Skin synthesis guarantees about 80% of vitamin D requirements. The remaining percentage (exogenous intake) derives from the dietary intake of the substance, contained in foods such as the meat of some fatty fish (salmon, mackerel and herring), the yolk egg, liver, fish oils (especially cod liver oil) and in artificially enriched foods.
The man is able to synthesize cholecalciferol starting from a precursor, with function of provitamin: dehydrocholesterol (derived from cholesterol by reduction). This provitamin is found in the skin, in order to absorb the solar radiant energy (above all the UVB radiations) which transform it into an intermediate and unstable compound called previtamina D3. This intermediate, known as previtamin D3, spontaneously converts over a 48-hour period into a thermodynamically more stable compound called Vitamin D3 or colecalciferol.
Promotes the absorption of calcium and phosphorus in the intestine.
Increases bone resorption (stimulates osteoclast differentiation).
Increases the ability of parathyroid to reabsorb calcium at the kidney level.
At our latitudes, the amount of sunlight required for the synthesis of vitamin D is relatively low, but in the summer months it is however very important to expose the face and arms to the sun for at least a few minutes a day in order to guarantee an adequate synthesis dermal vitamin D and set aside reserves for the winter. UVB radiation does not penetrate the glass, so exposure to the sun through a window is not functional to the synthesis of vitamin D.
