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Synthesis of endogenous vitamin D
The only known positive biological effect of UV radiation is the UV-B-induced synthesis of endogenous vitamin D. UV-B radiation initiates the transformation of 7-dehydrocholesterol, which is present in our skin cells, into cholecalciferol (pre-vitamin D3). Further metabolic processes are finally leading to the biologically active form of vitamin D.
The vitamin D level in the blood serum accepted by specialist scientific societies for bone health is around 20 nanograms per millilitre (50 nanomols per litre) or higher. These specialist societies define a vitamin D deficiency only when the vitamin D blood serum level is below 12 nanograms per millilitre (30 nanomols per litre).
Only UV-B radiation enables vitamin D formation
The intensity of UV-B radiation is the crucial factor for the endogenous vitamin D synthesis: at too low levels, the endogenous vitamin D synthesis comes to a halt. This is the situation in Germany during wintertime. If the UV-B radiation is high – in spring, summer and autumn - a sufficient quantity of endogenous vitamin D is synthesized in a short time.
No extra sun exposure is needed for vitamin D synthesis
Based on current scientific knowledge, sufficient vitamin D synthesis is achieved when exposing the face, hands, and arms uncovered and without sunscreen to half of the minimum erythemal UV dose (0.5 MED) two to three times a week, i.e. half the time it would usually take for unprotected skin to develop a sunburn (see also "acute damage to the skin"). In purely mathematical terms, this would correspond to about 12 minutes of exposure to high erythemal UV radiation (UV index 7) for people with skin type II. According to scientific studies, longer exposure will not lead to more vitamin D synthesis, but will only increase the risk of UV-related health damage.
The vitamin D blood serum level decreases in winter
Due to the low UV-B radiation in winter (October to March in Germany), a reduction of vitamin D levels in the blood serum can occur even with a balanced diet. Vitamin D reservoirs formed during summer contribute to the vitamin D supply in winter. From the spring, the vitamin D reservoirs depleted during the winter months can be easily replenished.
Vitamin D deficiency and risk groups
Groups at particularly high risk of vitamin D deficiency are people with certain chronic diseases for example of the liver, kidneys and the gastrointestinal tract. But also people with lacking or insufficient sun exposure, such as immobile elderly people, people with a dark skin type living in Germany or traditionally veiled people are at risk.
Babies are a particular risk group. Because of their very sensitive skin and the still insufficient heat regulation, they should never be exposed to direct sunlight. Therefore, it is recommended in Germany that babies and toddlers shall be treated with vitamin D supplements up to their second early summer, i.e. for 12 to 18 months depending on their date of birth, as a rickets prophylaxis.
Medically diagnosed vitamin D deficiency must be treated by a doctor
Only a doctor can ascertain whether there is a health-relevant vitamin D deficiency. If vitamin D deficiency is diagnosed, vitamin D supplementation or medication should be carried out under medical supervision. It is strongly advised against a treatment of vitamin D deficiency with non-medically controlled additional UV exposure (sun or sunbed).
Sunbed use contributes very little to endogenous vitamin D synthesis
In comparison to the sun, different UV radiation levels exist in sunbeds. In the UV-B range, the UV radiation in sunbeds is about the same level or less than the Mediterranean midday sun. In the UVA range, the radiation in sunbeds can be up to six times as strong as the sun, in facial tanners even up to ten times.
UV-A radiation does not contribute to endogenous vitamin D synthesis but to an increase of the skin cancer risk. Scientific findings furthermore indicate that UV-A even supports the degradation of vitamin D. In addition, first-time use of a sunbed at younger ages (under 35 years) roughly doubles the risk for malignant melanoma. Reason enough not to expose oneself to the health-damaging effects of artificial UV radiation of sunbeds for the purpose of endogenous vitamin D synthesis.
State of 2017.11.23