Vitamin D is a fat-soluble vitamin and one of the few nutrients that our body is able to produce on its own, in response to the skin's exposure to the sun's UVB rays. It can also be obtained from several food sources and, where necessary, through supplements. Unlike other nutrients, it can be stored in the liver and fat and used as needed.
Unfortunately, due to our lifestyles and work schedules and the resulting lack of sun exposure, vitamin D deficiency is a common problem today. Yet it plays an essential role in the body's overall balance.
The main functions of vitamin D
- It helps maintain normal bones by promoting the absorption of calcium, which is essential for maintaining skeletal integrity. It also contributes to the absorption of phosphorus, another key mineral for bone health.
- It improves immune function: it acts on the innate immune system by stimulating macrophages and dendritic cells, the body's first line of defense. It also stimulates the adaptive immune system by increasing the number of T lymphocytes, thereby producing an anti-inflammatory effect.
- It improves mood disorders: vitamin D plays a key role in regulating the synthesis of serotonin, the happiness hormone, and effectively prevents seasonal depression.
- It promotes weight loss: the brain cells that control metabolism, hunger hormones, and satiety need vitamin D to function properly. In addition, it also helps reduce the systemic inflammation associated with being overweight.
Because of its various roles, particularly in bone density and the immune system, vitamin D is necessary for everyone, regardless of age or physiological characteristics (pregnant women, children, athletes, and older adults).
Sources of vitamin D
The main source of vitamin D is sunlight. It acts on the body and enables vitamin D production in the skin. This reaction, known as photolysis, leads to the production of provitamin D3, which is then converted into vitamin D3 (also known as cholecalciferol) under the action of the sun's rays. It is only after this reaction that the active form of vitamin D is released.
The amount of vitamin D synthesized varies from person to person because it depends on age, skin pigmentation, the duration of sun exposure, the intensity of UV radiation, and the area of exposed skin.
For example, pale skin synthesizes vitamin D more quickly.
Vitamin D is synthesized in the skin when it receives doses of ultraviolet rays. It is therefore logical to wonder whether sunscreen would still allow us to receive a sufficient dose of vitamin D and, if not, how to limit the risk of skin cancer …
When the UV index is high, often between 12 p.m. and 3 p.m., you should protect your skin by staying in the shade and applying sunscreen with a high sun protection factor. When the UV index is low, less sun protection may be needed. Under these conditions, vitamin D synthesis is optimal.
For most people, 5 to 30 minutes of sun exposure twice a week is enough to meet daily needs.
However, in winter or in certain geographical areas, sun exposure is not always accessible. In addition, older people and those with dark skin do not synthesize vitamin D as efficiently as others. To prevent deficiencies, it is therefore advisable to regularly consume foods rich in vitamin D. The vitamin D found in food exists in two forms: D2 (or ergocalciferol) and D3 (or cholecalciferol). Vitamin D3 is better metabolized and therefore leads more efficiently to the active form of vitamin D than vitamin D2.
Foods rich in vitamin D include animal products such as oily fish, eggs, beef liver, cheese, and butter.
Here are a few examples:
- Cod liver oil (1 tablespoon provides more than 100% of the recommended daily value)
- Wild salmon (85 g provides more than 100% of the recommended daily value)
- Mackerel (85 g provides 76% of the recommended daily value)
- Tuna (85 g provides 39% of the recommended daily value)
- Sardines (85 g provides 12% of the recommended daily value)
- Beef liver (85 g provides 11% of the recommended daily value)
- Eggs (85 g provides 1o% of the recommended daily value)
Among plant-based foods, chanterelles and white mushrooms contain a relatively high amount of vitamin D, but in the less bioavailable form known as vitamin D2.
Recommended vitamin D intake
Several studies have shown that spending 15 to 20 minutes outdoors provides sufficient vitamin D. However, care should still be taken not to expose yourself to direct sunlight between 12 p.m. and 3 p.m., particularly in summer.
To ensure an adequate intake, it is also recommended to maintain a balanced and varied diet and consume 2 servings of fish per week (preferably 1 oily fish).
According to ANSES, the recommended nutritional intake for the population is 15 µg per day for adults, or 600 IU. For other population groups, these recommendations are still being studied.
This nutritional reference for the population is defined by considering only vitamin D intake from food and not the contribution of sun exposure.
Risks of vitamin D deficiency
In some cases, vitamin D deficiency may occur. Indeed, certain factors can increase the risk of deficiency:
- Spending less than 20 minutes outdoors per day,
- Very dark skin, for which vitamin D synthesis through sun exposure is less efficient,
- Insufficient dietary intake, such as with specific diets that exclude meat, fish, eggs, and dairy products,
- Chronic kidney disease (reduced kidney function),
- Aging, because the body's ability to absorb or synthesize vitamin D decreases with age,
- And finally, conditions that cause poor absorption of vitamin D in the intestines (such as celiac disease or Crohn's disease)
These deficiencies should not be taken lightly, as they can cause muscle disorders with reduced muscle tone, tetany attacks, or convulsions. They can also lead to bone disorders. Indeed, vitamin D deficiency reduces bone mineralization and can cause bones to weaken over the long term: osteomalacia (in adults), rickets (in growing children), or osteoporosis (bone demineralization).
Symptoms of vitamin D deficiency may include a weakened immune system, dizziness, fatigue, or difficulty concentrating.
For at-risk groups, it is important to monitor vitamin D levels and discuss supplementation with a doctor or pharmacist.
Since vitamin D production occurs mainly through sun exposure, there is a greater risk of deficiency during the winter and fall. Yet these are periods when the body must fight viral infections. It is therefore recommended to choose plant-based milks enriched with vitamin D, or to take vitamin D supplements and undergo a course of treatment between October and March.
Vitamin D supplements are available in different forms and doses: drops and ampoules. Drops allow you to take a small amount every day, whereas ampoules provide a high dose less frequently. The first option seems preferable because it helps maintain a more stable vitamin D level in the blood.
Excess vitamin D
It is not possible to develop excess vitamin D naturally. On the contrary, excess is possible through oral intake. Since vitamin D is fat-soluble, it can accumulate in the body. Naturally, doses should not be exceeded. Vitamin D would lose its benefits and cause health consequences such as nausea and vomiting, muscle weakness, severe fatigue, headaches, hypercalciuria (increased urinary calcium excretion), dehydration, and eventually kidney failure.
