Vitamin D deficiency is extremely common — affecting over 1 billion people globally. Key symptoms include fatigue, bone and muscle pain, frequent infections, depression, hair loss, slow wound healing, and in severe cases rickets (children) or osteomalacia (adults). However, up to 50% of deficient people have no symptoms — deficiency is often discovered only on blood testing.
Key Facts at a Glance
| Global prevalence | >1 billion people deficient |
| Deficiency threshold | <20 ng/mL (50 nmol/L) |
| Insufficiency threshold | 20–29 ng/mL |
| Optimal level | 40–60 ng/mL (100–150 nmol/L) |
| Correct test | Serum 25-hydroxyvitamin D [25(OH)D] |
| Groups most at risk | Older adults, dark skin, northern latitudes, indoor workers |
Most Common Vitamin D Deficiency Symptoms
Fatigue and low energy — vitamin D is required for mitochondrial ATP production; deficiency impairs cellular energy. This is often the most prominent symptom. Bone and muscle pain — vitamin D deficiency causes impaired calcium absorption, leading to softening of bone matrix (osteomalacia in adults) that presents as deep, aching bone pain (often in the ribs, hips, and legs). Muscle weakness and pain are common. Frequent infections — vitamin D receptors (VDRs) are on every immune cell. Deficiency impairs both innate and adaptive immune responses, increasing susceptibility to respiratory infections, particularly. Depression and low mood — VDRs are expressed throughout the limbic system. Multiple meta-analyses link low vitamin D to depression. Seasonal affective disorder (SAD) is partly attributed to reduced winter sunlight and falling vitamin D. Hair loss — vitamin D stimulates hair follicle cycling. Severe deficiency is associated with alopecia areata. Slow wound healing — vitamin D promotes keratinocyte migration and differentiation essential for wound closure. Sweating of the head — a classic early sign mentioned in older medical literature, less commonly referenced today.
Blood Levels and What They Mean
Serum 25(OH)D interpretation: • <10 ng/mL (25 nmol/L): Severe deficiency — risk of rickets/osteomalacia • 10–19 ng/mL (25–50 nmol/L): Deficiency — immune and bone function impaired • 20–29 ng/mL (50–75 nmol/L): Insufficiency — suboptimal for many functions • 30–39 ng/mL (75–100 nmol/L): Adequate by NIH standards • 40–60 ng/mL (100–150 nmol/L): Optimal for immune, bone, mood, and cancer protection per most functional medicine guidelines • >150 ng/mL (375 nmol/L): Toxic — hypercalcaemia risk
Risk Factors for Vitamin D Deficiency
You are at higher risk of deficiency if you: • Live above 37° latitude (north of Los Angeles, Rome, Seoul — insufficient winter UVB) • Have darker skin (more melanin reduces cutaneous D synthesis) • Are over 60 (skin synthesis capacity reduces with age) • Work primarily indoors • Use high SPF sunscreen consistently • Have obesity (vitamin D is sequestered in adipose tissue) • Have malabsorption conditions (Crohn's, celiac, bariatric surgery) • Take medications reducing D absorption (corticosteroids, anticonvulsants) • Are vegetarian or vegan (limited dietary D sources)
How to Correct Vitamin D Deficiency
For deficiency (<20 ng/mL): 4,000 IU/day for 3 months, then retest. Some physicians prescribe weekly 50,000 IU doses for severe deficiency. For insufficiency (20–30 ng/mL): 2,000–3,000 IU/day. For maintenance once optimal: 2,000 IU/day for most people. Always take vitamin D3 (cholecalciferol), not D2 (ergocalciferol) — D3 is 87% more effective at raising 25(OH)D. Take with fat-containing food for best absorption. Pair with vitamin K2 (100–200 mcg MK-7) to direct calcium to bones.
Frequently Asked Questions
Clinical References
- Holick MF. (2007). N Engl J Med. Vitamin D deficiency — comprehensive review. → Source
- NIH Office of Dietary Supplements. Vitamin D Fact Sheet for Health Professionals. → Source
- Martineau AR et al. (2017). BMJ. Vitamin D supplementation to prevent acute respiratory tract infections. → Source