Medical disclaimer: This article does not replace medical advice, diagnosis, or treatment. If you suspect a vitamin D deficiency, plan to take high-dose supplements, or have an existing condition (kidney, thyroid, sarcoidosis, pregnancy), please consult a clinician. Lab values must be interpreted individually.
Vitamin D deficiency is common in higher-latitude countries. Surveys suggest that 40–60 % of adults in northern Europe and roughly 1 billion people worldwide do not reach the threshold of 20 ng/mL of 25-hydroxy-vitamin D considered sufficient — particularly in late winter. The complaints are diffuse: fatigue, bone pain, low mood, frequent infections.
This article explains what vitamin D does, which symptoms point to a deficiency, how the 25-OH-D blood test works, how many IU per day are reasonable, and where the safety upper limit sits.
The role of vitamin D in the body
Strictly speaking, vitamin D is not a vitamin but a prohormone. Through the active form calcitriol, it regulates more than 200 genes and influences calcium and phosphate balance, immune cells, muscles, the nervous system, and gene expression in nearly every cell.
Bones and teeth: Drives intestinal calcium absorption and bone mineralization.
Muscle function: Affects strength and coordination — relevant for falls and fractures in older adults.
Immunity: Activates antimicrobial peptides and modulates inflammation.
Mood and sleep: Low levels are associated with depressive symptoms and disturbed sleep — whether causal or a consequence is not fully resolved.
If you want to understand how much sun the body needs to make its own vitamin D, see the background article on vitamin D synthesis.
Symptoms of vitamin D deficiency
The symptoms are non-specific and easily attributed to other causes. When several appear together, a blood test is worth ordering:
Persistent fatigue
Exhaustion despite adequate sleep and quick fatigue during everyday tasks. Trials in people with confirmed deficiency show that supplementation measurably reduces fatigue.
Bone and muscle pain
Diffuse bone or back pain, weakness in the thighs and shoulders. Severe deficiency can lead to osteomalacia (soft bones) — known as rickets in children.
Low mood
Reduced mood, especially during winter. The link between vitamin D and seasonal affective symptoms is well documented; supplementing in deficient individuals can ease symptoms.
Frequent infections
More colds and prolonged respiratory infections than usual. A BMJ 2017 meta-analysis found that supplementing deficient individuals significantly reduces the rate of acute respiratory infections.
Hair loss and slow wound healing
Diffuse hair shedding, slow-healing wounds, brittle nails. Vitamin D contributes to skin and hair follicle differentiation.
Who is at higher risk?
Some groups rarely reach adequate levels even in summer. Routine monitoring and year-round supplementation are usually appropriate:
| Risk group | Reason |
|---|---|
| Infants (0–12 months) | Limited sun exposure, low stores — 400–500 IU/day recommended |
| Adults 65+ | Reduced skin synthesis, less time outdoors |
| Darker skin types (V/VI) | Melanin reduces UV-B-driven synthesis 3–8× |
| Obesity (BMI > 30) | Vitamin D is sequestered in adipose tissue |
| Veiled clothing or little outdoor time | UV-B barely reaches the skin |
| Chronic gut or liver disease | Impaired absorption or activation |
| Pregnant and breastfeeding | Increased need, supports the baby |
If you also have bone pain, falls, or fractures in your history, it is worth checking your individual osteoporosis risk.
Blood test: interpreting 25-OH-D
The standard test measures 25-hydroxy-vitamin D (25-OH-D, calcidiol) in serum. This value reflects vitamin D status over the previous weeks. Calcitriol (1,25-OH-D) is the active form — measured only in special cases, since it usually stays normal even in deficiency.
Values are reported in ng/mL or nmol/L. Conversion: 1 ng/mL ≈ 2.5 nmol/L.
| Status | ng/mL | nmol/L |
|---|---|---|
| Severe deficiency | < 10 | < 25 |
| Deficient | 10–20 | 25–50 |
| Insufficient | 20–30 | 50–75 |
| Sufficient | 30–60 | 75–150 |
| High | 60–100 | 150–250 |
| Toxic range | > 100 | > 250 |
Ideally, test at the end of winter (February/March) and again after a sunny stretch (September) — that gives an honest picture of seasonal swings.
When deficiency is found, related labs help: calcium, phosphate, parathyroid hormone (PTH), and kidney function. If you already have a lab report, calculate your corrected calcium.
How much sun do you actually need?
Skin type, latitude, season — the free vitamin D calculator estimates your needs.
Open the vitamin D calculator →Supplementation: choosing the right IU per day
The International Unit (IU) is the standard dose unit. Conversion: 40 IU = 1 µg of vitamin D3. The right daily dose depends on starting level, body weight, and goal.
| Group | Maintenance dose |
|---|---|
| Infants 0–12 months | 400–500 IU/day |
| Children 1–18 years | 600–1,000 IU/day |
| Adults | 800–2,000 IU/day |
| Older adults / obesity | 1,500–4,000 IU/day |
| Pregnant / breastfeeding | 1,000–2,000 IU/day (under medical supervision) |
A practical rule of thumb when correcting a deficiency: 30–50 IU per kg of body weight per day. A 70 kg person therefore needs about 2,100–3,500 IU/day, a 100 kg person 3,000–5,000 IU/day, until the target of 30–50 ng/mL is reached. After that, a lower maintenance dose usually suffices.
High-dose loading regimens (for example 50,000 IU once weekly) should only be used after a lab result and under medical supervision.
D3 vs. D2 — which is better?
Two main forms are used in supplements:
Vitamin D3 (cholecalciferol)
Animal-derived form (often from lanolin, or from lichen for vegan products). It raises 25-OH-D more reliably and lasts longer than D2 — studies show roughly 1.7× greater effectiveness. The default choice for supplementation.
Vitamin D2 (ergocalciferol)
Plant-derived form from yeast or fungi. Cleared from the body more quickly and less effective at raising 25-OH-D. Available by prescription in some countries; in everyday use, D2 plays only a small role.
Vitamin D3 + K2
Vitamin K2 (menaquinone) is often combined because it directs calcium into bone. Evidence is reasonable for osteoporosis prevention but K2 is not mandatory. Anyone on vitamin K antagonists (e.g. warfarin) should not start K2 without medical advice.
Vitamin D is fat-soluble. Take supplements with a meal that contains some fat — absorption improves substantially.
Food sources of vitamin D
Diet alone rarely covers the requirement — even cod liver oil, the dietary champion, only provides part of what sun or supplements deliver. Still, these foods help:
| Food | Vitamin D per 100 g |
|---|---|
| Cod liver oil | ~10,000 IU |
| Wild salmon, cooked | ~600–1,000 IU |
| Herring, mackerel | ~400–800 IU |
| Sardines, tuna | ~200–400 IU |
| Egg yolk (1) | ~40 IU |
| UV-treated mushrooms | ~400–1,200 IU (D2) |
| Fortified cow milk | ~40–50 IU |
Two to three portions of fatty fish per week provide a baseline supply. In northern winters this is rarely enough — a low-dose supplement is the simplest fix.
Upper limit and overdose
Vitamin D is fat-soluble and accumulates in body fat. Acute toxicity is rare but possible. Authorities define the Tolerable Upper Intake Level (UL) as:
Infants: 1,000 IU/day (EFSA), 1,500 IU/day (IOM)
Children up to age 10: 2,000–2,500 IU/day
Adolescents and adults: 4,000 IU/day (EFSA, IOM)
Toxicity typically appears only after very high, prolonged intakes — usually above 50,000 IU/day for weeks or sustained 25-OH-D levels > 100 ng/mL. Symptoms reflect hypercalcemia: nausea, vomiting, confusion, increased thirst, kidney stones, cardiac arrhythmia.
Take extra care with sarcoidosis, tuberculosis, primary hyperparathyroidism, severe kidney disease, or while taking thiazide diuretics or digitalis. In these situations, medical supervision is mandatory.
With long-term intake above 4,000 IU/day, plan annual checks of 25-OH-D, calcium, and kidney function.
Bottom line
Vitamin D deficiency is common, often silent, and easy to fix. If you notice several typical symptoms or belong to a risk group, ask for a 25-OH-D test. Levels below 20 ng/mL are deficient; the goal is 30–50 ng/mL.
For adults without deficiency, 800–2,000 IU of vitamin D3 per day usually suffices. If you are deficient, dose by body weight (30–50 IU/kg/day). Loading doses, very high doses, and special conditions belong in medical hands. In parallel, use the vitamin D calculator to see how much your own skin synthesis can realistically contribute.
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