Vitamin D deficiency: the ache and tiredness half the world shares
Last updated September 3, 2026.
Vitamin D deficiency is common, usually subtle, and easy to fix once it is measured. Vitamin D is less a vitamin than a hormone your skin makes from sunlight, and modern indoor life plus northern latitudes leave a large share of adults below optimal, especially in winter. The symptoms are vague enough that guessing fails; a blood test settles it.
What does it actually cause?
Severe, prolonged deficiency softens bones (osteomalacia in adults, rickets in children) and causes deep aching bone pain, especially in the hips, pelvis, and legs, plus genuine muscle weakness that makes stairs and rising from a chair hard. Mild deficiency is murkier: fatigue, low mood, and general aches are often attributed to it, and treating a confirmed low level is reasonable, but these symptoms have many causes and vitamin D is not a cure-all. Where the evidence is solid: deficiency worsens osteoporosis, raises fall risk in older adults, and severe lack directly weakens muscle and bone. Where it is not: routine supplementation has not shown clear benefits for colds, heart disease, or cancer in large trials.
Who should be tested?
Risk concentrates predictably: darker skin (more melanin means less D from the same sun), little sun exposure, covering clothing, living far from the equator, age over 65, obesity (D gets sequestered in fat), malabsorption from celiac or Crohn's disease, and kidney or liver disease. Testing makes sense with symptoms (bone pain, muscle weakness, frequent falls) or strong risk factors. The test is 25-hydroxyvitamin D: under 12 ng/mL (30 nmol/L) is deficient, 12-20 is insufficient, above 20 is generally sufficient for bone health. Screening everyone without symptoms is not recommended; treating a number nobody measured is guesswork.
What actually helps?
- Daily D3 maintenance: 600-800 IU daily covers most adults; 1,000-2,000 IU daily is a common, safe choice for risk groups and winter months.
- Repletion for true deficiency: clinicians commonly prescribe 50,000 IU weekly for 6-8 weeks (or 6,000-8,000 IU daily), then maintenance. Recheck the level after 2-3 months.
- Sunlight, dosed sensibly: brief midday exposure on arms and legs a few times a week generates meaningful D; it stops working through glass and works poorly in winter at northern latitudes. Never tan for it.
- Food helps at the margins: oily fish, fortified milk and cereals, and egg yolks contribute, but diet alone rarely fixes deficiency.
- Do not megadose blindly: the tolerable ceiling for adults is 4,000 IU daily long-term. Very high unsupervised doses can push calcium up dangerously.
When is it an emergency?
Deficiency itself is a slow problem, not an emergency. The urgent scenarios are the extremes at either end. In a baby or toddler: bowed legs, a soft skull, delayed teeth, or seizures (from low calcium) need prompt assessment. In someone taking high-dose supplements: nausea, vomiting, confusion, and intense thirst suggest calcium toxicity from too much D, and that needs same-day labs. Pymander's escalation routing is built and tested specifically for this class of decision; see the safety architecture working paper.
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Common questions
What is a normal vitamin D level?
The test measures 25-hydroxyvitamin D. Below 12 ng/mL (30 nmol/L) is deficiency, 12-20 ng/mL is insufficient for bone health, and 20-50 ng/mL is the generally accepted sufficient range. Some labs flag up to 30 ng/mL as ideal, and expert groups genuinely disagree on the optimal cutoff, which is why the range matters more than chasing a precise number. Levels above 100 ng/mL from oversupplementation are harmful, usually through raised calcium.
Can I get enough vitamin D from sunlight alone?
It depends on your skin, latitude, and season. Fair skin at mid-latitudes in summer can make a full day's requirement in 10-30 minutes of midday sun on arms and legs. Darker skin needs several times longer. Above about 37 degrees latitude, winter sunlight is too weak to make meaningful D at all, which is why winter supplementation is standard advice in northern countries. Glass blocks the relevant UV, and sunscreen reduces but does not eliminate production.
Is vitamin D2 or D3 better?
D3 (cholecalciferol, the form skin makes) raises and sustains blood levels better than D2 (ergocalciferol), so D3 is the usual recommendation when there is a choice. Both work, and prescription repletion courses in some countries still use D2 effectively. The bigger variables are the dose and taking it with a meal containing fat, which improves absorption substantially.
Can you take too much vitamin D?
Yes, and the harm runs through calcium. Very high doses (typically 10,000 IU or more daily for months, or accidental megadosing) can push blood calcium up, causing nausea, vomiting, confusion, kidney stones, and in severe cases heart rhythm problems. The long-term tolerable ceiling for adults is 4,000 IU daily. Standard doses of 1,000-2,000 IU daily are far below any risk zone. More is not better; it is just more.
Does vitamin D prevent colds and flu?
The evidence is modest at best. A large analysis found daily supplementation reduced respiratory infections slightly, with the benefit concentrated in people who were deficient to start. For people with adequate levels, extra D did not show meaningful protection. Fixing a real deficiency is worthwhile for bone and muscle reasons; taking it primarily to dodge colds overpromises what the trials showed.
Should I take vitamin D with calcium?
For bone health in older adults and people with osteoporosis, yes, they work as a pair: vitamin D enables calcium absorption, and the trials that cut fracture rates used them together. Get calcium mostly from food (dairy, fortified alternatives, leafy greens, sardines) targeting about 1,000-1,200mg daily, with supplements filling only the gap. Taking calcium supplements without a D check, or D without adequate calcium, is half a plan.