Vitamin D-The Basics

Vitamin D-The Basics

By Dr Sherry

Vitamin D is a fat‑soluble vitamin (and prohormone) essential for calcium and phosphorus balance, bone health, immune function, and muscle strength.
The “hype” is partly justified: strong evidence links adequate vitamin D status to better bone health and reduced risk of fractures when combined with calcium; evidence for broader outcomes (cardiovascular disease, cancer prevention) is mixed or inconclusive.
Best approach: sensible sun exposure plus diet and targeted supplementation when needed to achieve and maintain recommended serum 25‑hydroxyvitamin D levels.

Roles of vitamin D

Promotes intestinal absorption of calcium and phosphorus, supporting bone mineralization.
Regulates bone remodeling and reduces risk of rickets in children and osteomalacia/osteoporosis in adults.
Modulates innate and adaptive immune responses.
Supports muscle function and may reduce fall risk in older adults.
Acts via its hormonal form (calcitriol) on multiple tissues influencing cell growth and inflammation.

What the evidence shows

Strong evidence: preventing rickets in infants/children; reducing osteomalacia; improving bone density and reducing fracture risk when vitamin D and calcium are combined in deficient or older adults.
Moderate evidence: vitamin D supplementation reduces falls in some older adults (usually when correcting deficiency).
Weak/inconclusive evidence: consistent benefit for cardiovascular disease, cancer prevention, or broad immune protection (RCTs show mixed results).
Conclusion: correcting and preventing deficiency is important, especially for bone and muscle health; routine high-dose supplementation for other outcomes is not strongly supported.

Good sources (sun, food, and supplements)

Sunlight: UVB exposure produces vitamin D3 in skin. Amount needed varies by latitude, season, skin pigmentation, and sunscreen/clothing use.
Food (naturally rich and fortified): oily fish (salmon, mackerel, sardines), cod liver oil, egg yolks, liver; fortified milk, fortified plant milks, fortified orange juice, fortified cereals.
Supplements: vitamin D3 (cholecalciferol) is more effective at raising and maintaining serum levels than D2 (ergocalciferol) in most studies. Prescription calcitriol is used for some medical conditions but not routine supplementation.

Typical needs and target levels

Intake guidance (common recommendations): Infants 0–12 months: 400 IU/day; Children 1–18 years: 600 IU/day; Adults up to 70 years: 600 IU/day; Adults 71+ years: 800 IU/day. Pregnant/lactating adults: typically 600 IU/day (some authorities recommend higher if deficient).
Serum 25‑hydroxyvitamin D (25[OH]D) targets: deficiency generally <20 ng/mL (50 nmol/L); insufficiency often 20–30 ng/mL (50–75 nmol/L); sufficiency commonly defined as 30–50 ng/mL (75–125 nmol/L) by many experts. Optimal target may vary by guideline and clinical context.
Upper limit: generally safe upper intake level for adults ≈4,000 IU/day (100 µg/day) without monitoring; higher therapeutic doses may be used short-term under medical supervision.

How to tell if you’re deficient

Symptoms: bone pain, muscle weakness, increased falls, delayed growth/rickets in children; symptoms can be subtle or absent.
Risk groups: limited sun exposure, darker skin, older adults, obesity, malabsorption (celiac, IBD), chronic kidney disease, liver disease, certain medications (anticonvulsants, glucocorticoids), bariatric surgery.
Testing: serum 25‑hydroxyvitamin D is the standard test for vitamin D status. Evaluate with calcium, phosphorus, PTH, and renal function if abnormal or if clinical concern for metabolic bone disease.
Interpretation: a low 25(OH)D (<20 ng/mL) indicates deficiency; values 20–30 ng/mL suggest insufficiency; aim for ≥30 ng/mL in many clinical situations, especially in people at high fracture risk.

Should people take vitamin D supplements? Safety and practical guidance

Food + sun first: try safe sun exposure and eating fortified/containing foods; many people still need supplements to reach targets.
When to supplement: evidence supports routine supplementation for breastfed infants (400 IU/day), people with limited sun exposure, older adults, and those with documented deficiency or at high fracture risk.
Typical supplemental doses: 400–2,000 IU/day commonly used; 600–800 IU/day meets general needs for many adults; 1,000–2,000 IU/day is frequently used to correct insufficiency and maintain levels. Higher therapeutic regimens (e.g., weekly high-dose loading) are sometimes used under supervision.
Monitoring: check serum 25(OH)D after 8–12 weeks when using higher doses or to confirm correction; adjust dose accordingly.
Safety: vitamin D toxicity (hypervitaminosis D) is rare but can cause hypercalcemia, nausea, weakness, polyuria, and kidney stones; usually from very high supplemental intakes (often >10,000 IU/day over time) or underlying disorders of calcium metabolism. Monitor calcium and renal function if using high doses.
Interactions/considerations: vitamin D increases calcium absorption — consider total calcium intake; be cautious in patients with granulomatous diseases (sarcoidosis, tuberculosis) or certain lymphomas that increase conversion to active vitamin D; review medications affecting vitamin D metabolism.

Bottom-line recommendations

Ensure infants get 400 IU/day; consider routine supplementation for older adults and people with limited sun exposure or risk factors.
Aim for serum 25(OH)D ≥20–30 ng/mL depending on risk; test if risk factors or symptoms present or before using high-dose therapy.
Typical safe maintenance doses for most adults: 600–2,000 IU/day. Use vitamin D3 (cholecalciferol) for supplementation unless a clinician advises otherwise.
Monitor and adjust doses under medical supervision for high doses, kidney disease, or conditions affecting vitamin D metabolism.