The body needs to be exposed to the sun to produce enough vitamin D to maintain the health of our bones and teeth. Although it can also be obtained through diet, there are few foods that contain it. Therefore, one of the simplest and most natural ways to meet the needs of the so-called ‘sun vitamin’ is to expose yourself to solar radiation, but do you know how much time is needed to recharge our batteries and do so safely?

Expert researchers have estimated the necessary time of exposure to solar radiation to obtain the recommended doses of vitamin D. Each year, studies on the benefits of sunbathing in moderate doses are interspersed with those that confirm the risks involved in doing so excessively. Thus, although solar ultraviolet (UV) radiation intervenes in the development of solar erythema, cancer and aging of the skin, it also reduces blood pressure, synthesizes vitamin D and improves the treatment of various pathologies.

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Now, the Solar Radiation Research Group of the Polytechnic University of Valencia (UPV) has analysed the necessary exposure time to obtain the recommended doses of vitamin D without damaging our health.  In Australia, despite being a country with many hours of sunshine, it has been reported that there is a high percentage of vitamin D deficiency among several strata of the Australian population.

Insufficient vitamin D is associated with an increased risk of various diseases in adulthood. Since very few foods contain this vitamin, its synthesis in the skin from sun exposure is the main natural source that exists. Experts have estimated the time necessary to obtain the recommended doses – equivalent to a daily intake of 1000 IU (international units) of vitamin D – in an area such as the city of Valencia, which receives a large dose of UV radiation during the entire year.

Burns in 30 minutes

A work analysed the ultraviolet solar irradiance (UVER) around midday (between 12:30 and 13:30 h) during four months of the year (one of each season) from 2003 to 2010. With these figures the time for produce erythema-reddening of the skin caused by burns. Thus, the data underline that, in July, an individual with skin type III (the usual one in the Spanish population) should not be more than 29 minutes under the sun if he wants to avoid it. However, in January, the same individual can remain in the sun for 150 minutes.

In the same way, the minimum exposure time was obtained to obtain the recommended daily dose of vitamin D. The problem can appear in winter due to low levels of UV radiation and the fact that people cover most of the body.

While in the spring and summer there are enough 10 to 20 minutes in the sun, in the winter months it would take almost two hours

In this sense, it was found that, around noon in January, with 10% body exposure, it takes about 130 minutes to get the recommended daily dose of vitamin D. Since this time is less than what would produce erythema, there is no risk of sunburn. In contrast, in April and July, with 25% body exposure would be enough about 10 minutes to acquire the vitamin. And in October, for example, it would take about 30 minutes.

These calculations were made for skin type III, but if it were clearer or darker the figures would change. We also have to take into account that we have considered a percentage of body exposed for the season, if it were higher, then the exposure time would be reduced.

Likewise, the time obtained to produce erythema has been calculated for average days. It should be taken with caution. For the extreme days, the permissible exposure times would be much shorter.

Keep vitamin D in the winter

The results show that, although there is enough radiation in countries such as Spain, it is difficult to reach the recommended doses of vitamin D in winter (from November to February) in a northern latitude, since the necessary time of exposure is excessive (130 minutes).

In these months, with 10% body exposure, during solar noon it would take two hours of exposure to the sun to obtain an optimal dose of vitamin D – but at 10:00 it would take approximately 9.7 hours and at 16:00 about 5.7 hours.

The radiation received depends on the posture, the shape of the body and the clothes

On the contrary, in the central hours of the spring and summer days, with 25% body exposure, about 10 minutes of solar exposure around 1:00 pm and about 20 minutes from 3:00 pm to 5:00 pm: 00 h would be enough to meet the daily needs of vitamin D.

In addition, the radiation received depends on the posture, the shape of the body and the clothing. It should also be borne in mind that all areas of the skin of the body do not synthesize vitamin D with the same efficiency.

The age of individuals also plays an important role in the synthesis of vitamin D from UV radiation, as over the years there is a decrease in the ability to produce vitamin D, with middle-aged adults having 66% of the potential in comparison with children.

These results can help the adoption of appropriate measures to compensate for the possible deficiency, such as informing the medical community about the convenience of increasing vitamin D intake through diet or supplements.

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