Vitamin D Deficiency Symptoms: What to Look For and How to Test
Short answer: The classic symptoms of vitamin D deficiency are fatigue, bone pain, muscle weakness, and frequent infections — but these are non-specific and easily missed. The only reliable diagnosis is a blood test measuring 25-hydroxyvitamin D (25(OH)D). Deficiency (<50 nmol/L) affects 40-60% of adults in northern Europe and North America who don’t supplement. Sub-optimal levels (<75 nmol/L) affect an estimated 1 billion people globally. If you haven’t tested, there’s roughly a coin-flip chance you’re deficient.
The symptoms — and why they’re unreliable
Vitamin D deficiency produces symptoms so non-specific that most people and many doctors don’t connect them. The documented symptoms from clinical studies of people with confirmed deficiency: persistent fatigue (present in ~70% of deficient people in a 2015 BMJ case reports analysis), bone ache particularly in the lower back, sternum, and shins (the bones most affected by impaired calcium mineralisation), proximal muscle weakness (difficulty rising from a chair, climbing stairs), depressed mood (multiple cross-sectional studies show correlation, causation less clear), and frequent respiratory infections.
Severe, prolonged deficiency causes rickets in children (bone deformity) and osteomalacia in adults (soft, painful bones). These are the unambiguous clinical presentations. Sub-clinical deficiency (50-75 nmol/L) may produce subtle fatigue and mood effects, or no symptoms at all. This is why testing is the only reliable approach — you cannot diagnose vitamin D deficiency by symptoms alone.
Who is at highest risk
People living above 37° latitude (approximately north of Madrid, or north of San Francisco in the US) cannot synthesise vitamin D from sunlight between October and March — the sun angle is too low for UVB rays to reach the earth’s surface. This includes all of the UK, Canada, most of Europe, and the northern US. Even in summer months, the 10-15 minutes of midday sun recommended in guidelines may be insufficient if you have darker skin (melanin reduces UVB penetration), use SPF 30+ sunscreen (reduces D3 synthesis by 97%), or spend most time indoors.
What blood level to aim for
The debate: 50 nmol/L (20 ng/mL) is the official minimum for bone health. Most vitamin D researchers consider 75-100 nmol/L (30-40 ng/mL) optimal for immune and non-skeletal functions. The VITAL trial used participants with mean baseline 77 nmol/L. Evidence for supplementation benefits is strongest in people below 50 nmol/L — above this, benefits become more modest and study-dependent.
Testing: what to ask for
Request: 25-hydroxyvitamin D (25(OH)D) blood test. Not 1,25-dihydroxyvitamin D (calcitriol) — this is the active form and stays normal even in deficiency. 25(OH)D is the correct marker for vitamin D status. Available through your GP (NHS in the UK, covered by many US insurers if deficiency is suspected) or private blood tests (typically £30-50 in the UK, $40-80 in the US).
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- Holick MF. (2007). Vitamin D deficiency. N Engl J Med, 357(3), 266-81. PubMed →
- Martineau AR et al. (2017). Vitamin D supplementation to prevent acute respiratory tract infections. BMJ, 356, i6583. PubMed →
- Spiro A & Buttriss JL. (2014). Vitamin D: An overview of vitamin D status and intake in Europe. Nutr Bull, 39(4), 322-350. PubMed →
