Vitamin D is a vitamin unlike any other: our skin makes it when exposed to sunlight, and in the body it behaves more like a hormone. Yet a shortfall affects a large share of the population, especially in winter.
In brief – Vitamin D is a hormone-like nutrient made by the skin in sunlight and needed for bone strength, immune function and muscle health. Weak sunlight between November and March makes deficiency common, particularly among older adults, people with darker skin, and those who spend little time outdoors. Typical maintenance doses range from 800 to 2000 IU a day, adjusted for age, sun exposure and blood levels. Bone benefits are well established, while effects on immunity and mood are still being studied. A blood test measuring 25-OH-D helps confirm a deficiency and guide safe supplementation, ideally alongside magnesium and vitamin K2, two cofactors that help the body use it properly.
What does vitamin D do?
Vitamin D is best known for its role in bone health: it enables the absorption of calcium and phosphorus. But its functions go far beyond the skeleton. Receptors for this vitamin exist in nearly every tissue in the body, which explains its involvement in immunity, muscle function, mood and the regulation of many genes.
Vitamin D deficiency: a very common shortfall
At our latitudes, sunlight is too weak from November to March to allow sufficient synthesis. Deficiency is therefore common, particularly among older adults, people with darker skin, or those who spend little time outdoors. A few signs can be a warning:
- Persistent fatigue and low energy;
- Bone or muscle pain, weakness;
- Recurrent infections and weakened immunity;
- Low mood, especially in winter;
- Slow wound healing and hair loss in marked cases.
What the science really says
Health authorities recognize the importance of adequate levels: the National Institutes of Health (NIH) note that it is essential for bone strength and that deficiency remains common worldwide.
Research also explores its other effects: see the studies on deficiency and long-term health and on supplementation and immunity. While the bone benefits are well established, other avenues, such as immunity and mood, still need confirmation.
In practice: meeting your needs
Monitoring your level is part of a sound prevention strategy: it is one of the blood biomarkers worth checking, alongside our 8 essential preventive health habits. Find all our guides in the Preventive Health pillar.
Protocol: sources and dosage
- Sunlight: 15 to 20 minutes of exposing the arms and face, several times a week in summer.
- Fatty fish (salmon, mackerel, sardines), the main dietary source.
- Eggs, liver and fortified foods as a complement.
- Winter supplementation, often useful at our latitudes, on medical advice.
- A blood test when in doubt, to adjust intake.
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Should you supplement?
For many people, supplementing during winter is a simple, safe measure recommended by many physicians. Typical doses range from 800 to 2000 IU a day depending on individual profile. Still, it is best not to self-supplement blindly at very high doses: excess, though rare, can lead to calcium overload. A blood test and medical advice allow the dose to be fine-tuned.
Sunlight: the primary source
No source rivals sunlight: a few minutes of exposure are enough to produce large amounts of it. Exposure should still stay moderate to protect the skin and avoid UV-related risks. The goal is not to tan, but to briefly and regularly expose a sufficient area of skin without burning. In winter, or for people who spend little time outdoors, diet and supplementation take over.
D2 or D3: which form of vitamin D to choose
Two forms of supplementation exist. D3 (cholecalciferol), of animal origin or derived from lichen in plant-based versions, is identical to the form our skin makes in sunlight. D2 (ergocalciferol), of plant origin, is somewhat less effective at raising and maintaining blood levels. At an equivalent dose, D3 remains the preferred form in most cases.
It is available in drops, ampoules or capsules. Daily or weekly intake at a moderate dose is generally preferred over very high, spaced-out doses, which produce less stable levels. The exact regimen is best chosen with a physician, based on starting levels.
Magnesium and vitamin K2: essential cofactors
It does not act alone. Magnesium is required to activate it in the body: a magnesium deficiency, which is common, can limit the benefits of supplementation. Vitamin K2, for its part, helps direct calcium toward the bones rather than the arteries, which is particularly relevant during long-term supplementation. A diet rich in leafy greens, nuts, legumes and fermented foods covers much of these needs. Paying attention to these cofactors helps maximize the benefits, rather than taking it in isolation.
Who is most at risk of deficiency
Some people are clearly more exposed to a shortfall:
- Older adults, whose skin synthesizes the vitamin less efficiently;
- People with darker skin, which filters more UV;
- People who spend little time in the sun (indoor work, covering clothing, hospitalization);
- Residents of northern regions, especially in winter;
- People with overweight, in whom the vitamin is more readily sequestered in fat tissue.
For these groups, screening and winter supplementation are often warranted, alongside the other blood biomarkers worth monitoring.
How to read your blood test result
The blood test measures 25-hydroxy-vitamin D, or 25-OH-D, expressed in nanograms per milliliter (ng/mL) or nanomoles per liter (nmol/L). Deficiency is generally defined below 20 ng/mL and insufficiency between 20 and 30 ng/mL, with the optimal range often set around 30 to 50 ng/mL depending on the guideline used. Beyond that, going higher has no demonstrated benefit, and very high levels are not desirable. This test is not needed for everyone as a matter of routine, but it is valuable for people at risk or with suggestive symptoms, to fine-tune supplementation.
| 25-OH-D level | ng/mL | nmol/L | Status |
|---|---|---|---|
| Deficiency | Below 20 | Below 50 | Supplementation usually advised |
| Insufficiency | 20 to 30 | 50 to 75 | Often warrants correction |
| Optimal range | 30 to 50 | 75 to 125 | Target under most guidelines |
Bone health and fall prevention
Its skeletal role is the best established. By enabling calcium absorption, it maintains bone strength and helps prevent osteoporosis. In older adults, adequate status is also associated with better muscle strength and a lower risk of falls, a major concern since fractures in later life durably affect independence. This is why supplementation is often recommended for seniors, alongside sufficient calcium intake and regular physical activity, which together protect bones and muscles.
Vitamin D and winter mood
Many people notice their mood dip in the cold season, when daylight is scarce. Vitamin D, nicknamed the “sunshine vitamin,” has been linked to mood regulation, although the cause-and-effect relationship remains debated. A deficiency is frequently observed in people with seasonal low mood. Correcting a confirmed deficiency can be part of the overall approach, without being a treatment for depression on its own. Getting daylight exposure, staying active and maintaining good levels form a useful trio for getting through winter more comfortably.
Frequently asked questions
What are the signs of a vitamin D deficiency?
Fatigue, bone or muscle pain, frequent infections and low mood can suggest a shortfall. Only a blood test can confirm a deficiency with certainty.
How much should you take per day?
Typical maintenance doses range from 800 to 2000 IU a day, adjusted for age, sun exposure and blood levels. It is best to ask a physician for guidance.
Can you get enough from diet alone?
It is difficult: diet provides only a fraction of what is needed. Sunlight remains the main source, which is why supplementation in winter is useful for many people.
Is too much vitamin D dangerous?
Overdose is rare but possible with very high, prolonged doses, leading to excess calcium. At recommended doses, it is safe. Avoid self-supplementing at very high doses.
Medical disclaimer. The information provided here is for informational purposes only and does not constitute medical advice. It does not replace a consultation. Ask a healthcare professional before changing your diet, taking dietary supplements or starting a new practice, especially if you have a medical condition, are pregnant or are under treatment. Dietary supplements do not replace a balanced diet or medical follow-up.
