Finally, I have the opportunity to write an article on this vitamin, which is by far my favorite, my darling, my favorite. In short, you understand, I looooove vitamin D. 

Vitamin D is a vitamin that we know how to make, with the sun, and which is involved in hundreds of internal reactions. Without it, everything seizes up: morale, sleep, intestinal functioning and even immunity. 

He is a discreet and essential conductor so that our organizations do not play a false note. 

Let's find out together who it is in detail, why we lack it and what to do in the event of deficiencies. 

Part 1: What is vitamin D?

1.1 | Structure

Vitamin D is one of the fat-soluble vitamins, meaning it dissolves easily in fats. It itself has a composition close to fats since it is derived from cholesterol.

It exists in our bodies in several forms:

• Vitamin D3 or cholecalciferol which is its inactive or storage form.

• Calcifediol which is the circulating form measured in the blood

• Calcitriol which is the form activated in the kidney.

Note that there is vitamin D2 or ergocalciferol, synthesized by plants, which can be transformed into D3.

Note : there is vitamin D2 or ergocalciferol, synthesized by plants, which can be transformed into D3.

1.2 | Synthesis and storage

Its synthesis is made in the skin from cholesterol under the action of UVB from the sun, dehydrocholesterol is transformed into vitamin D3. This will then be transformed into calcifediol in the liver then into calcitriol in the kidney.

Skin synthesis represents 90% of our vitamin D levels.

If we are not deficient, vitamin D is stored in our muscles and in our liver.

In deficiency, the circulating form is distributed throughout the body and used immediately.

There are vitamin D receptors on almost all of our organs, which is why vitamin D behaves almost like a hormone.

1.3 | Vitamin D requirements and intakes

The circulating level can be measured by a simple blood test , the serum level of calcifediol is obtained.

This level should be included in healthy adults above 50 ng/l and in people over 65 years of age or at risk above 75 ng/l.

Concerning the recommended daily intakes:

• in infants: 400 to 800 IU/day to consult your pediatrician

• in children and adolescents: 400 to 800 IU/day

• in pregnant and breastfeeding women: 800 IU/day

• in people with black skin and people who do not expose themselves to the sun: 800 to 1600/day to assess with your health professional


These needs are daily and we will see that our contributions are often insufficient to meet them. Vitamin D deficiency then appears.

To put it simply: This organization requires a lot of equipment to function well, good nutrients, this is where the plate becomes an essential tool to fight against inflammation with so-called “anti-inflammatory” foods.

Part 2: What is vitamin D used for?

This little vitamin D plays many roles and contributes to our overall health.

radiologie d'une main qui a le pouce levé

2.1 | Bones-teeth

Vitamin D is well known for its action on bones since it regulates calcium and phosphorus levels at this level and ensures the strength of our bones.

The same goes for our teeth where its action is also essential for good oral health.

2.2 | Intestines – Immunity

I often say that vitamin D is the vitamin of our intestines and therefore of our immunity. I have already told you about it in this article.

Vitamin D helps guarantee the tightness of the intestinal mucosa and ensures the barrier role between the self and the non-self, essential for good immune functioning.

60% of our immune cells are found in our intestines . Immunity is therefore closely linked to good intestinal functioning and therefore a good level of vitamin D. Everything fits together in the human body and each parameter influences another.

visage d'une femme de profil qui prend le soleil

2.3 | Morale and fatigue

Vitamin D plays an important role in the regulation of our neurotransmitters: dopamine and serotonin which are the ups and downs of our morale (to simplify a lot).

As a result, vitamin D contributes to good sleep , because melatonin, our sleep hormone, is made from serotonin.

une femme en jaune avec des tranches de citron sur les yeux

Signs of vitamin D deficiency

When our vitamin D synthesis is insufficient and our needs are not met, we become vitamin D deficient.

Initially almost “silent”, this vitamin D deficiency will gradually manifest itself:

Poor immunity: repeated infectious episodes: colds, viruses, etc.

• Chronic fatigue

• Low morale

• Sleeping troubles

• Cramps

• Fractures

• Joint pain


If this deficiency persists over time, particularly in young children, rickets or osteomalacia can occur in adults. Without vitamin

D, bone regulation no longer occurs and disorders appear, causing disabling bone fragility.

un étalage de marché avec des piments rouges

Categories of people more exposed to a deficiency

Some people are more exposed to vitamin D deficiency. This is the case:

• menopausal women

• people with dark, mixed or black skin

• pregnant women

• infants and babies

• patients suffering from autoimmune diseases

• patients on long-term corticosteroid therapy

• people after 75 years

2.4 |Why these vitamin D deficiencies?

Several factors will explain this vitamin D deficiency which is widely observed in the general population:

Insufficient synthesis

We consider that at our latitudes in France, the synthesis is insufficient from October to April. The too little winter sunshine and the fact that we are very covered during this period make the synthesis of vitamin D insufficient.

Food intake

Food intake only represents 25% of our needs . Vitamin D is found in offal, fatty fish, oysters, egg yolk, etc.

Our ancients consumed cod liver oil in winter, very rich in vitamin D.

Insufficient exposure

In the fall and winter, we're not outside enough. In spring and autumn, we often wear sunscreen or avoid sunburn. The sun has been much criticized with the increase in skin cancers.

Vitamin D synthesis co-factors

For vitamin D to be well assimilated and absorbed, it requires co-factors which we also lack, such as magnesium which intervenes in the activation stage of vitamin D.

Vitamin K2 and calcium also play a role.

2.4 | What to do in case of vitamin D deficiency?

Several factors will explain this vitamin D deficiency which is widely observed in the general population:

How do you know if you are deficient in vitamin D?

If you have any doubts about your vitamin D level, it is possible to have it measured by a simple blood test. There is no need for a prescription and this analysis is not reimbursed. You can ask your testing laboratory to measure your blood vitamin D level.

You will know precisely if you are deficient with the help of a health professional: biologist, pharmacist, doctor, nurse, etc.

For babies, children and adolescents, you must be accompanied by a health professional. There have been cases of vitamin D overdose, linked to misuse of vitamin D food supplements.

What to do in case of vitamin D deficiency?

If the deficiency is proven , contact your doctor who will be able to prescribe vitamin D ampoules to raise your levels.

For babies, children and teenagers, the same thing, your doctor will tell you what to do.

No supplementation without medical advice for children.

For the people at risk mentioned above, it is with medical monitoring that vitamin D will need to be addressed.

For healthy adults , you can supplement with a physiological intake of 15 μgrams or 600 IU (International Units) per day from October to April. Beware of the proliferation of food supplements which accumulate doses of vitamin D.

In the summer, sunbathe for 5 to 10 minutes when the sun is mild (early morning for example), without sunscreen, with bare arms and legs if possible, avoiding any reddening of the skin.

We remind you that high doses of UV rays from the sun are harmful to our skin.

So, you will have understood, vitamin D is the small, ultra-powerful vitamin that we almost all lack, especially in winter. Do not hesitate to talk about it with your healthcare professionals, they will be able to help you.

Auteur

Dr. Estel BARRELLON

Docteur en Pharmacie
Naturopathe