Vitamin D: why (almost) everyone is short on it
Vitamin D deficiency affects a large share of the population, especially in winter. Roles, symptoms, sources, sun and supplementation: the complete guide. Evidence-based.
Key takeaways
- Vitamin D is made by the skin in sunlight; food provides only a small share.
- In winter, at our latitudes, synthesis is near zero: deficiency becomes very common.
- It's essential for bones, immunity and muscles; its deficiency is often silent.
- Winter supplementation is often warranted, with medical advice and the right dose.
It's the most paradoxical deficiency: vitamin D comes almost not at all from food, but mainly from our skin under sunlight. As a result, as soon as days get shorter, a large part of the population slips into insufficiency without knowing it. Holick (NEJM 2007) already called the deficit “pandemic,” and European data (Cashman et al., 2016) confirm its scale. Here's what you need to know.
What vitamin D does
Its best-known role is for bones: vitamin D enables calcium and phosphorus absorption in the gut, and thus skeleton mineralisation. Prolonged deficiency weakens bones (rickets in children, osteomalacia/osteoporosis in adults). But its functions go further: it supports the immune system (supplementation modestly reduces the risk of respiratory infections, Martineau et al., BMJ 2017), contributes to muscle function and has receptors in most of our tissues.
Why deficiency is so widespread
Most of our vitamin D comes from skin synthesis: the sun's UVB rays turn a precursor into vitamin D in the skin. But this synthesis depends on season, latitude, time of day, clothing cover, age and skin colour. In France, from October to March, the sun is too low to produce effective UVB: synthesis is near zero. Add indoor work, sunscreen and darker skin (which synthesises more slowly), and winter insufficiency becomes the norm rather than the exception.
An often silent deficiency
Unlike iron, vitamin D insufficiency doesn't always give a clear sign. Fatigue, diffuse aches, recurrent infections or low mood can nonetheless alert you, especially in late winter.
Food sources (limited)
Very few foods contain vitamin D in useful amounts. The best sources are fatty fish (salmon, mackerel, sardine, herring), cod liver oil, egg yolk and some fortified products. Eating fatty fish 2 to 3 times a week helps, but in practice it's hard to cover your needs through food alone, especially in winter.
| Food | Why it helps |
|---|---|
| Cod liver oil | The most concentrated source |
| Salmon, mackerel, herring, sardine | Rich + omega-3 |
| Egg yolk | Regular top-up |
| Fortified dairy | Useful if consumed often |
| UV-exposed mushrooms | Plant option (vitamin D2) |
Should you supplement?
Given the small dietary share and the lack of effective sun in winter, supplementation is often recommended from October to March, particularly for at-risk people (darker skin, little exposure, the elderly, infants). Dose and form (daily or quarterly doses) should ideally be tailored to a blood level and medical advice. Vitamin D being fat-soluble, it's stored: at very high prolonged doses, excess is possible, hence the value of monitoring rather than self-megadosing.
The right winter habit
Fatty fish 2–3×/week, some sun exposure as soon as possible in spring, and tailored winter supplementation. A blood test in late winter takes stock.
Tracking your vitamin D (and the omega-3 that comes with it in fish) helps spot weeks that are too poor and correct course before insufficiency. That's the whole point of daily micronutrient tracking.
Frequently asked questions
How much sun do you need to make vitamin D?
In summer, ten to twenty minutes of exposure of arms and face several times a week is often enough. In winter at our latitudes, the sun allows almost no synthesis, whatever the duration.
Vitamin D2 or D3?
D3 (animal-derived, or lichen-derived for vegans) is generally more effective at raising and maintaining blood levels than D2. Most quality supplements are D3.
Can you overdose on vitamin D?
At usual doses, no. But since it's stored in fat, prolonged megadoses can cause calcium excess. Hence the value of a tailored dose rather than blind self-supplementation.
What blood level should you aim for?
Thresholds vary between expert bodies, but a 25-OH-vitamin D level above ~50 nmol/L (20 ng/mL) is generally considered sufficient. Your doctor interprets the result based on your profile.
Scientific sources
- 1.Holick MF. Vitamin D Deficiency. N Engl J Med. 2007.
- 2.Cashman KD, et al. Vitamin D deficiency in Europe: pandemic? Am J Clin Nutr. 2016.
- 3.Martineau AR, et al. Vitamin D supplementation to prevent acute respiratory infections. BMJ. 2017.
- 4.ANSES. Références nutritionnelles en vitamines et minéraux (vitamine D).
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