calcium supplements should be taken in divided doses of no more than 600 mg at once for optimal absorption 3, 2
• Separate calcium from iron-containing supplements by at least 2 hours to prevent absorption interference 3
• Take vitamin D with the largest, fattiest meal of the day to maximize absorption 3

Vitamin K2 Co-Supplementation

The evidence does not support routine co-prescription of vitamin K2 with vitamin D supplementation. 3, 1

• Current guidelines do not recommend vitamin K2 as a standard adjunct to vitamin D therapy 3, 1
• Focus should remain on ensuring adequate calcium intake alongside vitamin D 3, 1, 2

Monitoring Guidelines

Initial Assessment

Measure serum 25(OH)D before initiating treatment to establish baseline status and guide dosing. 3

• Deficiency: <20 ng/mL 3, 1
• Insufficiency: 20–30 ng/mL 3, 1
• Sufficiency: ≥30 ng/mL 3, 1
• Optimal range: 30–44 ng/mL 3
• Upper safety limit: 100 ng/mL 3

During Treatment

Recheck serum 25(OH)D at 3 months after initiating supplementation to allow levels to plateau and accurately reflect treatment response. 3, 1, 2

• If using intermittent dosing (weekly or monthly), measure just prior to the next scheduled dose 3
• Monitor serum calcium and phosphorus every 3 months during high-dose therapy to detect hypercalcemia early 3

Long-Term Monitoring

Once target levels (≥30 ng/mL) are achieved and stable, recheck serum 25(OH)D annually. 3

• Continue monitoring serum calcium every 3 months if on maintenance therapy 3
• For post-bariatric surgery patients, monitor every 3–6 months in the first year, then annually 3

Special Populations Requiring Modified Dosing

Obesity

Obese patients require higher doses—consider 7,000 IU daily or 30,000 IU weekly for prolonged maintenance, or 50,000 IU weekly for 6–8 weeks to treat deficiency. 5

• Vitamin D is sequestered in adipose tissue, requiring higher doses to achieve target levels 3, 5

Malabsorption Syndromes

For patients with inflammatory bowel disease, celiac disease, pancreatic insufficiency, or short-bowel syndrome, intramuscular vitamin D3 50,000 IU is the preferred route when available. 3

• IM administration results in significantly higher 25(OH)D levels and lower rates of persistent deficiency compared to oral supplementation 3
• When IM is unavailable, use substantially higher oral doses: 4,000–5,000 IU daily for 2 months 3

Post-Bariatric Surgery

Post-bariatric surgery patients require at least 2,000 IU daily for maintenance, with IM administration preferred if oral supplementation fails to achieve target levels. 3

• Malabsorptive procedures (Roux-en-Y gastric bypass) dramatically reduce vitamin D absorption 3
• Monitor more frequently (every 3–6 months) in the first year 3

Chronic Kidney Disease (CKD Stages 3–4)

For CKD patients with GFR 20–60 mL/min/1.73m², use standard nutritional vitamin D (cholecalciferol or ergocalciferol), not active vitamin D analogs. 3, 1, 2

• CKD patients are at high risk due to reduced sun exposure, dietary restrictions, and urinary losses 3
• Monitor serum calcium and phosphorus every 3 months 3
• Reserve active vitamin D analogs (calcitriol) only for advanced CKD with PTH >300 pg/mL 3

Safety Thresholds and Toxicity

Daily doses up to 4,000 IU are completely safe for adults; limited evidence supports up to 10,000 IU daily for several months without adverse effects. 3, 1, 4

• Toxicity is rare and typically occurs only with prolonged daily doses >10,000 IU or serum 25(OH)D >100 ng/mL 3, 1
• Symptoms of toxicity include hypercalcemia, hyperphosphatemia, suppressed PTH, and hypercalciuria 3

Immediately discontinue all vitamin D supplementation if serum calcium exceeds 10.2 mg/dL (2.54 mmol/L). 3

Avoid single ultra-high loading doses (>300,000 IU), as they may paradoxically increase fall and fracture risk. 3, 1

Critical Pitfalls to Avoid

Never use active vitamin D analogs (calcitriol, alfacalcidol, doxercalciferol, paricalcitol) to treat nutritional vitamin D deficiency, as they bypass normal regulatory mechanisms and dramatically increase hypercalcemia risk. 3, 1, 2

• Active analogs do not correct 25(OH)D levels and are reserved only for advanced CKD with impaired 1α-hydroxylase activity 3, 2

Do not rely on sun exposure for vitamin D repletion due to increased skin cancer risk from UVB radiation. 3

Ensure adequate calcium intake alongside vitamin D; vitamin D alone is less effective for bone health. 3, 1, 2

Verify patient adherence before increasing doses for inadequate response; poor compliance is a common reason for treatment failure. 3

Practical Dosing Equivalents

• 50,000 IU monthly ≈ 1,600 IU daily 3, 1, 2
• 1,000 IU daily increases serum 25(OH)D by approximately 10 ng/mL (rule of thumb, though individual responses vary) 3
• For calculating loading dose: dose (IU) = 40 × (75 − serum 25(OH)D) × body weight (kg) 6