In France, vitamin D deficiency affects more than 80% of the population to varying degrees. Yet this vitamin plays a fundamental role in immunity, energy, mood, bone health and many other functions. And unlike other vitamins, our bodies can synthesize it themselves - provided we are exposed to enough sunlight.
The problem is that in France, except in summer and in the south of the country, the sun is not strong enough to allow sufficient skin synthesis from September to April. As a result, most of us are deficient without knowing it.
Vitamin D: what exactly is it used for?
Vitamin D is actually a prohormone that acts on nearly 200 genes in the body. Its roles are numerous and crucial:
- Immunity: it contributes to the normal functioning of the immune system, both in defending against infections and regulating inflammation
- Bones and teeth: it is essential for the absorption of calcium and phosphorus, contributing to the maintenance of normal bones and teeth
- Energy and reduction of fatigue: it helps maintain normal energy levels
- Muscles: it helps maintain normal muscle function
- Mood and cognitive function: vitamin D receptors are present in the brain - a deficiency is associated with seasonal low mood
- Cardiovascular health: it helps maintain normal blood pressure
Symptoms of vitamin D deficiency
Vitamin D deficiency is often silent, especially in its early stages. Here are the most common signs:
General signs
Persistent fatigue
Chronic fatigue with no obvious cause is one of the most common signs. It may be accompanied by daytime sleepiness and difficulty recovering after exertion.
Frequent infections
Recurrent colds, sinusitis, bronchitis... A poorly functioning immune system is often linked to vitamin D deficiency, which directly modulates the immune response.
Bone and joint pain
Diffuse pain in the bones (back, hips, legs) or joints, with no identifiable cause, may indicate a deficiency. In children, severe deficiency causes rickets.
Mood disorders and seasonal depression
Seasonal affective disorder (the winter blues) is closely linked to vitamin D levels. Low mood, irritability and lack of motivation often worsen between October and March.
Muscle weakness
Difficulty climbing stairs or getting up from a chair, diffuse muscle pain - common symptoms in people with a deficiency.
Increased hair loss
Vitamin D plays a role in the hair follicle growth cycle. A deficiency may contribute to hair loss.
Who is most at risk?
- People who work indoors: little or no sun exposure during the week
- People with dark skin: melanin filters the UV rays needed for synthesis
- Older people: skin synthesis capacity decreases by 70% after age 70
- Pregnant and breastfeeding women: increased needs
- People with obesity: vitamin D is fat-soluble and stored in adipose tissue
- People who avoid the sun or systematically use high-SPF sunscreen
How can you get enough vitamin D?
The sun: the main source
In summer, 15 to 20 minutes of daily exposure of the arms and legs (without sunscreen) between 11 a.m. and 3 p.m. is enough to produce 1,000 to 4,000 IU of vitamin D. From September to April, in France, this is insufficient.
Foods rich in vitamin D
- Fatty fish: salmon (600-1000 IU/100g), mackerel, sardines, herring
- Cod liver oil: the most concentrated dietary source (8000-10000 IU/tbsp)
- Egg yolk: 40-50 IU per egg
- Sun-exposed mushrooms
- Fortified milk and dairy products
Supplementation
This is the most effective and simplest strategy from September to April. General recommendations:
- Healthy adults: 1,000 to 2,000 IU per day
- People at risk of deficiency: 2,000 to 4,000 IU per day
- Children: 400 to 1,000 IU per day depending on age
Vitamin D3 (cholecalciferol) is significantly more effective than D2 at raising and maintaining blood levels. Combining vitamin D3 with vitamin K2 (which directs calcium toward the bones rather than the arteries) further optimizes effectiveness.
Which blood test should be done to check your level?
The reference test is 25(OH)D (25-hydroxyvitamin D) measured in the blood:
- < 20 ng/mL: deficiency
- 20-30 ng/mL: insufficiency
- 30-60 ng/mL: optimal level
- > 100 ng/mL: risk of toxicity (rare with standard oral supplementation)
FAQ — Vitamin D
Can you overdose on vitamin D?
Toxicity is possible but rare with standard doses (< 4,000 IU/day over the long term). It generally occurs with massive doses taken without medical advice over several months. Always consult a doctor before taking doses above 4,000 IU/day.
Vitamin D: morning or evening?
Ideally in the morning with a meal containing fat (vitamin D is fat-soluble) to optimize absorption.
Should you stop supplementation in summer?
If you are regularly exposed to the sun (15-20 min per day), you can reduce or suspend supplementation in summer. A blood test at the end of the summer season can provide an update.
