Vitamin D is probably the vitamin that has been the most talked about in scientific research for 20 years. And for good reason: its biological roles go far beyond the bone health for which it is classically known. Involved in immune regulation, muscle health, mental health, prevention of certain cancers and glucose metabolism, it acts more like a hormone than like a vitamin in the classic sense. However, according to ANSES, more than 80% of French people have a vitamin D deficiency at the end of winter. It is the most widespread nutritional deficiency in France, and yet one of the simplest to correct. Here's everything I need to know.
1. Vitamin D: a hormone more than a vitamin
The name vitamin D is actually incorrect. Vitamin D behaves biologically like a pro-hormone: it is synthesized by the body (via the skin under the effect of UV rays), stored in adipose tissue and activated in two stages by the liver and kidneys to give the active form, calcitriol (1,25-dihydroxyvitamin D3).
There are two main forms:
- ●Vitamin D2 (ergocalciferol): of plant origin, present in certain mushrooms exposed to UV. Less powerful and less stable than the D3.
- ●Vitamin D3 (cholecalciferol): of animal origin and cutaneous synthesis. It is the preferred form for supplementation, more effective in increasing blood levels of 25-OH-vitamin D.
The vitamin D receptor (VDR) is present in virtually every cell in the body - more than 200 different cell types - and regulates the expression of more than 1,000 genes. This ubiquity explains the extent of its biological effects.
| Rate of 25-OH-D | Status | Clinical implications | |
|---|---|---|---|
| Less than 25 nmol/L (10 ng/mL) | Severe deficiency | Risk of rickets, osteomalacia, severe muscle weakness. Urgent supplementation. | |
| 25 to 50 nmol/L (10-20 ng/mL) | 50 to 75 nmol/L (20-30 ng/mL) | Moderate insufficiency | Gray area. Recommended supplementation with recontrolee. |
| 75 to 125 nmol/L (30-50 ng/mL) | Optimal level | Target area for the majority of healthy adults. | |
| More than 150 nmol/L (60 ng/mL) | High Level | No additional benefit established. Monitor if intense supplementation. | |
| More than 375 nmol/L (150 ng/mL) | Possible toxicity | Hypercalcemia possible. Cannot occur with sun exposure alone. |
2. The biological roles of vitamin D: well beyond bones
Vitamin D was known for its role in calcium absorption and bone mineralization. Research over the last 20 years has profoundly enriched this picture:
- ●Bone health: vitamin D is essential for the intestinal absorption of calcium and phosphorus. Its severedore deficiency causes rickets in children and osteomalaceous disease in adults. Its chronic insufficiency contributes to osteoporosis.
- ●Immune system: vitamin D modulates the innate and adaptive immune response. It stimulates the production of antimicrobial peptides (defensins, cathelicidins) and regulates inflammation. Sufficient levels are associated with better resistance to respiratory infections.
- ●Muscle health: vitamin D is involved in muscle protein synthesis and muscle contraction. Its deficiency is associated with muscle weakness, pain and an increased risk of falls in the elderly.- Mental health: Low vitamin D levels are associated with an increased risk of seasonal and non-seasonal depression, anxiety and cognitive decline. Vitamin D is involved in the synthesis of serotonin and dopamine.
- ●Cardiovascular prevention: Vitamin D receptors are present in cardiac and vascular cells. Insufficient levels are associated with increased risk of hypertension, heart failure, and cardiovascular mortality in prospective studies.
- ●Cancer prevention: several meta-analyses suggest an inverse association between vitamin D levels and risk of certain cancers (colorectal, breast, prostate). The mechanisms involve the regulation of cell proliferation and apoptosis.
- ●Carbohydrate metabolism: the beta cells of the pancreas producing insulin express vitamin D receptors. Its insufficiency is associated with insulin resistance and an increased risk of type 2 diabetes.
Food Vitamin D / 100 g Frequency to contribute to requirements Cod liver oil (1 tbsp) ~250 µg (10,000 IU) 1 spoon per week = therapeutic dose Fresh salmon (100 g) ~10-20 µg (400-800 UI) 2 times a week contributes significantly Mackerel (100 g) ~10-15 µg (400-600 IU) 2 times a week Canned Sardines (100 g) ~5-10 µg (200-400 UI) 2 to 3 times a week Egg yolk (1 yolk) ~1-2 µg (40-80 IU) Modest contribution. Complements other sources. Mushrooms exposed to the sun (100 g) ~5-10 µg variable variable. Milk and fortified dairy products ~1-2 µg/100 ml Modest but regular contribution.
3. Why 80% of French people are insufficient
The French paradox: we live in a relatively sunny southern European country, and yet vitamin D deficiency is almost universal there in winter. The reasons are structural:
- ●Geographic latitude: in mainland France (between 42 and 51 degrees north), the sun's UVB rays have an insufficient angle of incidence to allow effective skin synthesis of vitamin D between October and April.
- ●Modern lifestyles: I spend the majority of my day indoors. A typical employee gets little exposure to the sun outside of weekends.
- ●The use of sunscreen: essential for the prevention of skin cancers, it significantly reduces the synthesis of vitamin D.
- ●Insufficient food sources: rare and little consumed (oily fish, liver, egg yolk). Food alone cannot cover needs.- The phototype: dark skin synthesizes less vitamin D under the same sunlight conditions as light skin, because melanin filters UV rays.
- ●Age: skin synthesis capacity decreases with age. At 70 years old, the skin synthesizes 4 times less vitamin D than at 20 years old under the same conditions.
4. Understand my vitamin D levels
The level of vitamin D is assessed by a blood dosage of 25-hydroxy-vitamin D (25-OH-D), a reflection of the body's reserves. Here are the reference thresholds:
5. Food sources and supplementation
Food sources of vitamin D are few in number and insufficient to cover needs without sun exposure:
Given the almost universal insufficiency in France and the difficulty of covering needs through diet alone, ANSES has recommended since 2021 systematic vitamin D supplementation for all adults in France, particularly between October and April. The recommended dosage is 800 to 1,000 IU (20 to 25 mcg) per day, or an equivalent monthly dose (25,000 to 50,000 IU). I consult my doctor for a preliminary blood test and personalized supplementation.
💡 tips to remember
- More than 80% of French people are vitamin deficient At the end of winter: it is the most widespread nutritional deficiency in France and one of the easiest to correct.
- Vitamin D acts like a hormone: its receptors are present in more than 200 cell types and it regulates the expression of more than 1,000 genes. Its roles go far beyond the bones.
- In France, between October and April, the angle of the sun is too low for effective skin synthesis: supplementation is necessary for almost the entire population.
- The optimal level of 25-OH-vitamin D is between 75 and 125 nmol/L (30-50 ng/mL). I have it measured during my annual biological check-up.
- Dosage recommended by ANSES: 800 to 1,000 IU per day from October to April, or an equivalent monthly dose. I take vitamin D3 (cholecalciferol) rather than D2 for better effectiveness.
- I eat fatty fish twice a week: salmon, mackerel, sardines. They are the only significant food sources of vitamin D.
- Persistent fatigue, repeated winter infections, low morale: consider getting your vitamin D measured before attributing these symptoms to something else.
