Vitamin D:why it matters,what the sun has to do with it

In nature, vitamin D exists in two variants:

  • vitamin D2 (also called ergocalciferol), mainly present in plants (fruit, vegetables and mushrooms)
  • vitamin D3 (also called cholecalciferol) which is synthesized by the skin starting from the precursor 7-dehydrocholesterol following exposure to sunlight.

To produce the quantity of vitamin D3 necessary for the efficient functioning of the organism, 15 minutes a day of sun exposure are sufficient, without having applied creams containing filters protecting against ultraviolet (UV) rays.

Chemical structure

Vitamin D3 is present in animals and humans and, compared to vitamin D2, it is more bioavailable, that is, more easily assimilated and usable by the human body.

Inside the body, vitamin D3 is transformed into two metabolites:

  • 25 (OH) -D (25-hydroxy-cholecalciferol or calcidiol)
  • 1,25 (OH) 2-D (1,25-dihydroxy-cholecalciferol or calcitriol).

1.25 (OH) 2-D represents the actually active form in the human body, responsible for all the biological properties of vitamin D3 , while the blood levels of 25 (OH) -D are normally taken as reference values ​​for the diagnosis of situations of sufficiency, insufficiency or deficit / deficiency of variable severity ( hypovitaminosis D).

The synthesis of vitamin D2 is instead induced by UVB solar rays starting from ergosterol, a compound present in the membranes of plant cells.


Vitamin D3 and vitamin D2, introduced with foods, respectively of animal and vegetable origin, or with nutritional supplements by mouth, are absorbed at the intestine level , through a passive transport process similar to that of the other fat-soluble vitamins .

In essence, once it arrives in the first tract of the small intestine (duodenum), vitamin D is included in vesicles formed by lipid hydrolyzed aggregates from bile acids and, in this form, it crosses the intestinal epithelium, where it is incorporated in other vesicles called chylomicrons, subsequently transferred to the lymphatic circulation and, therefore, to the general blood circulation.

Vitamin D is indispensable to support an efficient intestinal absorption of the two fundamental minerals for the formation of bones and teeth, namely calcium and phosphate.

The levels of calcium and phosphate in the blood are regulated, in addition to vitamin D, also by two other hormones:

  • parathyroid hormone, which modulates the renal excretion of phosphate and promotes the absorption of calcium, in conditions of hypocalcaemia
  • calcitonin, which stimulates the excretion of calcium and phosphate in the urine and the deposition of calcium in the bones, in the presence of hypercalcaemia.


Compared to other vitamins, vitamin D is characterized by an action more similar to that of hormones and for the ability to modulate gene expression , as well as for direct or indirect involvement in a multiplicity of metabolic and functional regulation processes, extremely diversified, at the level of countless organs and systems.

The actions of vitamin D in the human body are mediated by the link to its specific receptor (vitamin D receptor, VDR), present on the membranes of the cells of the target tissues and organs (bones, teeth, cardiovascular system, immune system cells, etc.) , and the subsequent internalization and transfer into the cell nucleus of the complex formed, which interacts with specific DNA sequences, triggering the synthesis of proteins characterized by well-defined activities.

The activities and functions of vitamin D are so numerous and refined that to date, despite intense research, they are still only partially known.

Properties and benefits

Clinical-practical experience and scientific studies conducted over several decades have shown that vitamin D is essential for proper mineralization of bones and teeth during growth and for maintaining adequate bone mass and the integrity of the nail polish during adult life.

In addition, vitamin D helps to maintain normal levels of calcium in the blood , through a fine regulation of the release and deposition of this mineral in the bones, which represent its main form of storage in the body.

In addition to ensuring the maintenance of bone health, vitamin D prevents calcium from depositing in other body tissues, such as the kidneys, arteries or bone cartilages, where it could cause dysfunctions and severe pathologies (arteriosclerosis, tissue calcifications, etc.). .

Calcium is also a fundamental neurotransmitter to ensure correct contraction of the heart and other muscles of the body . The action of regulation of calcium levels in the blood by vitamin D is therefore also important to contribute to the maintenance of normal muscle function.

Several studies have shown that vitamin D also contributes to the normal function of the immune system , thus being able to contribute to protection from infections .

Food sources of Vitamin D

The foods commonly included in the diet, in general, are not good sources of vitamin D . However, regular consumption of those who are richer in it can, in part, help offset the insufficient production of vitamin D3 by the skin during periods of lower sun exposure, such as in winter or when not enough time can be spent in the open air during the day due to diseases, unfavorable weather conditions, professional commitments etc.

Among the foods that contain the greatest quantities of vitamin D we mention: – some types of fish (salmon, herring, mackerel, sardines and in general all the fish of the North Seas, also rich in omega-3 fats beneficial for the nervous system and l ‘cardiovascular system – the liver of pig – milk and yogurt whole – butter – fat cheeses – eggs – the milk-based creams and / or eggs.

The main plant source of vitamin D is, however, represented by mushrooms, while vegetables and fruit contain very little and always in the least bioavailable variant (vitamin D2).

Cod liver oil , a traditional remedy against rickets before the development of nutritional supplements and specific drugs based on vitamin D3, should not be considered as much a food source as a real supplement , to be taken with criteria and following the doctor’s instructions so as not to incur overdose and toxicity.

The following table shows the quantity of vitamin D in foods that are richer in it.

Food Amount of vitamin D
Cod liver oil 210 µg / 100 grams
Salmon and herring 25 µg / 100 grams
eggs 5 µg / 100 grams
mushrooms 3 µg / 100 grams
Bran 3 µg / 100 grams
Butter 0.75 µg / 100 grams
Liver 0.5 µg / 100 grams

If in the Mediterranean countries the amount of ultraviolet radiation absorbed is normally sufficient to cover the need for vitamin D, those who have less chance of sun exposure and / or play outdoor sports due to climatic or health conditions or habits of life, he often needs to resort to supplementation.


Since vitamin D intervenes in the regulation of innumerable fundamental metabolic functions, its insufficient intake protracted long enough and the low levels of vitamin D in the blood that derive from it can lead to the development of innumerable disorders and dysfunctions.

The most severe and possible symptoms of a severe vitamin D deficiency in children are mainly rickets and the development of fragility and bone malformations .

In adulthood , especially in women aged 40-45 years and in both sexes after 55-60 years, low levels of vitamin D in the blood promote bone loss (osteopenia) and the development of osteoporosis , resulting in greater propensity to fractures and skeletal deformations, as well as muscle weakness and intercostal pain.

Today, in western countries, situations of severe vitamin D deficiency are rare. Unfortunately, the same cannot be said for relative deficits, however harmful to bone health and well-being of the whole body.

On the contrary, the probability of encountering situations of vitamin D overdose through diet alone or with food supplements properly taken and formulated is remote . In fact, there are no commonly used foods that can cause an excess of vitamin D in the blood and all the supplements authorized by the Ministry of Health, taken according to the indications on the packs, contain quantities of vitamin D3 absolutely in line with the needs of people. healthy adults.

Intoxication cases, very rare, can occur as a result of incorrect and frankly excessive supplementation (also with liver oil from North Sea fish, such as cod) or an inappropriate intake of vitamin D through high-level drugs dosage prescribed by the doctor for therapeutic purposes.

To avoid these risks, it is sufficient to follow the dosage indications provided by the manufacturers and / or the doctor and possibly repeat the measurement of the vitamin D values ​​in the blood after a first period of supplementation.

The symptoms and disorders resulting from an excessive intake of vitamin D can be generic and include, for example, nausea , diarrhea and weakness, or more specific, such as hypercalcaemia, kidney changes (nephrocalcinosis) and calcification of soft tissues.

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Recommended doses

In October 2016, the European Food Safety Authority (EFSA) Panel on Dietetic Products, Nutrition and Allergies (NDA) established a recommended intake of vitamin for healthy people older than one year. D3 corresponds to 15 µg per day.

This amount is also considered adequate for pregnant women and breastfeeding women, while for both women and men over 75 years of age intake levels rise to 20 µg per day.

For infants aged between 6 and 12 months, adequate intake values ​​were set at 10 µg per day.

Higher daily dosages may be necessary in situations of ascertained deficit and in the context of the treatment of specific pathologies (such as rickets and osteoporosis), but they must always be defined and prescribed by the doctor.

The requirement setting for vitamin D3, which is part of the revision of the reference values ​​for the intake of nutrients and calories previously identified in 1993, is important for making specific prevention recommendations on the intake of nutrients that allow citizens of European countries to implement healthy dietary choices and to identify situations at possible risk of deficiency on a nutritional basis and consequent need for specific integration, in addition to deficiencies and shortcomings that are evident and verified through the dosage of 25-OH-D levels in the blood.

In this regard, it should be remembered that a category of people particularly at risk of food deficiency of vitamin D and consequent organic ailments / damages is that of the elderly, the main recipients of supplementation, through nutritional supplements or real higher-dose drugs, chosen by doctor in relation to any copatologies present (in particular, osteoporosis) and to the general clinical picture.

Supplementation of vitamin D3 , in addition to those of folic acid (specifically recommended for all pregnant women) and omega-3 (especially valuable for the development of the nervous system of the baby), can also be useful during pregnancy , especially in women who have suboptimal 25 (OH) -D levels in the blood, since it has been observed that exposure of the fetus to insufficient amounts of vitamin D during development can adversely affect skeletal development in the early years of life.

Additional risk factors for developing vitamin D deficiency include:

  • poor exposure to the sun / outdoor life
  • l ‘ obesity
  • eating disorders (especially if associated with frequent vomiting) or in any case poor nutrition of foods that can provide vitamin D3 , such as milk and derivatives, salmon and other fatty fish
by Abdullah Sam
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