Vitamin D

What is vitamin D?

Vitamin D is a fat-soluble vitamin that is found in two forms: vitamin D2 (or ergocalciferol) produced by plants, and vitamin D3 (or cholecalciferol) produced by animals (and certain lichens), the latter being the main form present in our bodies. It has the particularity of being able to be synthesized by the body, at the level of the skin, from a cholesterol derivative under the action of the sun's UVB rays, from which it derives its nickname of "sunshine vitamin": this makes it a vitamin that is a little bit different. It behaves almost like a hormone, and can be stored in the fat and liver, to be released according to the needs of the body. Once in the body, vitamin D is converted into its final active form, called calcitriol.

What is its function and what are its benefits?

Vitamin D is mainly known for its major role in bone health. It contributes both to improving the absorption of calcium (and indirectly of phosphorus), but also, along with vitamin K2, is one of the main actors in bone remodeling (by releasing calcium from aging parts of the bone, and via other mechanisms of action, by allowing the creation of new bone structures). In this sense, it contributes to keeping the bones healthy, and would thus prevent fractures and osteoporosis.

It also plays a role in maintaining the immune system and is essential for neuromuscular function.

It is given to children to prevent rickets, a growth disorder that manifests itself in particular by a lack of calcification of the skeleton.

Finally, it is the subject of more and more research for its potential anti-cancer role (colon, rectum, breast, prostate).

Where to find it?

There are relatively few foods rich in vitamin D: it is found in fatty fish (salmon, mackerel, sardines, herring, cod liver), and to a lesser extent, in milk, butter, cheese, etc.

As a result, our diet only covers a small part of our needs (generally around 10 to 20% maximum), and the rest must be covered by exposing ourselves to the sun on sunny days (from March to October, for about 20 minutes a day, but not more than that so as not to get sunburned), or by taking vitamin D supplements during the cold season (from October to March, in the northern hemisphere). Indeed, during these cold season months, the sun is too "low", and the wavelength of ultraviolet rays does not allow the skin to absorb vitamin D.

Deficiency and excess

Since vitamin D is fat-soluble and stored in the body, it can be toxic in high doses. However, it seems that only doses higher than 10,000 IU/d over the long term can have this effect.

Cases of deficiency, on the other hand, are much more widespread: it is estimated that they affect up to 91% of the population living in the northern hemisphere in winter. Apart from newborns, in order to prevent rickets, it could therefore be recommended that all adults take vitamin D supplements during the cold season.

People with insufficient sun exposure or dark skin should also be aware of their vitamin D blood levels and take supplements if necessary.

The same is true for people who are overweight. This subject is quite complex and for a long time it was not known why overweight people were almost systematically deficient in vitamin D, but it is now known that it is a combination of factors, including a volumetric dilution of vitamin D in fatty tissues (lowering the concentration of this vitamin in the blood) as well as "trapping" of vitamin D by adipose tissues which, in certain cases (in particular in the case of resistance to insulin), is not sufficiently quickly put back into circulation in the blood.

Finally, the capacity of the body to synthesize vitamin D decreases with age.