What is Vitamin D and what is it for? Here's when to take supplements

We know it as a vitamin, but in reality, it is a pro-hormone. And contrary to what we might think, its main source is not food but the sun. It is vitamin D, a micronutrient needed in very small amounts but essential for our body’s health. We get it mainly from the sun because foods containing vitamin D are scarce, but we can store it thanks to the action of UV-B radiation present in sunlight. Its precursor, present in our skin, is first activated by ultraviolet rays (UV-B) and then modified in the liver and kidneys to produce the active form of vitamin D: cholecalciferol, also known as vitamin D3. But if the human body can synthesize it, why is vitamin D deficiency so widespread? How can we notice if our vitamin D3 is low? And how can we prevent or correct it?

Causes of vitamin D deficiency

In many cases, adequate sun exposure combined with the small amount of vitamin D found in foods could prevent deficiency. Unfortunately, our lifestyle often forces us to spend many hours indoors, reducing the amount of UV-B rays that reach our skin. Moreover, we have to consider that ultraviolet radiation promotes the development of skin cancers. For this reason, we protect ourselves (or at least should protect ourselves) from UV rays in at least two ways: avoiding exposure during the hours when the sun’s rays are strongest and applying appropriate sunscreens on the skin. All this significantly reduces the trigger for vitamin D production in the skin. If that were not enough, other factors can also reduce the amount of vitamin D3 synthesized by our body:

• Our skin type: people with darker skin naturally block UV rays with the pigments in their skin;
• The latitude where we live: the closer to the poles, the less intense the solar radiation;
• The season: in winter months, when daylight hours are fewer and sunlight is less intense, the possibility of synthesizing vitamin D in the skin is reduced.

Finally, circulating vitamin D can also depend on our body composition. Being a lipophilic molecule, vitamin D tends to accumulate in adipose tissue; the more abundant this is, the more vitamin D can be stored within it. All this, combined with the low availability of vitamin D in foods, can lead to vitamin D deficiency, a problem that can have numerous health consequences.   Consequences of vitamin D deficiency The first to suffer from inadequate vitamin D availability are the bones and teeth. Not surprisingly, the most frequent manifestation of vitamin D deficiency in the past was rickets, a condition characterized by severe skeletal problems in children. Vitamin D promotes bone health by supporting calcium and phosphorus absorption. Given this role, other problems associated with its deficiency include osteoporosis, osteomalacia (a condition due to inadequate bone mineralization), increased risk of fractures, and secondary hyperparathyroidism (increased production of hormones in the parathyroid glands due to low blood calcium levels). However, since the functions and benefits of vitamin D3 extend beyond this, its deficiency can have other consequences. In particular, the vitamin D receptor is also present in cells of the immune system, small intestine, as well as in skin and brain cells. Moreover, this precious molecule is involved in stimulating insulin production (the hormone regulating glucose entry into cells) and in the contraction of heart cells. For this reason, vitamin D deficiencies can be associated, for example, with immune defense alterations.

Symptoms of vitamin D deficiency 

Often vitamin D deficiency goes unnoticed without specific symptoms. However, there are cases when deficiency manifests with:

• bone pain
• muscle pain
• muscle weakness

  When to measure vitamin D blood levels and reference values A simple blood test is enough to discover vitamin D deficiency. According to experts, cases where it is necessary to measure levels include:

• rickets, osteomalacia, and osteoporosis
• chronic renal failure
• liver failure
• cystic fibrosis
• inflammatory bowel diseases
• Crohn's disease
• bariatric surgery
• radiation enteritis
• hyperparathyroidism
• pregnancy and breastfeeding
• elderly with history of falls or non-traumatic fractures
• obesity (also in children)
• diseases associated with granuloma formation, such as sarcoidosis
• use of some drugs (antiepileptics, glucocorticoids, antifungals, cholestyramine, medicines used in AIDS)
• some lymphomas

Currently, vitamin D insufficiency is defined when levels are between 20 and 30 ng/ml, while levels below 20 ng/ml indicate deficiency. There are also cases where vitamin D is very low (below 10 ng/ml) or too high. When it exceeds 150 ng/ml, intoxication and problems related to excess calcium may occur, such as:

• formation of calcium deposits in soft tissues (for example in the heart and lungs)
• confusion and disorientation
• kidney damage
• kidney stones
• nausea, vomiting, constipation, loss of appetite, weakness, and weight loss

 When to take vitamin D supplements

As mentioned, there are few dietary sources of vitamin D. Where is it found? Essentially in fatty fish, the same that provide Omega 3, for example mackerel, sardines, herring, salmon, and tuna. Not surprisingly, cod liver oil supplements (another rich source of these essential fats) can provide both Omega 3 and vitamin D; compared to the past, products currently on the market provide standardized doses of this micronutrient, added ad hoc after purification of the oil containing Omega 3. Other vitamin D-containing foods include fortified foods (i.e., those with added vitamin D), such as breakfast cereals or milk. Finally, some mushrooms and chocolate also contain vitamin D; in these cases, however, the form present is vitamin D2 (or ergocalciferol) rather than vitamin D3. What is its role? It actually performs the same functions as vitamin D3 but is not the form synthesized by our body.

The best vitamin D supplement varies case by case. In cases of particularly low blood levels, a doctor may prescribe very high doses of vitamin D, sometimes in the form of single one-time injections. In all other situations, vitamin D supplements that do not require a prescription may be sufficient. Due to the high frequency of vitamin D deficiency, it may happen that a doctor or nutritionist suggests taking supplements even without blood level testing, especially if risk factors like obesity, advanced age, or low outdoor activity are present.

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