Vitamin D is the sun’s vitamin. It has a characteristic that makes it different from the rest since it is a vitamin that our body can synthesize in an autonomous way by the simple incidence of the sun rays on our skin.
Vitamin D is one of the most important vitamins that our body needs because of the many processes in which it is involved. One of the most known processes in which vitamin D intervenes is in the absorption of calcium and phosphorus at intestinal level, but it also plays a very important role in other processes such as the regulation of our immune system.
What is Vitamin D?
Like any other vitamin, it is a vital compound for the body (the term vitamin comes from “vital amine”). Vitamin D is frequently associated with the concept of hormone, because of its molecular structure and because it is a compound that acts by regulating certain fundamental processes in our body.
There are two types of Vitamin D:
- Vitamin D2 o Ergocalciferol.
- Vitamin D3 o Cholecalciferol.
Both are provitamins, that is to say, they have no activity as such and must undergo a series of metabolic processes that will end up transforming it into the active form that is 1.25 Dihydroxy Vitamin D or Calcitriol.
Vitamin D in food
Vitamin D can be obtained from two sources. On the one hand we have the intake of Vitamin D from the diet or food and on the other hand the Vitamin D that our body synthesizes from the solar radiation on our skin.
Vitamin D obtained through food, diet or dietary supplements
This type of source provides us with both Vitamin D or Ergocalciferol (from vegetable products) and Vitamin D if the source is of animal origin
There are not many foods rich in vitamin D, although there are some where we can find a greater amount of vitamin D. It is the case of some fishes such as salmon, sardines, tuna as well as in eggs (mainly in the yolk) or in some mushrooms.
Vitamin D obtained from sun exposure
It is the main source of Vitamin D, specifically Vitamin D3. This synthesis is due to a compound present in the skin called 7-dehydrocholesterol that due to the incidence of UV-B rays from the sun, is transformed into Vitamin D3 or Cholecalciferol.
Sun exposure time for Vitamin D synthesis.
It takes about 10 – 15 minutes of daily exposure to the sun – which can produce a certain redness of the skin – to get enough Vitamin D due to the action of the sun. It is important to emphasize that this exposure should be done without using any sunscreen because they filter out UV-B rays, responsible for this endogenous synthesis of Vitamin D3 or Cholecalciferol. Obviously we should not exceed this exposure time since we are doing it without any type of sun protection and this could have other consequences in our dermis.
There are factors that influence this type of synthesis: skin color, (the more melanin we have and the darker the skin, there will be less synthesis of Vitamin D). It also influences the time of year in which we are (generally greater synthesis in summer than in winter) or the altitude and latitude of our location because as we approach the equator, there is no longer that difference between summer and winter as UV-B radiation is produced in a similar way throughout almost all the year.
Vitamin D Metabolism
As we mentioned before, both vitamin D2 and D3 are provitamins, that is, inactive forms that need to undergo two processes followed by hydroxylation to become 1.25 dihydroxy Vitamin D or Calcitriol which is the active substance.
- The first hydroxylation occurs at the hepatic level where both Vitamin D2 and Vitamin D2 are converted to 25-hydroxy Vitamin D or Calcidiol which is still an inactive molecule. This 25-hydroxy vitamin D or Calcidiol molecule circulates in the blood and it is precisely this form that is identified and quantified in the plasma when a blood test is done to determine the level of Vitamin D that we have.
- The second hydroxylation is produced at the renal level by the action of 1-alpha-hydroxyalase. Here, in the kidney, the 25-hydroxy vitamin D or Calcidiol is converted into 1.25 dihydroxy vitamin D or Calcitriol which is already the active molecule.
This 1.25 dihydroxy Vitamin D or Calcitriol, once in the blood, binds to specific receptors (VDR or Vitamin D Receptor) on the cells where it can exercise its activity. In this way it reaches the cell nucleus where it binds to the RxR receptor (retinoid receptor), where it acts at the DNA level to express certain genes so that the synthesis of a series of proteins in charge of regulating functions such as those related to osteoblasts and osteoclasts, bone mineralization, calcium and phosphorus reabsorption at the intestinal and renal levels takes place.
Vitamin D and Calcium
One of the most important functions of the vitamin D is the regulation of the serum levels of calcium, favoring its absorption at intestinal level and its reabsorption at renal level.
This influence of Vitamin D on serum calcium levels implies that Vitamin D deficiency has very important consequences on bone mineralization
n addition to bone metabolism, Vitamin D influences other factors and functions such as the regulation of the immune system
More specifically, at the intestinal level, the Vitamin D acts by facilitating the entry of Calcium into the bloodstream through the regulation it exerts on the channels of Calcium (TRPV6) and on Calbindine, a Calcium transport protein found in the cytoplasm of the cells of the intestinal mucosa and which is regulated by Vitamin D. This transport mechanism facilitates the entry into the blood of the Calcium that we ingest with the diet
Similarly, the Vitamin D acts at the renal level, promoting the absorption of calcium and preventing its loss through urine. This mechanism of reabsorption is very similar to the one that follows at intestinal level
Vitamin D deficiency
Currently there is a high percentage of the population, around 60% – 70%, that shows Vitamin D deficiency. There are several causes for this low level of Vitamin D, among them are
- Low sun exposure by spending excessive time indoors, in the office, at home, in the car, etc.
- High use of sun protection creams. Although obviously the use of these creams is totally necessary, we must know that, if we sunbathe with them, we will not absorb any UV-B rays and therefore there will be no synthesis of Vitamin D.
- There are not many foods rich in Vitamin D, which means that through the diet we do not have many opportunities to ingest Vitamin D.
- The senile population, much of it in residences, with little sun exposure and therefore less synthesis of Vitamin D.
- Obesity. Vitamin D is liposoluble, it is retained in the adipose tissue and reduces its bioavailability.
- People with intestinal absorption problems.
- Intake of drugs that promote the catabolism of vitamin D, as is the case of antiretroviral treatments.
This Vitamin D deficiency has several consequences
Hypocalcemia due to reduced intestinal calcium absorption and reduced reabsorption at the renal level. This hypocalcemia can lead to a demineralization of the bone and can produce a reduction in bone mineral density (BMD) with increased bone weakness that can result in
- Rickets in children
- Osteomalacia in adults.
How to increase Vitamin D
It is considered a normal value of Vitamin D in blood when it has a concentration of 30 ng/ml or more, without reaching 150 ng/ml since values of this magnitude are considered toxic.
Values below 20 ng/ml are considered too low and require an intake of Vitamin D to raise their blood concentration to optimal values.
The best way to increase the Vitamin D is to take 15-20 minutes of sun daily without using sunscreen, although if this is not possible, you can always increase your concentration by taking Vitamin D with your diet, using foods rich in Vitamin D (unfortunately, not very often) and / or Vitamin D supplements that we can find in pharmacies or online.
Vitamin D in Pharmacies
We commented before that there are not many foods rich in Vitamin D so we often have to resort to pharmacological Vitamin D that we can find in pharmacies or online.
Vitamin D and COVID-19
In the wake of the SARS-CoV-2 pandemic, numerous studies are being conducted to try to address this difficult global situation. Some of these studies claim a link between higher concentrations of Vitamin D and a lower risk of infection
One of these studies, conducted this year, revealed a direct and inversely proportional relationship between population vitamin D levels and mortality cases due to COVID-19 (1)
Another study relates the regulatory capacity of Vitamin D on the immune system and its protective role against viral infections of the respiratory tract through various mechanisms. It has also correlated the population likely to have lower levels of vitamin D with a higher number of cases of COVID-19 (2).
Although they are data based studies, it is necessary to design and carry out clinical trials to confirm this hypothesis and establish the appropriate doses of Vitamin D for this pathology.
(1)Ilie PC, Stefanescu S, Smith L. The role of vitamin D in the prevention of coronavirus disease 2019 infection and mortality. Aging Clin Exp Res. 2020;32(7):1195-1198. doi:10.1007/s40520-020-01570-8
(2)Grant WB, Lahore H, McDonnell SL, et al. Evidence that Vitamin D Supplementation Could Reduce Risk of Influenza and COVID-19 Infections and Deaths. Nutrients. 2020;12(4):988. Published 2020 Apr 2. doi:10.3390/nu12040988