Vitamin D

Zusammenfassung

Vitamin D nimmt unter den Vitaminen eine Sonderstellung ein, da es auch endogen aus 7-Dehydrocholesterol in der Haut durch UV-Strahlung gebildet werden kann. Auf alimentäre Vitamin-D-Quellen ist man erst angewiesen, wenn die endogene Synthese an Vitamin D nicht ausreicht. Zahlreiche Faktoren können die körpereigene Bildung von Vitamin D beeinträchtigen. Dazu zählen u. a. zu wenig Aufenthalt im Freien, geringe UV-Intensität in den Wintermonaten, eine starke Hautpigmentierung, Sonnenschutzmittel, Bekleidung, die keine adäquate UV-Exposition der Haut zulässt sowie die nachlassende Bildung von 7-Dehydrocholesterol im Alter. Da unsere Lebensmittel nur sehr wenig Vitamin D enthalten (Ausnahme: fettreicher Seefisch), können viele Menschen ihr endogenes Vitamin-D-Defizit nicht über die Ernährung ausgleichen und sind daher auf Vitamin-D-Supplemente angewiesen. Ein Biomarker für den Vitamin-D-Versorgungsstatus ist das 25-Hydroxyvitamin D. Allerdings herrscht noch Uneinigkeit über die Höhe des optimalen 25-Hydroxyvitamin-D-Spiegels der zur Minimierung von Krankheitsrisiken erforderlich ist. Derzeit sind beim Menschen genomweit 2776 Vitamin-D-Rezeptor-Bindungsstellen und 229 Gene identifiziert, die durch aktives Vitamin-D-Hormon reguliert werden. Das Wirkungsspektrum von Vitamin D reicht damit weit über die Regulation des Kalzium- und Phosphatstoffwechsels hinaus. Studien haben gezeigt, dass Vitamin D auch vaskuläre Zellen sowie Immunzellen beeinflusst. Hinweise mehren sich, dass nicht nur toxische Vitamin-D-Mengen, sondern auch eine unzureichende Vitamin-D-Versorgung die vaskuläre Kalzifizierung fördern. Des Weiteren scheint Vitamin D die Immuntoleranz zu fördern. Die ungebremste Entwicklung von T-Helferzellen 1 bei einer unzureichenden Vitamin-D-Versorgung wird derzeit mit dem vermehrten Auftreten von Autoimmunerkrankungen in Verbindung gebracht. Auf der anderen Seite wird diskutiert, ob Vitamin D auch das Risiko für Allergien beeinflussen kann. Die Studienergebnisse sind dazu aber äußerst widersprüchlich. Obgleich sich für die Prävention der vaskulären Kalzifizierung und Allergie keine konkreten Empfehlungen ableiten lassen, sollten zumindest im Hinblick auf die Knochengesundheit 25-Hydroxyvitamin-D-Spiegel von mindestens 50 nmol/l angestrebt werden.

Abstract

Vitamin D occupies a special position among vitamins as it can be synthesized in skin from 7-dehydrocholesterol in response to UV-radiation. Dietary sources of vitamin D are only necessary in cases of inadequate vitamin D synthesis in skin. A series of factors adversely affect the cutaneous vitamin D synthesis. These are limited outdoor activities, seasonal variations of UV radiation, dark skin pigmentation, use of sunscreens, clothing which does not enable UV exposure, and aging. Since vitamin D is found naturally in only a few foods (except fatty fish from the sea), many individuals depend on supplements as a source of vitamin D. The plasma concentration of 25-hydroxyvitamin D is used as a biomarker for vitamin D status. However, the optimum levels of circulating 25-hydroxyvitamin D for minimization of diseases are still uncertain. Currently, 2776 genomic positions are identified in humans that are capable of binding vitamin D receptor, and 229 genes are found to be transcriptionally regulated by vitamin D. Besides the central role of vitamin D for regulation of calcium and phosphate metabolism, vitamin D appears to have many other functions. Specific vascular and immune cells are supposed to be controlled by vitamin D. Hypervitaminosis D has long been known to cause vascular calcification. Recent data also reveal calcium deposits in the arterial blood vessels in response to an inadequate vitamin D status. Vitamin D has further been shown to modulate immune system by stimulating the immune tolerance. Regulatory T cells and T helper cells are in particular under the control of vitamin D, and vitamin D deficiency has been assumed to promote the development of T helper cells type 1 in favor of other T cells. This alteration in T cell balance has been linked to the development of autoimmune diseases. The proposed loss of immune tolerance in the absence of vitamin D is also suggested to modulate the allergy risk. However, allergy data from vitamin D studies are inconsistent, and specific recommendations for vitamin D to reduce the risk of these diseases cannot be made at present. However, despite the uncertainties, it is generally agreed that the plasma concentrations of 25-hydroxyvitamin D should be, at least, 50 nmol/l to reduce the risk of fractures and falls.

Schlüsselwörter

Vitamin-D-Status - Biomarker - Vitamin-D-Synthese - alimentäre Versorgung - vaskuläre Kalzifizierung - Allergien

Keywords

vitamin D status - biomarker - vitamin D synthesis - nutrition - vascular calcification - allergies

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