vitamin D

Vitamin D: Functions, Deficiency and How to Avoid It

Vitamin D is considered one of the most important micronutrients. Being involved in hundreds of metabolic reactions, it plays an important role in our health. Among all, some functions of vitamin D are regulating bone and calcium metabolism, muscle function, inmunity, and cognitive function.

However, deficiency of this vitamin is very common, showing that roughly 40% of the entire population in Europe are deficient. Since foods are low in vitamin D and sun exposure is often limited, supplements of vitamin D are a great tool to avoid deficiency.

In this post we'll go through the main risks of vitamin D deficiency and why, if not yet, you should consider take a daily supplement to avoid health risks


Vitamin and minerals are key factors in our metabolism. They play different functions depending on their chemical structure and biochemical properties. Vitamin D is one of the most important micronutrients. Its popularity arose when it was discovered the role it had on bone health and absorption of calcium (source)(source)

However, a wide variety of different effects have been associated with vitamin D status, including muscle function, inmunity, cardiovacular events and risk of cancer (source). It is now clear by the scientific society how important is vitamin D for our health, and more interest is being shown to evaluate and investigate health risks associated with vitamin deficiency.


Vitamin D is a liposoluble vitamin considered an steroid hormone due to its similar chemical structure. It's transported through the plasma membrane by a protein transporter, and attached to the vitamin D receptor (VDR) to trigger genomic and non-genomic actions (study)

It's found in two different forms, vit.D2 (ergocalciferol) and vit.D3 (cholecalciferol). Independently of the form, both show the same activity and metabolism. During sunlight exposure, UVB rays reach our skin, alo called epidermis. Here, 7-dehydrocholesterol catches the UVB rays and it's converted into pre-D3, which is the inactive form.

Once we have the prohormone in our organism, two conversions take place before we have the active form. First, the prohormone is converted into 25(OH)D3 in the liver by adding a OH group in the carbon nÂș25. Secondly, another hydroxylation takes place, but in this case in the kidneys, synthesizing 1,25(OH)2D3. This is the active form, which will bind to the VDR to trigger different actions (source)

Vitamin D Metabolism


To understand the metabolism and actions of vit.D we need to know what's VDR and why it's important here. VDR is a nuclear receptor specific for vitamin D. When vit. D attaches to VDR, both genomic and non-genomic actions are triggered. Genomic actions occur by the formation of a dimer between VDR and retinoid x receptor (RXR). This conjugation VDR-RXR will regulate gene expression and the consequent protein synthesis (study).

The presence of this receptor in many of our tissues is what gives vitamin D such an importal role and a long list of metabolic functions. Among these, we know that VDR is expressed in the intestine, brain, muscle and inmune cells, among others (study)


We have two different sources from where we can obtain our vitamin D. These are diet and sunlight exposure. As you may have guessed, sunlight exposure is the main source, and is almost impossible to obtain the required amounts of vitamin D from dietary sources. But some foods contain fair amounts of this vitamin.

Vitamin D2 is found in foods from plants sources such as fungi, yeast. If we talk about Vitamin D3, we can find fair amounts in fatty fishes, fish oils, liver, or enriched dairy product. In the table below you can give a quick insight of the amounts coming in each product (source).

Last, but not least, is supplementation. Vitamin D supplements are widely used nowadays due to the high prevalence of defficiency in modern society. As you can see in the table, vitamin D content of dietary sources compared to UV exposure is very small, and sometimes is not enough to get the required amounts.



  • Vitamin D undergoes two conversions in the liver and kidneys, respectively before becoming active.

  • There are two types of vitamin D. Vitamin D2 is found in plant sources, while Vitamin D3 is found in animal sources

  • Supplementation of vitamin D is an easy way of getting enough vitamin D


As we said earlier, vitamin D is associated with many different systems and functions in our system. Being crucial for our health and quality of life, we'll go through the main functions of vitamin D..


This was what made this hormone popular. Its presence was associated with falls and fractures (study). Not only that, but vitamin D deficiency increased the risk of diseases related to bone health (rickets, osteoporosis,..).

Vitamin D benefits bone health by being involved in calcium and potassium metabolism. It enhances absorption of calcium in the intestine and mobilization to the bone. It also acts on the kidneys, promoting reabsorption of the mineral priorly mentioned (study). Because of this, vitamin D is an important substance to mantain calcium homeostasis and proper bone health (study)

Not only in calcium homeostasis, but vitamin D acts directly on bone tissue, promoting osteoclast differentiation and calcium reabsorption on the bone (study).


Inmunity is an important field to investigate due to high prevalence of infectious diseases taking place lately (more now with Covid pandemic). The inmune system is a very complex net made of different cells and molecules with connected functions working towards the same goal; destroy the infectious agent and protect our organism from foreign pathogens.

Vitamin D deficiency has been associated wit higher risk of infection from influenza (study) and covid-19 (study). These findings prove the importance of vit. D on inmunity and protection from infections.

Vitamin D has been shown to inhibit T-cells proliferation and facilitates regulation of T regulatory cells. In fact, vit. D regulates both innate and adaptive inmune system. And research have shown that inmune cells are able to synthesize and respond to vitamin D actions (study)

Vitamin D and Inmune System


VItamin D has been shown to regulate cardiovascular outcomes such as cardiac death and combined cardiovascular events (study).

Research has been done to investigate wether vitamin D supplementation could potentiate and lower the risk of cardiovascular events. In this systematic review, we can see that serum levels of vitamin D show a moderate effect improving cardiovacular health (study).

Vitamin D acts on the cardiovacular system by improving insulin sensitivity, smooth muscle cell proliferation, and modulating inflammatory processes (study). However, the effect concluded in the meta-analysis is very low, and although positive is not significant. The lack of studies and heterogeneity lead to these non-concluding findings, and we'll have to wait for more evidence regarding this topic.


Metabolites of vitamin D have been found in the brain (study). This finding and the presence of VDR in brain cells suggest that vit.D could have a benefitial effect lowering mental disordes and improving cognitive performance.

In this interesting study done in vitamin D deficient rats, it was found that they showed increased neurotrophic factor levels, decreased mitosis and increased apoptis (study). In other words, brain cells divided less efficiently and higher death rate was observed. These rats also showed different behaviour, being more sensitive to external factors and a more inconsistent behaviour.

Vitamin D deficiency has been associated with many mental disorders, mainly schizophrenia (study). In a long term study performed with danish babies, they observed that low levels of vitamin D were associated with higher prevalence of schizophrenia and mental disorders (study).


VDR has also been found in muscle tissue, in where proper serum levels of vitamin D could have a benefitial effects of muscle outcomes such as power, strength or muscle hypertrophy.

Muscle health and function is important when it comes to optimize performance and induce higher structural adaptations. Systematic reviews found an association between vitamin D and muscle strenght, while no significant effect on muscle mass or power were found (meta-analysis).

Vitamin D has been directly associated with testosterone levels. Correct serum levels of vit. D lead to higher levels of testosterone, which potentially improves muscle growth and muscle hypertrophy (study).

With this evidence we can say that vitamin D has an impact on muscle hypertrophy by inducing greater levels of testosterone. Additionally, you could benefit from vit. D if practising strenght sports such as powerlifting or weightlifting.


  • Vitamin D improves calcium metabolism and absorption in the bone

  • Inmune system can be potentiated by correct serum levels of vitamin D

  • Vitamin D has shown to have a moderate effect on improving cardiovascular events

  • During sports performance, vitamin D could improve muscle strenght and boost testosterone levels
Functions of Vitamin D


Vitamin Deficiency is a big pandemic nowadays. We know that roughly 40-45% of the population shows deficiency or insuficiency (source), compromisign health and with a higher risk of showing any of the adverse effect you saw before.

Vitamin D comes mainly through sunlight exposure. If this one is limited due to season, lifestyle, latitude or any other factor, vitamin D serum levels will be lower than we need. The (almost) absence of this vitamin on dietary sources make it hard to reach the required levels.

Latitude and vitamin D

In this picture we can see and evaluate the importance of where you live on your serum levels of vitamin D. If you above the orange line, you'll be insuficieny at least one month of the year, mostly during winter. If you are above the red line, vitamin D insuficiency and/or deficiency appears most of the year, with the only months with enough sunlight exposure during summer (June-August).


Based on our everyday life and latitude we may have an idea wether we are vitamin D deficient or not. But to be 100% sure, you could take a blood analytics and ask for serum levels of 25(OH)D, which is the first metabolite of vitamin D.

Once you know your serum levels you can analyse and evaluate wether you need to raise your vitamin D consumption or start supplementing. Serum levels of <10 ng/ml are considered deficiency, while between 10-30 ng/ml is an insuficiency, still under optimum levels. Between 30-100 ng/ml is known to be the 'sweet spot' in where we could benefit from all the effects of this molecule.

Serum Levels Vitamin D

Vitamin D has been shown to be safe even in large acute amounts of 50.000-100.000 UI (study). But befe you take any supplement, please talk with your doctor or a health professional to get proper assesment and follow-up.


If you are finally considering supplementing with vitamin D because you are deficient or you suspect you are, then I have good news for you. Vitamin D supplements are cheap, fairly reliable and the protocol of use is so simple.

Daily doses of 2000-4000 UI are considered and showed an increase of serum levels over time. Vitamin D should be taken with at least 2g of fat for a better absorption, since it's a liposoluble substance. It is suggested that benefits could be potentiated if you take vitamin D in the morning, but evidence regarding timing is non-existent. It could help regulating our circadian rythms, highly associated with health and quality of life.

Magnesium, Boro and Calcium interfere with Vitamin D metabolism (study)(study). Because of this, a correct consumption of foods containing these minerals will help making vitamin D metabolism more efficient. If efficiency of conversion is higher, risk of Vitamin D deficiency is lower and we don't have to supplement as much as if we were also deficient in any of these minerals.


Vitamin D is an important vitamin considered an hormone because of its synthesis from 7-dehydrocholesterol and the similar structure to steroids hormones. We have seen the importance and the main functions of vitamin D, mainly by interacting with VDR. Among others, we mentioned bone health and calcium metabolism, skeletal muscle, inmunity, and mental disorders. However, more research is being done of this molecule and we expect to know more about it in the coming years.

Vitamin D deficiency is common nowadays. Due to the limited sunlight exposure and the absence of food sources, supplementing is, in many cases, the easiest way of correcting the deficiency. Supplementing 2000-4000 UI with at least 2g of fat is known at the date to be the optimum protocol to increase serum levels.

I hope you enjoyed the post of the week. If you have any doubts, please leave your comment down on the comments section!

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