While 10 to 20 minutes in the sun is enough in spring and summer, almost two hours would be needed in the winter months, the researchers found, concluding that it is difficult to achieve the optimal values year-round for the vast majority of the population.
Vitamin D deficiency
While it's true that too much ultraviolet radiation can contribute to cancer, aging of the skin, and other health issues, it also reduces blood pressure, synthesises vitamin D, and improves the treatment of several diseases.
Vitamin D deficiency is linked in adults to a higher risk of suffering from various diseases. Since very few foods contain this vitamin, its synthesis in the skin as a result of sun exposure is the main natural source that exists.
So researchers at the Solar Radiation Research Group at the Polytechnic University of Valencia (UPV) set out to find the exposure time needed to obtain the recommended doses of vitamin D without damaging our health.
"In Spain, despite being a country with many hours of sunlight, several articles have reported a high percentage of vitamin D deficiency among various strata of the Spanish population," said María Antonia Serrano, a scientist at the UPV and main author of the study.
29 minutes
Serrano and her colleagues estimated the time needed to obtain the recommended doses -- which is equivalent to a daily intake of 1,000 IUs (international units) of vitamin D -- in an area such as the city of Valencia, which receives a large dose of UV radiation throughout the year.
They analyzed ultraviolet solar irradiance (UVER) around midday (between 12:30 and 1:30) for four months of the year (one in each season) from 2003 to 2010. With these figures, the time taken to cause sunburn was calculated.
They found that in July, an individual with normal skin could spend only 29 minutes in the sun without erythema, the technical name for sunburn. But in January, the time increases to more than two hours -- 150 minutes.
Maintaining vitamin D in winter
The study found that, even in sunny countries like Spain, it is difficult to attain recommended doses of vitamin D in winter because of the excessive time required.
On the other hand, in the middle hours of the day in spring and summer, with 25% of the body exposed, around 10 minutes of sun exposure in early afternoon would be sufficient to meet daily vitamin D requirements.
"Radiation received also depends on posture, body shape, and clothing. It should also be remembered that not all areas of the body synthesise vitamin D with the same efficiency," Serrano noted.
An individual's age also plays an important role in synthesising vitamin D from UV radiation, because the older one gets the less able one becomes to produce vitamin D: middle-aged adults have 66% of the potential children have to do this.
"These results can help to adopt the right measures to make up for any deficiency, such as informing the medical profession about the utility of increasing vitamin D intake in the diet or through supplements," the Spanish researcher concludes
The results have been published in the journal Science of the Total Environment.