Vitamin D in winter: how much, which form and do you need a test?

From October to March, vitamin D becomes the topic of the season: in France, the winter sun barely allows the skin to make any, and dietary intake remains low. Should you take it, at what dose, in what form, and do you need a blood test first? Official requirements, foods, blood testing, forms, dosing schedules and the risks of excess: here is what to know before winter.

What does vitamin D do?

Vitamin D mainly contributes to the absorption of calcium and phosphorus from food, two minerals essential for bones and teeth. It comes in two forms: D3 (cholecalciferol), made by the skin under the action of UVB and found in animal products, and D2 (ergocalciferol), produced by mushrooms and yeasts exposed to UV light. The liver converts it into 25-hydroxyvitamin D, or 25(OH)D, the form measured in the blood.

European Regulation 432/2012 authorises seven claims for vitamin D:

• “Vitamin D contributes to normal absorption/utilisation of calcium and phosphorus”;
• “Vitamin D contributes to normal blood calcium levels”;
• “Vitamin D contributes to the maintenance of normal bones”;
• “Vitamin D contributes to the maintenance of normal muscle function”;
• “Vitamin D contributes to the maintenance of normal teeth”;
• “Vitamin D contributes to the normal function of the immune system”;
• “Vitamin D has a role in the process of cell division”.

These claims may only be used for a food that is at least a “source” of vitamin D. Other nutrients share some of these functions, such as magnesium for bones, or zinc and selenium for the immune system.

Why do vitamin D levels drop in winter?

Because the winter sun, too low in the sky, no longer provides enough UVB. A landmark study exposed human skin to sunlight on clear days: in Boston (42° north), no previtamin D3 was produced from November to February; in Edmonton (52° north), this period stretched from October to March. Paris lies at about 49° north, between the two.

Other factors matter:

• age: the skin of people aged 77 and 82 produced less than half as much previtamin D3 as that of young people aged 8 and 18;
• darker skin: in a European analysis, darker-skinned subgroups had a prevalence of deficiency 3 to 71 times higher than fair-skinned populations;
• spending little time outdoors, covering clothing or routine use of sunscreen.

This analysis, which standardised the blood tests of 55,844 Europeans, found 13% of levels below 30nmol/L on average over the year: 17.7% during the October-to-March period and 8.3% in summer. With a threshold of 50nmol/L, the proportion rose to 40.4%.

How much vitamin D do you need per day?

EFSA (2016) and the French food safety agency ANSES set 15µg per day for an adult, or 600IU (1µg = 40IU). These values assume low sun exposure. In France, the INCA 3 survey estimates average dietary intake in adults at 3.1µg per day: according to ANSES, most people in France do not get enough.

Which foods contain vitamin D?

Few foods contain much. ANSES lists cod liver oil and cod liver, fish roe, oily fish (herring, sardine, salmon, anchovy, mackerel), egg yolk, certain mushrooms and fortified products. The amounts below come from the reference fact sheet of the US National Institutes of Health (NIH).

A portion of oily fish once or twice a week helps, but on average it struggles to cover 15µg per day. Fruit, vegetables, cereals and pulses provide practically none, with the exception of certain mushrooms.

Is sunlight enough?

In spring and summer, often. ANSES lists among the sources of vitamin D 15 to 20 minutes of sun exposure in the late morning or afternoon, without specifying the area of skin or the skin type. In winter, at these latitudes, sun exposure no longer makes up the difference.

Should you have your vitamin D level tested?

Not routinely. In 2013, the French National Authority for Health (HAS) concluded that the usefulness of testing had not been demonstrated in the general population. In France, the test is still reimbursed in six situations: suspected rickets, suspected osteomalacia, follow-up of an adult kidney transplant recipient beyond three months after the transplant, before and after obesity surgery, older people prone to repeated falls, and when a medicine’s summary of product characteristics requires it. The Endocrine Society guidelines published in 2024 also advise against routine testing, including in people with obesity or darker skin.

When a test is done, results are expressed in nmol/L or ng/mL (1ng/mL = 2.5nmol/L). Common benchmarks:

• below 30nmol/L (12ng/mL): deficiency, the threshold used in the European analysis cited above;
• 50nmol/L (20ng/mL): the level that covers the needs of at least 97.5% of the population according to the US Institute of Medicine;
• above about 375nmol/L (150ng/mL): the range associated with toxicity according to the NIH.

Vitamin D2 or D3: which should you choose?

D3 has the edge. A meta-analysis of trials directly comparing the two forms concluded that D3 raises blood 25(OH)D levels more. The difference was clear with infrequent doses but disappeared with daily doses.

Vitamin D is fat-soluble: an oil-based form in drops or a softgel is best taken with a meal (see capsule, softgel or tablet).

Daily dose or high-dose vial: what is the difference?

In France, vitamin D is often prescribed as high-dose vials taken several weeks or several months apart. It is convenient, but several findings point towards daily or weekly doses instead:

• in a trial in 2,256 women aged 70 and over at high risk of fracture, a single annual dose of 500,000IU increased falls by 15% and fractures by 26% compared with placebo;
• in a 2017 meta-analysis (10,933 participants), the risk of acute respiratory infection was lower in those taking vitamin D daily or weekly, but not in those receiving large, infrequent doses;
• the Endocrine Society suggests, after the age of 50 outside pregnancy and when vitamin D is indicated, a daily dose rather than large intermittent doses.

The 2025 update of the meta-analysis on respiratory infections (61,589 participants) does, however, qualify the picture: the overall effect is no longer statistically significant, and the analysis no longer finds a difference according to dosing schedule.

What do the major supplementation trials show?

They have tempered many hopes. The US VITAL trial followed 25,871 men aged 50 and over and women aged 55 and over for a median of 5.3 years. 2,000IU of vitamin D3 per day reduced neither cancers (hazard ratio 0.96) nor major cardiovascular events (0.97). A further analysis showed no effect on fractures (0.98) in these participants, who had not been recruited for a deficiency.

These results concern healthy adults, often without deficiency. For the Endocrine Society, supplementation without a prior test is mainly justified in children and adolescents, people aged 75 and over, during pregnancy and in high-risk prediabetes. In healthy adults under 75, it advises against exceeding the reference intakes. The message is therefore not that vitamin D is useless, but that the aim is to meet your needs, without seeking very high doses.

Vitamin D overdose: what are the risks?

Excess is rare from food, but possible with supplements. It causes hypercalcaemia (too much calcium in the blood), with effects on the heart and kidneys. ANSES also lists headaches, nausea, vomiting, weight loss and severe fatigue. It has recorded overdoses in infants after the use of food supplements: for them, it recommends medicines prescribed by a healthcare professional.

For adults, the limit of 100µg per day includes all sources. As with vitamin B6, the risk comes mainly from combining several products: add up the vitamin D from each supplement and from fortified foods.

Frequently asked questions

Should you take vitamin D in the morning or at night?

The time of day matters less than the meal. In a small study without a control group, in 17 patients whose levels remained insufficient despite treatment, taking vitamin D with the largest meal of the day raised blood levels by about 50% in two to three months. Choose a fixed time, with a meal.

Should vitamin D be combined with vitamin K2?

It is not essential. Vitamin K has its own claims: “Vitamin K contributes to the maintenance of normal bones” and “Vitamin K contributes to normal blood clotting”. If you are taking anticoagulant treatment, ask your doctor before taking any vitamin K supplement.

Which fruits contain vitamin D?

None in useful amounts. In the NIH table, the fruits, vegetables, cereals and pulses listed (broccoli, carrot, brown rice, lentils…) show 0µg. The sources are oily fish, egg yolk, certain mushrooms and fortified products.

Does vitamin D help with tiredness?

Vitamin D has no authorised claim relating to tiredness. The vitamins and minerals that do have one are presented in our comparison which vitamin for tiredness. Persistent tiredness warrants medical advice.

Can you take vitamin D during pregnancy?

Requirements remain 15µg per day, and the 100µg limit also applies during pregnancy and breastfeeding. The Endocrine Society suggests supplementation during pregnancy; the dose and form are decided with your doctor or midwife.

Which vitamin D for a baby?

In infants, vitamin D is prescribed by the doctor. ANSES recommends using medicines rather than food supplements, after several cases of overdose linked to supplements. Do not change product or dose without medical advice.

Conclusion

Vitamin D contributes to the maintenance of normal bones and muscle function and to the normal function of the immune system, and in France winter alone is enough to bring levels down. The reference intake is modest, 15µg per day, and the safe upper limit, 100µg, leaves some margin without allowing excess. In adults who had not been selected for a deficiency, the major trials showed no benefit on cancers, cardiovascular events or fractures. For winter: oily fish regularly, daylight whenever possible and, if needed, small daily doses of D3 rather than large, infrequent doses.

Sources

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Food supplements are not a substitute for a varied and balanced diet and a healthy lifestyle. Ask a healthcare professional for advice if you are taking medication, are pregnant or are breastfeeding.