Vitamin D

What is the purpose of vitamin D in the body?

Vitamin D isn't really a vitamin; it's a prohormone. Ingested or synthesized by the skin, it becomes 25-hydroxyvitamin D in the liver—the storage form measured in the blood—and then calcitriol in the kidneys, which is the active form. Calcitriol acts on receptors throughout the body and, notably, controls intestinal calcium absorption. Seven health claims are authorized at the European level, with regulated wording:

• "Vitamin D contributes to the normal absorption and utilization 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 plays a role in the process of cell division."

These claims can only be used if the product is at least a "source of vitamin D," meaning 15% of the nutrient reference value: since the NRV is 5 µg, this means 0.75 µg per serving. Note what they do not say: they speak of maintenance and normal function, never of treating illnesses.

Why isn't the sun enough from October to March?

The skin produces vitamin D3 from a cholesterol precursor, 7-dehydrocholesterol, under the effect of UVB rays—the fraction of solar radiation between 290 and 315 nanometers, which is most easily blocked by the atmosphere. This is where latitude comes in. Metropolitan France extends from approximately 41° to 51° north: in winter, the sun never rises high above the horizon there, its rays pass through a much greater thickness of atmosphere than in summer, and the ozone layer absorbs most of the UVB before it reaches the ground. From October to March, cutaneous synthesis thus becomes marginal in Lille as in Lyon, a little less early and for a shorter time in Montpellier. It's not a matter of temperature: a bright and freezing winter day doesn't produce more vitamin D than a gray day.

Three additional factors play a role all year round: glass blocks UVB, melanin filters radiation in darker skin, and synthesis capacity decreases with age.

ANSES recommends exposing arms and face for 15 to 20 minutes in the late morning or afternoon during the warmer months, without negating skin cancer prevention guidelines. In terms of diet, intake remains low—fatty fish, cod liver oil, egg yolk, fortified dairy products—and ANSES estimates the average intake for adults in France at around 3.1 µg per day. It is this deficiency that raises the question of winter supplementation.

Vitamin D2 or D3, and where does D3 come from?

Vitamin D2 (ergocalciferol) comes from yeasts or fungi exposed to UV. Vitamin D3 (cholecalciferol) is the form the body produces itself: at the same dose, it raises and maintains blood levels of 25-hydroxyvitamin D more effectively than D2, which has made it the standard in supplements.

Its origin remains. Classic D3 is extracted from lanolin, the wool grease from sheep shorn: the animal is not slaughtered, but the product is not vegan for all that. The alternative is D3 from lichen (Cladonia rangiferina), chemically identical but of plant origin; relevant labels specify this, often with the mention "vegan." In its absence, it is almost always lanolin. As the extraction yield from lichen is lower, these versions cost more.

What are the official intake benchmarks in France?

The nutritional reference for the population established by ANSES is 15 µg per day for adults, or 600 IU. The conversion to remember when reading a label: 1 µg = 40 IU. A dosage of 1,000 IU is therefore 25 µg, 2,000 IU is 50 µg.

Regarding the upper limit, the safe upper limit adopted at the European level is 100 µg per day (4,000 IU) for adults, from all sources combined: including food, supplements, and medications. It is lower for children, and much lower for infants. This is not a target but a threshold not to be crossed without medical supervision.

Two additional claims come with specific conditions. For seniors: "Vitamin D helps reduce the risk of falling associated with postural instability and muscle weakness. Falling is a risk factor for bone fractures in men and women aged 60 and over" — starting from 20 µg per day, for this age group only. For children: "Vitamin D is necessary for normal bone growth and development in children." This last one does not concern us: ANSES recommends using prescribed medications for infants and young children. This section is for adults.

Why is vitamin D combined with K2 and magnesium?

Vitamin K2 (menaquinone, most often MK-7) activates vitamin K-dependent proteins, including osteocalcin, involved in bone mineralization. The accepted claim is "Vitamin K contributes to the maintenance of normal bones," usable from 11.25 µg per serving. The widespread idea that K2 "directs" calcium towards bones rather than arteries remains a research hypothesis. One safety point, however, is established: vitamin K interferes with vitamin K antagonist anticoagulants. Anyone concerned should avoid D3+K2 formulas and discuss it with their doctor.

Magnesium acts as a cofactor for the enzymes that hydroxylate vitamin D in the liver and kidneys—this is biochemistry, not a promise of effect. The authorized claims for it are "Magnesium contributes to the maintenance of normal bones" and "Magnesium contributes to normal muscle function," from 56.25 mg per serving. As intake is often just right in France, the magnesium section has its place here, as do ZMA formulas that combine it with zinc. Nothing is mandatory: D3 alone remains consistent. If you combine, do the math—many multivitamins already contain it.

Oily drops or tablets: which to choose?

Vitamin D is fat-soluble: regardless of the form, take it with a meal containing fats. On an empty stomach or with a fat-free breakfast, absorption is less effective.

Oily drops and sprays deliver the vitamin already dissolved in a fatty substance—olive oil, coconut oil, medium-chain triglycerides. This is the most flexible form for dosing and the simplest for those who have difficulty swallowing capsules; the bottle should be stored away from light. Dry tablets and capsules are easier to transport and offer perfectly reproducible dosing, with good absorption if taken with a proper meal. Daily intake is easier to maintain than a large weekly dose. Highly dosed ampoules sold in pharmacies are prescription medicines: they do not fall under this category.

What risks should be known before supplementing?

The risk of overdose is real, precisely because vitamin D is fat-soluble. Unlike vitamin C or B vitamins, whose excess is excreted in the urine, it accumulates in fatty tissues and the liver. Prolonged excessive intake leads to hypercalcemia, with a risk of calcification of kidney and cardiovascular tissues. The rule: respect the dose on the label, do not stack sources, and never exceed 4,000 IU per day on your own initiative.

Medical advice is essential before any supplementation in case of pregnancy or breastfeeding; for infants, children, and adolescents; in case of kidney failure, kidney stones, known hypercalcemia, sarcoidosis, or another granulomatous disease; when taking vitamin K antagonist anticoagulants, especially with a formula containing K2; when taking thiazide diuretics, corticosteroids, or digitalis; and for individuals being treated for a bone pathology.

Finally, the most important point on this page: anything related to deficiency is a matter for a doctor. Deciding on a blood test, interpreting it, choosing a corrective dose, and monitoring it are medical acts. A supplement is not a diagnostic tool.

Is vitamin D of interest to athletes?

What we know: vitamin D status is, on average, lower in athletes who train indoors—strength training, gymnastics, combat sports, indoor swimming—and it decreases in almost everyone in winter. The claims "normal muscle function" and "normal bones" are not insignificant in high-impact activities.

What we don't know: there is no evidence to suggest that additional intake improves strength, endurance, or recovery in an athlete whose status is already adequate. Studies on performance are heterogeneous, and the effect, when it appears, concerns individuals who started from very low levels. Vitamin D is part of the nutritional foundation, alongside omega-3s, not a performance arsenal. It does not replace glucosamine or collagen, and for muscle mass, training and proteins do the work—our sports nutrition guide positions each product.

Frequently Asked Questions

What vitamin does the sun provide?

Vitamin D, and only vitamin D. Under the effect of UVB rays, the skin converts a cholesterol precursor into vitamin D3—exactly the same molecule as in supplements. This is the only vitamin the body produces this way, which explains its special status: neither quite a vitamin, nor quite a hormone. Three useful clarifications. It's UVB that counts, not heat or brightness: a bright winter day produces almost none. Glass blocks them: behind a window or windshield, synthesis is nil. And in our latitudes, it becomes marginal from October to March, no matter the weather.

What is the role of vitamin D?

Its central role is to control the intestinal absorption of calcium and phosphorus, and thus the mineralization of bones and teeth. However, its receptors are present in almost all tissues, which explains the seven authorized European claims: normal absorption of calcium and phosphorus, normal blood calcium levels, maintenance of normal bones, teeth, and muscle function, normal immune system function, and a role in cell division. Technically, it is not a vitamin but a prohormone: transformed in the liver and then in the kidneys, it then acts as a hormone throughout the body.

What is the daily dose of vitamin D for adults?

The ANSES nutritional reference for adults is 15 µg per day, or 600 IU. Remember the conversion, it allows you to read any label: 1 µg is 40 IU, so 1,000 IU is 25 µg and 2,000 IU is 50 µg. The European upper safety limit is set at 100 µg per day, or 4,000 IU, from all sources combined—including food, supplements, and medications. This is not a target, but a ceiling not to be exceeded without supervision. A dose intended to correct a low level should be decided with a doctor and not from a product sheet.

What time of day should vitamin D be taken?

The time of day matters little, the meal matters a lot. Vitamin D is fat-soluble: its absorption depends on the lipids present in the stomach at the same time. Take it with lunch or dinner, as long as the meal contains fats; on an empty stomach or with a fat-free breakfast, part of the dose is lost. Morning or evening makes no proven difference, contrary to what is sometimes read. A regular daily intake is easier to maintain over time than a large weekly dose, and it is regularity that counts over several months.

How to get vitamin D?

Through three channels, only one of which is truly effective in winter. First, the sun: exposing arms and face for 15 to 20 minutes in the late morning or afternoon during the warmer months is enough to cover a good part of the needs—but synthesis becomes marginal from October to March in our latitudes, and sun protection guidelines remain valid. Next, diet: fatty fish twice a week, egg yolk, liver, fortified products. This is the most neglected channel, but also the most limited—ANSES estimates the average dietary intake for adults in France at about 3.1 µg per day, far from the reference 15 µg. Finally, supplementation, which makes sense during the autumn-winter period. None of these three levers precludes medical advice if you suspect a real deficiency.

What foods contain vitamin D?

A short list, and that's the problem. At the top, cod liver oil, far above everything else. Then come fatty fish—herring, mackerel, sardine, salmon, anchovies—then liver, egg yolk, some cheeses, and fortified dairy products. Mushrooms exposed to ultraviolet light provide vitamin D2, which is less effective than D3 in raising blood levels. That's all: no other food family contains it in useful quantities. Two servings of fatty fish per week are the most effective dietary lever. Exact contents can be checked on the ANSES Ciqual table.

What fruit or vegetable contains vitamin D?

None, and that's the honest answer to a frequently asked question. Neither fruits nor vegetables provide vitamin D in useful quantities: the molecule is of animal or fungal origin. The only plant exception: mushrooms exposed to ultraviolet light, which produce vitamin D2. A vegan diet therefore particularly exposes one to deficiency, and supplementation becomes a full-fledged topic—with lichen-derived D3 if you exclude lanolin.

What is the best vitamin D?

D3 (cholecalciferol) rather than D2: at the same dose, it raises and maintains blood levels better. Beyond this choice, three label lines differentiate two products. The dose displayed in micrograms AND IU, without which comparison is impossible. The origin of D3: lanolin in the vast majority of cases, lichen for plant-based versions—chemically identical, more expensive. Finally, the galenic form: oily drops allow for fine dose adjustment, while capsules are easier to transport. A very high-dose product is not "better": beyond 4,000 IU per day, it falls outside the scope of self-medication.

How to fix vitamin D, and can it be combined with magnesium?

Two concrete levers. The first is a fatty meal: without lipids, absorption drops, regardless of product quality. The second is magnesium, a cofactor for enzymes that transform vitamin D in the liver and then in the kidneys. Taking them together poses no problem, and there's no need to space them out. Beyond that, no problematic interaction is documented at usual doses with common micronutrients; the only precaution is accounting—add up the vitamin D intake from all your products. Our iron and vitamin E sections complete the range.

What are the signs of vitamin D deficiency?

This is the most frequently asked question on the subject, and one we will not answer with a list. The signs mentioned everywhere—fatigue, widespread pain, low mood, muscle weakness—are too common and shared with other conditions to allow anyone to conclude. Only a doctor, with a 25-hydroxyvitamin D blood test if deemed useful, can answer. Interpreting the result, choosing a corrective dose, and monitoring it are medical acts. A food supplement ensures adequate intake for a healthy person; it diagnoses nothing and never replaces a prescription.

Does a lack of vitamin D cause weight gain?

The link exists in the data, but not in the way one might imagine. Vitamin D is stored in adipose tissue: for the same intake, blood levels are on average lower in overweight individuals because some of the vitamin is sequestered in fat. Thus, it is weight that influences the level, and not the other way around. No data shows that supplementation causes weight loss, and no European claim supports this. This does not, however, authorize increasing the dose on one's own initiative: the question should be posed to a doctor.

Can you overdose on vitamin D?

Yes, and this is the real difference from water-soluble vitamins. Vitamin C and B vitamins are excreted in the urine if there's an excess; vitamin D, being fat-soluble, accumulates in the liver and adipose tissues. Prolonged excessive intake leads to hypercalcemia, with a risk of renal and cardiovascular calcification. Reported cases almost always involve very high doses taken for a long time, or multiple products combined without realizing it. Respect the dose on the label, add up your sources, and do not exceed 4,000 IU per day on your own initiative.

Is K2 mandatory with vitamin D?

No. D3 alone covers bone, muscle, and immune claims. K2 is a complementary approach, to be avoided with vitamin K antagonist anticoagulants without medical advice.

Should it be taken all year round?

The common reasoning is to cover the period when skin synthesis is low or non-existent, from October to March. Continuous intake is discussed depending on lifestyle, and this decision, as well as the dosage, should be made with a healthcare professional.

Where to buy Vitamin D?

In pharmacies and parapharmacies, in specialized stores, and online — the regulatory framework is the same everywhere for a food supplement; only the formula changes. One distinction is worth noting: highly dosed ampoules dispensed in pharmacies are medicines, prescribed and monitored by a doctor; they do not fall under the same regime as drops and capsules sold freely. For the latter, compare three things: the dose in µg and IU, the origin of cholecalciferol, and the price per daily dose — a bottle of drops often lasts several months. Our products are available online, shipped everywhere in France and Europe, and in-store in Vendargues, about ten minutes from Montpellier.

What brands can be found in this section?

We select formulas with clear labels: dose in micrograms and in IU, specified origin of D3. Nutriting offers A-D-K synergies in drops and capsules. IO.Genix covers the D3+K2 combination in a daily format. TrainedByJP offers a plant-based D3+K2 for those who avoid lanolin. Now Foods provides liquid formats, practical for adjusting the dose. The rest of our micronutrition products are in the probiotics and sports nutrition sections.

Need advice in a store near Montpellier?

Our store in Vendargues, about ten minutes from Montpellier, allows you to compare labels in hand: for vitamin D, everything can be read in three lines — dose per drop or capsule, origin of cholecalciferol, presence or absence of K2. We also ship everywhere in France and Europe. We will gladly tell you what a product contains and how to take it; we will not tell you if you are deficient, as that question belongs to your doctor.

Sources: ANSES for nutritional references, sun exposure, and warnings related to infants; Ciqual for food content; EFSA for opinions supporting authorized claims and the safety limit.

To learn more: why combine vitamins A, D, and K2.