From November to March, the UVB intensity is insufficient for the synthesis of vitamin D by the skin. Where fatigue often felt in the heart of winter.
For healthy people, insufficient vitamin D is spoken when the blood concentration falls below 20 nanograms (20 billionths of a gram) per milliliter of blood. "All the experts agree that it's a decent minimum," says Professor Jean-Claude Souberbielle, researcher at the Necker Hospital in Paris.
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