Vitamin D Deficiency: Symptoms, Causes, Testing, and Treatment

Vitamin D deficiency is remarkably common—affecting ~35% of U.S. adults, higher in northern latitudes, darker skin tones, older adults, and those with limited sun exposure. Recognizing signs and correcting deficiency supports bone, immune, metabolic, and mental health.

Signs and Symptoms

• Fatigue and weakness: Vitamin D receptors in muscle; deficiency impairs mitochondrial function and energy production.
• Bone pain and muscle aches: Impaired calcium absorption → secondary hyperparathyroidism → bone resorption, osteomalacia (adult rickets), myopathy.
• Frequent infections: Vitamin D modulates innate and adaptive immunity; deficiency increases respiratory infection susceptibility.
• Slow wound healing: Involved in keratinocyte differentiation, antimicrobial peptide production.
• Skin conditions: Linked to psoriasis, atopic dermatitis, vitiligo severity.
• Mood changes: Receptors in brain regions regulating mood; deficiency associated with depression, seasonal affective disorder, anxiety.
• Hair loss: Vitamin D receptors in hair follicles; deficiency linked to alopecia areata, telogen effluvium.
• Cognitive concerns: Low levels associated with cognitive decline, Alzheimer's risk (observational).

Primary Causes

• Inadequate sun exposure (latitude >37°, sunscreen, indoor lifestyle, covering clothing)
• Darker skin (melanin reduces cutaneous synthesis)
• Aging (↓ 7-dehydrocholesterol, ↓ renal 1α-hydroxylase)
• Obesity (sequestration in adipose tissue)
• Malabsorption (celiac, Crohn's, cystic fibrosis, bariatric surgery)
• Medications (glucocorticoids, anticonvulsants, antiretrovirals, cholestyramine)
• Kidney/liver disease (impaired hydroxylation)

Testing and Interpretation

• Serum 25-hydroxyvitamin D [25(OH)D] is the standard marker.
• Deficiency: <20 ng/mL (50 nmol/L)
• Insufficiency: 20–29 ng/mL
• Sufficiency: ≥30 ng/mL (Endocrine Society); ≥20 ng/mL (IOM)
• Optimal for extraskeletal benefits: 40–60 ng/mL (many experts)

Treatment and Repletion

• Deficiency: 50,000 IU D2 or D3 weekly × 8 weeks, then maintenance 1,500–2,000 IU/day (adults).
• Insufficiency: 2,000–4,000 IU/day × 8–12 weeks, then maintenance.
• Maintenance: 1,000–2,000 IU/day (adults); higher for obesity, malabsorption, medications.
• D3 (cholecalciferol) preferred over D2 (ergocalciferol)—more potent, longer half-life.
• Co-factors: Magnesium (required for enzymatic activation), K2 (directs calcium to bone).

Prevention

• Sensible sun exposure (10–30 min midday, arms/legs, 2–3×/week, season-dependent).
• Dietary sources: fatty fish, egg yolks, UV-exposed mushrooms, fortified foods (milk, OJ, cereals).
• Supplementation per above, individualized.

If you experience these symptoms or have risk factors, ask your doctor for a 25(OH)D test. Correction is safe, inexpensive, and impactful.