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Im Gegensatz zu anderen Vitaminen kann der Körper Vitamin D mithilfe der Sonne selbst herstellen. Aber wie funktioniert das eigentlich? Wie lange sollte ein Sonnenbad dauern? Und beeinflusst Sonnencreme die Vitamin-D-Synthese negativ?
Vitamin D ist kein essenzielles Vitamin. Der Körper muss es nicht zwangsläufig über die Nahrung aufnehmen. Durch Sonneneinstrahlung stellt die Haut das wichtige Vitamin stattdessen selbst her. Mithilfe der UV-B-Strahlung wird in diesem Prozess eine Vorstufe des Vitamins, das Provitamin D3, zunächst in Prävitamin D3 und schließlich in Vitamin D3 umgewandelt. Auf diese Weise kann der Körper unter optimalen Bedingungen bis zu 90 Prozent des Vitamin-D-Bedarfs decken.
Für eine ausreichende Versorgung mit Vitamin D genügt es, Hände, Arme und Gesicht etwa dreimal pro Woche der Sonne auszusetzen. Als optimale Dauer gilt in etwa die Hälfte der Zeitspanne, in der die Haut ungeschützt einen Sonnenbrand bekäme. Diese sogenannte minimale erythemwirksame Dosis (MED) ist individuell verschieden. Zudem hängt die Vitamin-D-Synthese durch Sonnenlicht von weiteren Faktoren ab:
Die optimale Expositionsdauer schwankt aufgrund dieser Einflussfaktoren zwischen maximal 10 und bis zu 30 Minuten.
Schon ein geringer Lichtschutzfaktor von 8 bis 14 schränkt die Vitamin-D-Synthese stark ein. Aus diesem Grund sollte die Haut der Sonne je nach Hauttyp bis zu 30 Minuten ohne Sonnenschutzmittel ausgesetzt werden. Je gebräunter sie ist, desto länger hält sie ungeschützter Sonneneinstrahlung stand.
Wichtig: Die Sonne sorgt nicht nur für volle Vitamin-D-Speicher. Ein längerer ungeschützter Aufenthalt in der Sonne verursacht ernsthafte Hautschäden. Ein Sonnenbrand sollte unbedingt vermieden werden.
Das über die Haut gebildete Vitamin D wird im menschlichen Organismus gespeichert. Im Körper liegt es hauptsächlich in zwei verschiedenen Formen vor: als Calcitriol und als Calcidiol. Während es sich beim Calcitriol um die aktive Form des Sonnenvitamins handelt, bezeichnet Calcidiol (auch Calcifediol) die inaktive Speicherform des Vitamins. Insbesondere in den Sommermonaten füllt der Körper die Vitamin-D-Speicher mithilfe des Sonnenlichts auf.
Bekommt die Haut über einen längeren Zeitraum zu wenig Sonnenlicht, verbraucht der Körper diese Reserven. Der Vitamin-D-Spiegel sinkt langsam ab. Ursächlich können zum Beispiel der tiefe Stand der Wintersonne, großflächige Kleidungsstücke wie Jacken, lange Hosen und Handschuhe oder der überwiegende Aufenthalt in geschlossenen Räumen sein.
Da der Vitamin-D-Bedarf nicht allein über die Nahrung gedeckt werden kann, helfen Nahrungsergänzungsmittel bei einem diagnostizierten Vitamin-D-Mangel, die Speicher wieder aufzufüllen. Sie sind in Form von Tropfen, Kapseln und Lutschtabletten erhältlich.