vitamin D

Vitamin D: Functions, Deficiency and How to Avoid It

Vitamin D is considered one of the most important micronutrients. It's involved in hundreds of metabolic reactions, and 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.

Deficiency of this vitamin is very common. Roughly 40% of the entire population in Europe are deficient.

Introduction to Vitamin D

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. It became popular because of its importance on bone health and absorption of calcium (source)(source) However, a wide variety of other effects have been associated with vitamin D.

Vitamin D affects muscle function, inmunity, cardiovacular events and risk of cancer (source). Vitamin D is important for our health, and health risks associated with vitamin deficiency are under research


Vitamin D Metabolism

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). This union trigger genomic and non-genomic actions (study)

We find vitamin D in two forms; vit.D2 (ergocalciferol) and vit.D3 (cholecalciferol). Both show the same activity and metabolism.

Vitamin D 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.

With the prohormone in our organism, two conversions take place before we get the active form. First, the prohormone is converted into 25(OH)D3 in the liver.

Then, another hydroxylation takes place, 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 Receptor (VDR)

VDR is a nuclear receptor specific for vitamin D. When vit. D attaches to VDR, it triggers genomic and non-genomic actions.

Genomic actions occur by the formation of a dimer between VDR and retinoid x receptor (RXR). The conjugation VDR-RXR will regulate gene expression and consequent protein synthesis (study).

VDR is present in a wide variety of tissues. This is what gives vitamin D such an importal role and a long list of metabolic functions.  VDR is expressed in the intestine, brain, muscle and inmune cells, among others (study)

Sources of Vitamin D

There are two main sources of vitamin D. These are diet and sunlight exposure.

Sunlight exposure is the main source, but some foods contain fair amounts of this vitamin as well

Vitamin D2 is found in foods from plants sources such as fungi, yeast. You can also find vitamin D3 in fatty fishes, fish oils, liver, or enriched dairy products (source).


Vitamin D supplements are widely used nowadays due to the high prevalence of defficiency in modern society.

Content of Vitamin D in  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

Functions of Vitamin D

Vitamin D is related to many different systems and functions in our system.


Skeletal Tissue

Vitamin D benefits bone health by being involved in calcium and potassium metabolism.

It increases absorption of calcium in the intestine and mobilization to the bone. Vitamin D also acts on the kidneys, promoting reabsorption of calcium (study). It maintains calcium homeostasis and proper bone health (study)

Vitamin D acts directly on bone tissue, promoting osteoclast differentiation and calcium reabsorption on the bone (study).

gut microbiota as a defense barrier

Immune System

The inmune system is a very complex net made of different cells and molecules working towards the same goal; destroy the infectious agent and protect our organism from foreign pathogens.

Vitamin D and Inmune System

Vitamin D deficiency increases the risk of infection from influenza (study) and covid-19 (study).

Vitamin D inhibits T-cells proliferation and facilitates regulation of T regulatory cells. It regulates both innate and adaptive inmune system.

Research shows that inmune cells are able to synthesize and respond to vitamin D actions (study)


Cardiovascular Health

VItamin D regulates cardiovascular outcomes such as cardiac death and combined cardiovascular events (study).

In this systematic review, 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).


Cognitive Function

In this study, vitamin D deficiency increased neurotrophic factor levels, decreased mitosis and increased apoptis (study).

Brain cells divided less efficiently and higher death rate was observed. These rats showed different behaviour, being more sensitive to external factors and a more inconsistent behaviour.

Vitamin D deficiency is associated with many mental disorders, mainly schizophrenia (study).

In a long term study performed with danish babies, low levels of vitamin D increased prevalence of schizophrenia and mental disorders (study).


Muscle Function

vitamin D could have a benefitial effects of muscle outcomes such as power, strength or muscle hypertrophy.

Systematic reviews found an association between vitamin D and muscle strength, but no significant effect on muscle mass or power were found (meta-analysis).

Correct serum levels of vitamin D lead to higher levels of testosterone. This potentially improves muscle growth and muscle hypertrophy (study).


  • 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 D Deficiency

Vitamin Deficiency is a big pandemic nowadays. Roughly 40-45% of the population shows deficiency or insuficiency (source).

Vitamin D comes mainly through sunlight exposure. This can be limited due to season, lifestyle, latitude or any other factor.

The (almost) absence of this vitamin on dietary sources make it hard to reach the required levels.

Latitude and vitamin D

If you are above the orange line, you'll be insuficient 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 only enough sunlight exposure during summer (June-August).


How do I Know if I’m Deficient in Vitamin D?

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 analysis and ask for serum levels of 25(OH)D.

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 the 'sweet spot'. Here we could benefit from all the effects of this molecule.

Serum Levels Vitamin D

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

Vitamin D Supplementation

Vitamin D supplementation is a great way to avoid Vitamin D deficiency. Vitamin D supplements are cheap, fairly reliable and the protocol of use is so simple.

Daily doses of 2000-4000 UI increase of serum levels over time. The pill should be taken with at least 2g of fat for a better absorption.

You could gain extra benefits taking Vitamin D in the morning. It could help regulating our circadian rythms,

Magnesium, Boro and Calcium interfere with Vitamin D metabolism (study)(study). These minerals will make vitamin D metabolism more efficient.


Vitamin D is an important vitamin almost considered a hormone. It interacts with VDR, triggering genomic and non-genomic actions.

Vitamin D plays a role in bone health and calcium metabolism, skeletal muscle, inmunity, and mental disorders.

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.

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