Vitamin D: what it is, how it works and what are the benefits

Vitamin D has fundamental functions for the wellbeing of the entire body. In particular, it can help the body absorb and use calcium and phosphorus, which play an important role in maintaining healthy bones.


Vitamin D is a fat-soluble vitamin produced by the human body. It’s synthesised through a chemical reaction that takes place in the skin following exposure to sunlight. Vitamin D can also be introduced through nutrition, particularly through the consumption of fish, liver, egg yolk or by taking specific dietary supplements.

Vitamin D has multiple important functions for the wellbeing of the body. First of all, it contributes to the maintenance of normal blood calcium levels. Vitamin D deficiency mainly affects bones and teeth. In children, it causes rickets, which is characterised by weak and misshapen bones. In young people and adults, vitamin D deficiency can cause osteomalacia, a disorder that causes brittle bones and muscle weakness.

Low levels of vitamin D are associated, especially in elderly people, with osteoporosis, characterised by an imbalance in bone metabolism and an increased risk of fractures. Scientific evidence has also shown that vitamin D can contribute to the proper functioning of the immune system. On the other hand, the role of vitamin D in the cardiovascular field remains to be clarified.

What we call vitamin D is actually a group of compounds of which the main representatives in humans are vitamin D2 and vitamin D3. Neither of these molecules, when produced or introduced into the body, has an immediate biological effect; they need to be ‘activated’.

Both forms of vitamin D are adequately absorbed in the small intestine. The concomitant presence of fat in the gut improves vitamin D absorption, but some vitamin D can also be absorbed without dietary fat. Neither ageing nor obesity alters vitamin D absorption in the gut.

The recommended daily dose of vitamin D varies with age.

The majority of the world’s population meets most of their daily vitamin D requirements through exposure to the sun, at least during periods when more time can be spent outdoors. The sun’s rays penetrate uncovered skin and, through several steps, allow vitamin D3 to form. The season, time of day, number of daylight hours, cloud cover, smog, melanin content of the skin and sun protection are among the factors that affect vitamin D synthesis. Keep in mind that older people and people with dark skin are less able to produce vitamin D from sunlight. UVB rays don’t penetrate through glass, so sun exposure through a window doesn’t activate vitamin D production.


In addition to the sun’s rays, diet can also contribute to vitamin D intake, although few foods are rich in it. Some fatty fish, such as trout, salmon, herring, tuna and mackerel, contain it, as does fish liver oil, particularly cod liver oil, which is the main dietary source of this vitamin. Beef liver, fatty cheese and egg yolks contain small amounts of vitamin D, and mushrooms also provide varying amounts of vitamin D2. In some countries, to overcome the problem, it’s common practice to add vitamin D to certain foods: these are known as “fortified foods”. For example, almost the entire supply of milk in the United States is voluntarily fortified, as are breakfast cereals, while in Italy this occurs mainly in baby food. Milk and its derivatives (dairy products in general and yoghurt) contain modest quantities of vitamin D. The label showing the nutritional values indicates the quantity actually present.


Diet alone may sometimes be insufficient to provide adequate levels of vitamin D, and it may be necessary to supplement it by taking dietary supplements of vitamin D2 or D3 (cholecalciferol) possibly associated with other vitamins, minerals or milk enzymes. Supplements can be taken orally, usually in the form of tablets or orodispersible films.

Of the two forms of vitamin D, vitamin D3 is the most common because it’s more easily absorbed.

Importantly, lifestyle can also help maintain sufficient levels of vitamin D. In fact, leading a healthy life, avoiding alcohol and excesses, and exercising in the open air promote vitamin D synthesis and prevent its loss.

Vitamin D deficiency

Vitamin D and sunshine

Vitamin D excess

Vitamin supplements

Vitamin D and bones

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