Vitamin D: Deficiency, Benefits and Supplementation (Complete Guide)

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What is vitamin D and what is it used for?

Vitamin D is not quite a vitamin like any other. Fat-soluble (soluble in fats), it actually behaves like a hormone : the body transforms it into an active form that acts on many tissues. It exists in two main forms: vitamin D2 (ergocalciferol, of plant origin) and vitamin D3 (cholecalciferol, the form our skin produces and the most effective).

Its most well-known role concerns bones. Vitamin D allows the body toabsorb calcium and phosphorus from food, and to fix them on the skeleton. Without it, calcium is poorly assimilated, and bones become weakened. This is why severe deficiency causes rickets in children and osteomalacia (demineralization) in adults.

But its functions go well beyond the skeleton. Vitamin D contributes to normal muscle function, to normal functioning of the immune system, and to the maintenance of normal teeth. These effects are officially recognized by EFSA, the European Food Safety Authority, as authorized health claims. In other words, these are not marketing promises, but established functions.

The paradox of this essential vitamin? It is one of the most deficient in the population. To understand why, you must first know how to recognize the signs of a deficiency.

Symptoms of vitamin D deficiency

The major trap of vitamin D deficiency is its subtlety. Its symptoms are poorly specific: they resemble those of simple fatigue or a difficult winter period. Many people live with a deficiency without knowing it. Here are the signals that should alert you.

The crucial point to remember: these symptoms are not enough to make a diagnosis. They can all have other causes. The only way to confirm a deficiency is a blood test for 25-hydroxyvitamin D (the circulating form). The generally accepted thresholds are: deficiency below 20 ng/mL (50 nmol/L), insufficiency between 20 and 30 ng/mL, and adequate status above 30 ng/mL. If you recognize yourself in several of these signs, especially with risk factors, talk to your doctor.

Why is deficiency so common?

If vitamin D deficiency is so widespread, it's due to a simple mismatch: our main source is the sun, but our lifestyle and latitude greatly limit synthesis. Several factors combine.

Latitude and season. In mainland France, from October to April, the sun is too low on the horizon: the UVB rays necessary for vitamin D synthesis poorly penetrate the atmosphere. During this long period, even with sun exposure, the skin produces virtually no vitamin D. This is the major cause of winter deficiencies.

Indoor lifestyle. Desk work, screens, transportation: we spend most of our days sheltered from the sun. Even in summer, a person who goes out little does not synthesize enough vitamin D.

Clothing and sun protection. Covering clothes and sunscreen (which are moreover essential against skin cancer) block UVB and thus synthesis. It's a trade-off: protecting yourself from the sun reduces vitamin D production.

Skin color. Melanin, which colors the skin, filters UV rays. People with dark or olive skin therefore synthesize less vitamin D at equal exposure, and are more at risk of deficiency at our latitudes.

Age. With age, the skin loses its ability to produce vitamin D, sometimes significantly. Elderly people, who also often go out less, are one of the main at-risk groups, with major bone and muscle issues at stake.

To these factors are added certain situations: overweight (vitamin D is trapped in fat tissue), intestinal absorption disorders (celiac disease, Crohn's), and pregnancy. Overall, vitamin D deficiency affects a very large part of the French population, particularly during the cold season.

The proven benefits of vitamin D

Unlike many supplements with vague promises, vitamin D benefits from officially recognized functions. EFSA has validated several health claims, meaning these effects are based on solid scientific consensus. Here are the main ones.

Strong bones and teeth

This is its historical and best-established role. Vitamin D enables theabsorption of calcium and phosphorus and their fixation on the skeleton. It thus contributes to the maintenance of normal bone structure and dentition. Prolonged deficiency weakens bones: rickets in children, osteomalacia and aggravation of osteoporosis in adults. For older people, maintaining good vitamin D status is an essential bone prevention issue.

Normal muscle function

Vitamin D contributes to normal muscle function. A deficiency can result in muscle weakness, particularly problematic in seniors, where it contributes to fall risk. Maintaining adequate status supports strength and balance.

Calcium absorption

Vitamin D improves the absorption and utilization of calcium and phosphorus. This is the keystone of its bone action: without sufficient vitamin D, even adequate calcium intake is poorly utilized by the body.

The immune system

Vitamin D contributes to the normal functioning of the immune system. This is a very active research area, which we return to in detail in the next section, as the clinical data deserve close attention.

Let's be clear about what science supports and what it doesn't. The benefits above are firmly established. Other areas (mood, prevention of certain chronic diseases) are being studied but remain less certain: vitamin D is not a cure-all, and supplementing is not a substitute for a healthy lifestyle. The objective is to correct a deficiency, not to treat just anything and everything.

Vitamin D and immunity: what the studies say

The link between vitamin D and immunity is one of the most studied topics in recent years. And the data are encouraging, provided you read them correctly.

According to PubMed, a landmark meta-analysis by Martineau et al. 2017, published in the BMJ, pooled individual data from 25 randomized trials, involving more than 11,000 participants. Its conclusion: vitamin D supplementation reduces the risk of acute respiratory infections (adjusted odds ratio 0.88), a modest but real effect at the population level (DOI: 10.1136/bmj.i6583).

The most interesting point is the following: the benefit was much greater in people with the most severe deficiency. Among those whose baseline level was very low (below 25 nmol/L), the risk reduction reached an odds ratio of 0.30, a considerable effect. Conversely, the effect was weaker in people with already adequate status. Another finding: supplementation daily or weekly was effective, unlike high single doses (bolus).

What should we take away, concretely? Vitamin D is not a magic shield against infections, but correcting a deficiency genuinely supports immune defenses. That's the whole point of maintaining good status during the cold season, precisely when sunlight is no longer enough and respiratory infections are circulating. This is where combining it with a natural source of vitamin C such as acerola makes perfect sense for supporting immunity.

Sources: sunlight and diet

To understand how to maintain good status, you need to know the three sources of vitamin D: sunlight, diet, and supplementation. The first two, first.

Sunlight, the primary source

Most of our vitamin D comes from sunlight. Under the action of UVB rays, the skin synthesizes vitamin D3. In theory, a few minutes of exposure of the arms and face in summer is enough. But as we've seen, this source dries up from autumn to spring in France, and depends on age, skin color and lifestyle. Sunlight is essential, but seasonal and uneven.

Diet, a limited contribution

This is one of the peculiarities of vitamin D: food provides little of it. Few foods contain it in significant quantities. The best sources are:

• Fatty fish: salmon, mackerel, herring, sardine, trout. These are by far the best dietary sources.
• Cod liver oil: very rich, the traditional remedy par excellence.
• Egg yolk and liver.
• Fortified products: certain milks, yogurts and margarines with added vitamin D.
• Mushrooms exposed to UV, the primary source for vegan diets.

The problem: even with careful eating, it is difficult to meet requirements through diet alone, especially if you consume little fish. This is why, when sunlight is lacking and diet is insufficient, supplementation becomes the most reliable solution. This is the subject of the next section.

Should you supplement? Dosage and form

Given all of the above (insufficient sunlight in winter, diet low in vitamin D, deficiency very common), supplementation is often the simplest and most reliable way to maintain good status. But it must be reasoned.

Who should consider supplementation?

Supplementation is particularly relevant for: people living in France from October to April, older adults, those with dark skin, people with limited sun exposure, who are overweight, pregnant women and infants (on medical advice), and people with absorption disorders. In case of doubt, a blood test can objectively determine the need.

How to take it properly

Here are general guidelines. They do not replace medical advice, especially for high doses or specific profiles.

The liquid form in drops has a practical advantage: it allows you to easily adjust the dose and suits the whole family. Our 100% plant-based vitamin D3 liquid follows this logic, with vitamin D3 of plant origin, which is rarer on the market (D3 is most often of animal origin).

Vitamin D2 or D3: which one to choose?

When choosing a supplement, one question keeps coming up: D2 or D3 ? The distinction is real and worth examining.

Vitamin D2 (ergocalciferol) is of plant origin, produced notably by mushrooms. Vitamin D3 (cholecalciferol) is the form that our skin synthesizes and that is found in animal sources. Most studies and authorities agree on one point: D3 is more effective than D2 for increasing and maintaining vitamin D blood levels. It is therefore the form to favor in most cases.

For a long time, D3 posed a problem for vegetarians and vegans, as it was extracted from lanolin (sheep's wool fat) or fish. Good news: there are now plant-based vitamin D3options, extracted from lichen, which combine the effectiveness of D3 with a 100% plant-based origin. This is the choice we made for our range, so that everyone can benefit from it.

Precautions and overdosage

Vitamin D is safe at usual doses, but it requires minimum precautions, precisely because it accumulates (unlike water-soluble vitamins eliminated in urine).

Vitamin D is part of an overall balance. To support your daily vitality, it is part of a broader approach that includes diet, physical activity, and if needed, other supplements. Discover our vitamins and minerals and our selection immunity.

Vitamin D: needs by age and profile

Vitamin D is not managed the same way at all ages. Certain profiles deserve particular attention.

Infants and children

In infants, vitamin D is crucial for bone growth: severe deficiency causes rickets, with bones that become deformed. This is why systematic supplementation is recommended from birth and during the first years, according to guidelines set by the pediatrician. Children and adolescents in full growth also have high needs. These supplements must always follow prescribed doses, never through self-medication.

Pregnant and nursing women

During pregnancy, good vitamin D status is important for the mother and the baby's bone development. Supplementation is frequently recommended, particularly in the third trimester, but always at doses indicated by the doctor or midwife.

Older adults

Older adults face multiple risk factors: skin that produces several times less vitamin D, often reduced sun exposure, and major bone and muscle concerns. Maintaining good status helps preserve bone strength and muscle force, two key factors in preventing falls and fractures. For this population, medical-supervised supplementation is often recommended year-round.

How long to raise your levels?

Vitamin D does not work in a day. After beginning appropriate supplementation, it generally takes several weeks to a few months to raise low blood levels to a satisfactory value. This is why it is advisable to plan ahead before winter and maintain regular intake. In case of confirmed deficiency, the doctor may prescribe a correction regimen followed by maintenance.

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