Vitamin D Deficiency

Overview.

Vitamin D deficiency affects approximately 40% of adults globally and up to 70-80% in some populations (elderly, dark-skinned individuals, northern latitudes). While best known for bone health, vitamin D functions as a hormone with receptors in nearly every tissue — affecting immune function, muscle strength, mood, cardiovascular health, and cancer risk. Deficiency is readily detected with a simple blood test (25-OH vitamin D) and corrected with inexpensive supplementation.

Vitamin D is a fat-soluble secosteroid hormone synthesised in the skin upon UVB exposure, then converted to its active form (1,25-dihydroxyvitamin D / calcitriol) through sequential hydroxylation in the liver and kidneys. It regulates calcium absorption, bone mineralisation, immune cell function, and gene expression. Deficiency reflects inadequate sun exposure, dietary intake, or impaired absorption/conversion.

Prevalence: ~40% of adults globally are deficient (<20 ng/mL). Up to 70-80% in elderly, institutionalised, dark-skinned, and northern-latitude populations. Prevalence increases with: latitude, winter season, obesity (vitamin D sequestered in fat), CKD (impaired activation), and malabsorption conditions (celiac, IBD).

Medical name: Vitamin D Deficiency / Insufficiency

Symptoms.

Early warnings

• Unexplained fatigue and low energy
• Bone pain or aching (especially lower back and legs)
• Muscle weakness (difficulty rising from chair, climbing stairs)
• Frequent infections (immune dysfunction)
• Low mood or seasonal depression
• Slow wound healing

Classic symptoms

• Bone pain and tenderness (adults) — osteomalacia if severe
• Proximal muscle weakness (difficulty with stairs, getting up from floor)
• Frequent respiratory infections
• Fatigue and low energy (non-specific but very common)
• Depression and mood changes (especially seasonal pattern)
• Hair loss (associated with severe deficiency)
• Rickets in children (bowed legs, growth failure, delayed milestones)
• Stress fractures and pathological fractures (osteoporosis/osteomalacia)

Progression

Mild deficiency (20-29 ng/mL) is often asymptomatic or causes vague fatigue. Moderate deficiency (10-19 ng/mL) may cause bone pain, muscle weakness, and increased infection susceptibility. Severe deficiency (<10 ng/mL) causes osteomalacia (adults) or rickets (children) — painful softening of bones, proximal myopathy, and secondary hyperparathyroidism with accelerated bone loss.

Risk factors.

• Limited sun exposure (indoor workers, institutionalised, full-body covering)
• Dark skin pigmentation (melanin reduces UVB absorption)
• Northern latitude (above ~35°N — insufficient UVB for synthesis October-March)
• Obesity (BMI >30 — vitamin D sequestered in adipose tissue)
• Age >65 (skin synthesis capacity declines ~50% by age 70)
• Chronic kidney disease (impaired 1-alpha hydroxylation)
• Malabsorption: celiac disease, IBD, gastric bypass, cystic fibrosis
• Medications: anticonvulsants, glucocorticoids, antiretrovirals, rifampin
• Breastfed infants without supplementation
• Vegan/vegetarian diet without fortified foods

Lab interpretation.

Key biomarkers

• 25-OH Vitamin D — <20 ng/mL = deficient; 20-29 = insufficient; ≥30 = sufficient; 40-60 = optimal (primary)
• Calcium — May be low-normal in severe deficiency (secondary hyperparathyroidism maintains calcium at expense of bone) (secondary)
• ALP — Elevated in osteomalacia (bone-origin ALP rises due to increased bone turnover) (supportive)

Diagnostic criteria

• 25-OH Vitamin D <20 ng/mL (<50 nmol/L) = deficiency
• 25-OH Vitamin D 20-29 ng/mL (50-72 nmol/L) = insufficiency
• 25-OH Vitamin D ≥30 ng/mL (≥75 nmol/L) = sufficiency
• Optimal range for prevention: 40-60 ng/mL (many experts and Endocrine Society)
• Check PTH if deficiency is severe — elevated PTH with low vitamin D confirms secondary hyperparathyroidism
• 1,25-dihydroxyvitamin D (calcitriol) should NOT be used for routine screening — it's regulated independently and can be normal/high even in deficiency

When & next steps.

When to test

• Osteoporosis or history of fractures
• Chronic kidney disease
• Malabsorption conditions (celiac, IBD, gastric bypass)
• Unexplained bone pain or muscle weakness
• Dark skin and limited sun exposure
• Obesity (BMI >30)
• Elderly (>65 years)
• Pregnant women
• Treatment-resistant iron or calcium deficiency
• Autoimmune conditions (may benefit from repletion)

If suspected

• Check 25-OH Vitamin D (the correct test — not 1,25-dihydroxyvitamin D)
• If <20 ng/mL: also check calcium, phosphate, ALP, and PTH
• If celiac suspected as cause of deficiency: check tTG-IgA
• If CKD suspected: check eGFR

If confirmed

• Severe deficiency (<10 ng/mL): loading dose 50,000 IU/week x 8-12 weeks, then maintenance
• Moderate deficiency (10-20 ng/mL): 2,000-4,000 IU/day or 50,000 IU/week x 6-8 weeks
• Insufficiency (20-29 ng/mL): 1,000-2,000 IU/day maintenance
• Recheck 25-OH vitamin D after 3 months of supplementation to confirm repletion
• Maintenance dose: most adults need 1,000-4,000 IU/day to maintain >30 ng/mL
• Obese individuals may need 2-3x standard doses (vitamin D sequestered in fat)
• Vitamin D3 (cholecalciferol) is preferred over D2 (ergocalciferol) — more effective at raising and maintaining levels
• Take with a fat-containing meal to improve absorption

FAQs.

Important note.