Vitamin D is a fat-soluble vitamin and one of the few nutrients our body is able to produce on its own, in response to skin exposure to UVB rays from the sun. It can also be obtained through several food sources and where appropriate, through supplements. Unlike other nutrients, it can be stored in the liver and fats, and used as needed.
Unfortunately, due to our pace of life and work and the consequent low exposure to sunlight, vitamin D deficiency is a common problem today. However, it plays an essential role in the overall balance of the body.
The main functions of vitamin D
- It helps maintain normal bones: by promoting the fixation of calcium, which is essential for maintaining the integrity of the skeleton. It also participates in 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 and thus leads to 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: brain cells that control metabolism, hunger hormones and satiety need vitamin D to function properly. Furthermore, it also contributes to the reduction of systemic inflammation which is associated with being overweight.
Due to its different roles, particularly on bone density and the immune system, vitamin D is necessary for everyone, regardless of age and physiological particularities (pregnant woman, child, athlete, elderly person).
Sources of vitamin D
The main source of vitamin D is sunlight. It acts on the body and allows the production of vitamin D inside the skin. This reaction, called photolysis, results in 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 as a result of this reaction that the active form of vitamin D is released.
The quantity of vitamin D synthesized varies from one person to another because it depends on age, skin pigmentation, duration of exposure to the sun but also the intensity of UV radiation and the exposed skin surface.
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 ask yourself whether sun protection would still allow you 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 must protect your skin by staying in the shade and applying sunscreen with a high protection index. When the UV index is low, sun protection may be less. Thus, the conditions are optimal for the synthesis of vitamin D.
For most people, 5 to 30 minutes of sun exposure twice a week is enough to cover daily needs.
However, in winter or in certain geographical areas, solar exposure is not necessarily accessible. Furthermore, older people and those with dark skin do not synthesize vitamin D as efficiently as others. To avoid deficiencies, it is therefore wise to regularly consume foods rich in vitamin D. The vitamin D found in the diet exists in two forms: the D2 form (or ergocalciferol) and the D3 form (or cholecalciferol). Vitamin D3 is better metabolized and thus leads more efficiently to the active form of vitamin D than vitamin D2.
Foods rich in vitamin D include animal products such as fatty fish, eggs, beef liver, cheese and butter.
Here are some 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 provide 12% of the recommended daily value)
- Beef liver (85 g provides 11% of the recommended daily value)
- Eggs (85 g provide 1o% of the recommended daily value)
On the plant level, chanterelle and button mushrooms are foods that contain a relatively high quantity of vitamin D but in a less known form, vitamin D2.
Recommended intakes of vitamin D
Several studies have shown that spending 15 to 20 minutes outdoors ensures a sufficient intake of vitamin D. You must still be careful not to expose yourself to direct sunlight between 12 p.m. and 3 p.m., particularly in summer.
To ensure your body has sufficient intake, it is also advisable to balance and vary your diet and consume 2 portions of fish per week (preferably 1 fatty fish).
According to ANSES, the recommended nutritional intake for the population is 15 mcg per day for adults, or 600 IU. For other populations, they are currently being studied.
This nutritional reference for the population is defined by only considering the intake of vitamin D through food and not the contribution of exposure to the sun.
The risks of vitamin D deficiency
In certain cases, it is possible to observe a vitamin D deficiency. Indeed, certain factors can increase the risk of deficiency:
- Spend less than 20 minutes per day outside,
- Very dark skin for which the synthesis of vitamin D through exposure to the sun is less effective,
- Insufficient intake through food, such as specific diets eliminating meat, fish, eggs and dairy products,
- Chronic kidney disease (decreased kidney function),
- Aging, because the body's ability to absorb or synthesize vitamin D decreases with age,
- And finally, pathologies leading to poor assimilation of vitamin D in the intestine (such as Celiac or Crohn's disease)
These deficiencies should not be taken lightly because they can lead to muscular disorders with a drop in muscle tone, tetany attacks or convulsions. They can also cause bone disorders. Indeed, a vitamin D deficiency reduces bone mineralization and can lead to long-term bone weakening: osteomalacia (in adults), rickets (in growing young people) or even osteoporosis (bone decalcification). .
Symptoms of vitamin D deficiency may be associated with a weakened immune system, dizziness, fatigue, or difficulty concentrating.
In populations at risk, it is important to monitor vitamin D levels and discuss supplementation with a doctor or pharmacist.
Knowing that the production of vitamin D is mainly carried out with exposure to the sun, we are more likely to lack it during the winter and autumn periods. However, these are periods during which the body must fight against viral infections. It is then recommended to turn to plant-based milks, a plant-based alternative enriched with vitamin D, or to vitamin D supplementation and to take a course of treatment between October and March.
Vitamin D supplementation is available in different forms and doses: drops and ampoules. The drops allow you to take a little every day while the ampoules offer a large dose more spaced out. The first option seems preferable since it allows you to maintain a more stable level of vitamin D in the blood.
Excess vitamin D
It is not possible to have a vitamin D surplus naturally. On the contrary, this excess is possible by mouth. Indeed, vitamin D being fat soluble, it can accumulate in the body. It is obviously necessary to avoid exceeding the doses. Vitamin D would lose its benefits and cause health consequences such as nausea and vomiting, muscle weakness, intense fatigue, headaches, hypercalciuria (increased urinary calcium excretion), dehydration, and then renal failure.