Did you know that... the Sun helps us to have strong bones? Vitamin D, the "sunshine vitamin".

We do not want to finish the summer without you to had our little contribution and so in this post the Sun will be the protagonist, whose function is not only to tan us but, among other things, help us to synthesize a small key molecule in our life: vitamin D. We are going to talk very briefly the main features of the sunlight, how it affect us and what precautions we have to take against it to, finally, explain how our star helps us producing vitamin D and why it is so important to do so.

We hope you enjoy and that it entertain you during a hot summer minutes.

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What are the sunlight?

The nearly 150 million kilometers separating our planet from the sun make sunlight takes about 8 minutes to reach us. These rays are the emission as energy from the fusion reactions in its core and consist of electromagnetic radiation of many wavelengths propagating as waves through space and which differ in energy content. Specifically, the electromagnetic spectrum of solar radiation is distributed from the infrared to ultraviolet (UV) radiation, the latter being of shorter wavelength and thus the most powerful. Here you can see an outline of the electromagnetic spectrum of sunlight and its distribution.

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UV radiation is only a small percentage of sunlight and is responsible of tanning us but in turn is the main cause of the damaging effects of sunlight on the skin from burns to skin cancer. The latter usually occurs due to UV radiation can produce mutations in DNA, altering genes controlling the growth of skin cells and making cell division uncontrollated. Our atmosphere absorbs UV radiation of higher energy, which are the most dangerous for us, but still there are certain things that we have to know to prevent the harmful effects of the small percentage of UV radiation that reaches us. There are three main types of UV rays:

1. Type A (UVA): are responsible for the immediate tanning and aging of the skin, usually being associated to damage it in the long term as wrinkles. But it is also considered to play a role in the development of some kind of cancers. It represent more than 90% of the UV radiation that reaches the surface.
2. Type B (UVB): more energetic than UVA rays and responsible for the lasting tan. Do not penetrate the skin as much as UVA but can directly damage the DNA of its cells. That is why they are the main cause of sunburn and most skin cancers.
3. Type C (UVC): more energetic than the other two and therefore most dangerous, but fortunately do not penetrate our atmosphere.

Modified from www.skincancer.org

What is the relationship between sunlight and vitamin D?

Specifically, UVB rays have an additional function which is to help synthesize vitamin D in the skin cells. So we are going to introduce a little what is this molecule, what it does, how it is synthesized and in which foods we can find it habitually. Vitamin D or calciferol is a micronutrient whose primary physiological role is to regulate calcium and phosphate homeostasis, components that constitute bone mineral matrix in the form of crystals of calcium phosphate, also called hydroxyapatite. That is why its main role is maintaining the integrity of the bones of our skeleton. But we can not also forget that calcium is essential for normal functioning of the nervous and muscular systems. For all the above, this molecule is responsible for:

• Intestinal absorption of calcium and phosphate from diet.
• Stimulating differentiation of progenitor cells to osteoclasts. Bones are continually being formed by osteoblasts and degraded by osteoclasts, in a process called bone remodeling. Osteoclasts are therefore the cells that degrade, remodel and reabsorb the bone.
• Recovery of the calcium released from bone degradation.
• Mineralization of bone matrix.

There are several analogs or forms of vitamin D, being the most relevant to human health the ergocalciferol or vitamin D2 and cholecalciferol or vitamin D3. The three main sources of vitamin D are:

1. UVB radiation exposure: the most significant source, by which is known as the "sunshine vitamin". In the epidermal layer of the skin, 7-dehydrocholesterol molecule can absorb UVB radiation from Sun and become vitamin D3, which will be detail later.
2. Natural sources: fish oils, fish rich in fats such as salmon or sardines, mushrooms, beef liver, cheese, whole grains and egg yolk. They allow to obtain both vitamin D2 and D3 forms from cholesterol and 7-dehydrocholesterol of diet, respectively.
3. Nutritional supplements.

How active vitamin D3 is synthesized in the skin from UVB rays?

Entering a little in chemical terms, in the cells of epidermis UVB radiation transforms 7-dehydrocholesterol or provitamin D into vitamin D3. This compound along with vitamin D2 have no effect for themselves, they must become on the active form. Both D2 and D3 analogs, no matter if synthesized in the skin or from diet, must pass epidermal cells into the blood torrent and then undergo two chemical modifications in two spatially separate places: the first occurs in the liver, where enzymes CYP27A1 and CYP2R1 convert this molecule into 25-hydroxycholecalciferol (25D), also called calcifediol or calcidiol, which passes into the blood. It is the circulating metabolite that is used clinically to determine the status of vitamin D. Vitamin D deficiency is diagnosed with a blood calcidiol level lower than 20 ng/mL. The second modification occurs primarily in the kidney, where blood calcidiol becomes, thanks to the CYP27B1 enzyme, in the active form 1,25-dihydroxyvitamin D (1,25D), also called calcitriol. It has been shown that this enzyme is also located elsewhere and helps producing calcitriol but a minority. The active form of the vitamin reaches the blood and triggers its actions when it binds to its receptor, called vitamin D receptor or VDR in target cells that express it.

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Why is it so important to maintain adequate levels of vitamin D?

Deficiency in this vitamin is often accompanied by hypocalcemia (low blood calcium levels), bone disorders and rickets. However, his lack is not only associated with bone diseases, but is also involved in cardiovascular and muscle diseases, type 1 and 2 diabetes, various types of cancers, epilepsy, asthma, rheumatoid arthritis, male hypogonadism, dementia, tetany, autism and multiple sclerosis, among others. Recently it has been observed that vitamin D is also important for a correct immune response, mainly against infections by microorganisms, so its deficiency has also been associated with immune diseases. With all this, it is demonstrated how important it is this molecule for us. There are several studies showing prevalence of vitamin D deficiency in various parts of the world, largely because there are many factors that can affect its synthesis:

• Geographical and climatic conditions affecting direct sun exposure. This is, tropics receive more solar radiation, as well as areas where generally cloudy all day, less.
• Nutritional factors: mainly 7-dehydrocholesterol intake, the precursor molecule.
• Skin melanin content: people with dark skin need a greater sun exposure to produce the same amount of vitamin D than a person with white skin.
• Time of year and time of day.
• Use of sunscreen and skin products that block UVB rays.
• Age.

Due to the fact that sun exposure allows us to synthesize this vitamin but nothing exact nor easy to predict amounts because of the factors mentioned above, the recommended daily intake by diet is controversial. The FDA, the Endocrine Society and the Food and Nutrition Board recommends a daily intake of 400-800 International Units (IU), which equals to about 10-20 micrograms. However, the Vitamin D Council recommends a 1000-5000 IU intake, increasing demand with increasing body weight.

Greater exposure to the Sun means more vitamin D?

It is true that the main source of vitamin D3 is the Sun, but we must never forget that it is also the main cause of developing skin cancer and, less important, other symptoms such as premature skin aging. Skin cancer is a very common type of cancer. In fact, statistics show that each year in the United States there are more new cases of skin cancer than the combination of all lung, breast, colon and prostate cancer together, and that 1 in 5 Americans will suffer some form of it during the course of his life. The most aggressive and severe type is malignant melanoma. As mentioned above, skin cancer occurs due to excessive exposure to UV radiation, which can cause DNA mutations that accumulate over the years. In other words, it is not an instantaneous process. We have to enjoy the sun safely and avoiding burn, looking for a good balance. Greater sun exposure does not mean a greater amount of vitamin D3 synthesized, because when our body reaches the right levels or when the precursor levels have been spent, an additional exposure is excessive and not recommended because the increased risk of burn and skin cancer. A sun exposure prolonged enough to not get burned, this is, before reaching a pink colour, can make our body to produce from 10,000 to 25,000 IU of vitamin D3. This can take from 15 minutes for a person with white skin to several hours for a person with dark skin. The amount of time that we stay in the sun depends on each person, the skin type, time of day and the season. That is why the Cancer Research UK entity has developed a very useful guide where you can find out your skin type in order to check the risk of burn under the Sun. Clicking HERE you can access to it.

With all the above data we hope not to get you fearing sunlight, we simply wanted to increase your curiosity and knowledge, and to understand that is a potentially both beneficial and harmful element, and that the best way to avoid any risk is to possess good information about. Besides of teaching you that it helps us to synthesize vitamin D in situ, a molecule whose importance is vital for us. With a responsible exposure to sunlight we can both synthesize optimal levels of vitamin D and achieve this skin tone that we like a lot healthily.

Thanks for reading us.

REFERENCES

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