Vitamin D from A to Z

Vitamin D from A to Z

Vitamin D maintains adequate levels of the calcium and phosphorus absorption capacity by the intestines. This keeps the concentration of calcium and phosphorus in the blood plasma and optimizes mineralization of bone, cartilage and teeth.


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What is vitamin D?

NC-Vitamin-DVitamin D is actually a family of vitamins, which consists of vitamin D2, D3, D4, D5 and D6. Vitamins D2 and D3 are the two main forms of vitamin D and therefore we do examine the others in the context of this article. Vitamin D2 is of vegetal origin, while vitamin D3 is of animal origin. Both are provided by the nutrition, but human beings are also capable of synthesizing vitamin D3 themselves, in proportions that are sometimes enough to meet their needs (thus, it is not a question of a true vitamin but a pro-hormone, but this is a mere detail).

Vitamins D2 and D3 both play equivalent roles. So much so that when we speak generically of “vitamin D” it usually refers to both vitamins at once, D2 and D3. This will also be the case in the rest of this article.

The main function of Vitamin D is to maintain adequate levels of calcium and phosphorus absorption capacity by the intestines. This keeps the concentration of calcium and phosphorus in the blood plasma and optimizes mineralization of bone, cartilage and teeth.


Vitamin D plays a particularly key role in growth.

It also has many other functions in the body. It is particularly involved in the regulation of cell growth, in neuromuscular function in immune function and reducing inflammation.

How does vitamin D work? How is it capable to have a broad spectrum of action? What are the causes and the consequences of a failure? This is what we’ll see in a comprehensive overview of everything related to vitamin D. Let’s start by exploring the different sources of vitamin D.




Sources of vitamin D

As we briefly mentioned in the introduction, there are two sources of vitamin D: alimentation and synthesis by the body itself.

Foods containing vitamin D are actually quite few. They are mostly fatty foods, since vitamin D is fat soluble. The food richest in vitamin D3 is the oil of cod liver (containing 200 to 250 mcg per 100 g). Oily fish (such as salmon, tune, mackerel) and bacon also bring a certain dose of vitamin D3 (containing about 8 to 20 micrograms per 100 g). A small amount of vitamin D3 is also found in beef liver, cheese, and egg yolk. Finally, some champignons can provide vitamin D2.


source vitamin d

But the contribution from food is only a part of the Vitamin D resources available for the organism. It is in fact capable of synthesizing the vitamin D3 by itself, at the level of the skin. It is a molecule derived from cholesterol (7-dehydrocholesterol) which, exposed to ultraviolet rays of type B (290 to 320 nm), enables synthesizing initially a pre-vitamin D3, which converts to vitamin D3 . For this cutaneous synthesis of vitamin D3 to take place, it is necessary to be exposed directly to the sunlight.  Moderate exposure of the face, arms and legs for five to thirty minutes, and twice a week, would produce vitamin D in sufficient quantity to meet the needs of the organism.  Nevertheless, vitamin D soleila number of factors are likely to moderate the amount of vitamin D produced: the case of the season, time of day, cloud cover, haze pollution; but also pigmentation of the skin and the use of sun protection creams, which reduce the amount of UV capable to penetrate the skin. A question comes to mind here: prolonged sun exposure can cause excessive production and intoxication with vitamin D?  The answer is no, because, in fact, UV destroys some vitamins D3 molecules at the same time as they promote their formation.  In other words, a balance is established between formation and destruction, which prevents excessive build up. This should not, however, dispense with caution.  Be sure to limit sun exposure because UV rays are carcinogenic and are particularly responsible for many cases of skin cancer.

Thus, a balanced diet combined with moderate sun exposure is therefore able to provide the amount of vitamin D that everyone needs.

But how is vitamin D really assimilated and what is its fate in the body?  This is what we will address now.

Metabolism of vitamin D

The body does not differentiate between vitamins D from the diet (and thus assimilate it by the terminal portion of the small intestine) and vitamin D synthesized in the skin. Whatever its provenance, vitamin D undergoes the same treatment.

This treatment aims to activate vitamin D, because in the state, vitamin D is biologically inert. It requires two successive transformations to be activated.

First, it accumulates in the liver where it is converted to an intermediate compound (25-hydroxy-vitamin D). This intermediate is then routed through the bloodstream to the kidneys, where it is converted into the active form of vitamin D (1,25-dihydroxyvitamin D). Activated vitamin D then passes into the bloodstream and is carried to the organs that need it. See in detail how this works:


vitamind hormonal



 Action Mechanism of vitamin D

It is at the cellular level that vitamin D is absorbed by the organs. Indeed, the target cells core has a specific receptor on which alone the activated vitamin D can be fixed . Once the complex vitamin D receptor has formed, it is the transcription of a given gene which is in turn activated or, in some cases inhibited. Since the receiver to the vitamin D is present in most of the organs (in particular in the brain, heart, skin, gonads, prostate and breast), vitamin D influences in all sense more than 200 different genes. This explains the broad spectrum of the vitamin D action.

We now know the mechanism of action of vitamin D.  Vitamin D can act with full effectiveness provided that it’s supplied in adequate quantities. With this in mind, we need to consider what the recommended dietary allowance of vitamin D is.



Recommended vitamin D Dietary Supply

According to ANSES (French National Agency for Sanitary Security of Food, Environment and Labor), the recommended Dietary supply for vitamin D is 5 micrograms per day for children over 3 years old and for adults, and 10 to 15 mcg per day for the elderly.  This recommendation was established by considering the cutaneous synthesis product 50 to 70% of the daily requirement of this vitamin (this explains why it is below the current recommendations in North America).

The average consumption of the French population is below this recommendation, since children from 3 to 17 years consume an average of 1.9 micrograms per day while adults 18 to 79 years consume 2.6 mg per day. However, most people don’t seem to suffer from any symptoms related to vitamin D deficiency, probably because skin synthesis compensates the low food intake. The question arises, however, what are the consequences of vitamin D deficiency and the possible causes of such deficiency.


Vitamin D deficiency: causes and consequences

A Chronic vitamin D deficiency has as its main consequence, rickets in children and young people, and osteomalacia in adults. In both cases, minerals do not accumulate correctly in the skeleton. Although the bones maintain a normal weight, they deform and this causes bone and muscle pain.

Another consequence relates specifically to the elderly: it is osteoporosis, which is characterized by loss of bone mass and thus a weakening of the bones.

Newborns, infants and pregnant women are at increased risk of vitamin D deficiency due to important needs and, in regards young children, due to low sun exposure.

In general, the causes of vitamin D deficiency may be:

  • a unsuitable diet, which underestimates the vitamin D supply (including vegetarian diets without meat, fish, egg-free or dairy-free);
  • too little sun exposure or a very pigmented skin
  • incorrect absorption of food in the small intestine
  • a poor kidney function which is not able to convert the 25-hydroxy vitamin D to 1,25-dihydroxyvitamin D.

It is also appropriate to mention the consequences of vitamin D excess, often due to ingestion in excessive amounts of foods rich in vitamin D or food supplements. The symptoms are headaches, hypertension, thirst, nausea, weight loss, or severe fatigue.


Vitamin D and testosterone

vitamin d and testosterone

  A scientific study by an Austrian team and published in 2011 suggests that taking vitamin D as a dietary supplement increases testosterone levels in men. This result was confirmed by a second German study published in 2012. Testosterone varies in the same way as the measured levels of vitamin D. The correlation between testosterone levels and vitamin D levels is however not yet been finalized and these results call for further studies to be confirmed and to establish cause and effect.  Nevertheless, it is likely that if this result is confirmed, it has something to do with the fact that testicular cells have specific receptors for vitamin D.

Because testosterone plays a major role in male sexuality (including sperm production and libido), it is also involved in maintaining muscle mass.  Any sportsman wishing to develop his muscles should ensure he has adequate intakes of vitamin D to ensure his testosterone level and thus facilitate the development of musculature. This is especially true in the context of bodybuilding.


The importance of vitamin D for bodybuilding

But it is not the only reason that could encourage bodybuilders to consume vitamin D.  The following examples show the importance of vitamin D for bodybuilding:

  • Vitamin D plays a role in neuromuscular function. As such, it increases the tone and muscle performance and therefore benefits training.
  • Vitamin D is involved in the immune function. Helping to fight against any kind of disease (including common colds), it helps to fight against fatigue and therefore, again, to make the training more effective.
  • Vitamin D helps bones to stay in good condition. The bones and muscles are related to each other, it is not possible to have strong muscles without strong bones.
  • Finally, according to a recent scientific study, vitamin D may help eliminate fat cells and thus promote more lean muscles. This effect has been shown at least in mice. The reality of this effect in humans is uncertain.

Taking vitamin D in appropriate quantities seems to be profitable for bodybuilders and for athletes in general. But complete studies must still be conducted on this subject in the future to establish the benefits of vitamin D supplementation.


To conclude this study, I invite you to read our article: The importance of the diet in bodybuilding that will tell you more about helpful contributions for our sport.


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