VITAMINA D: a cosa serve e alimenti che la contengono
Table of Contents
Definizione
La vitamina D comporta una decina di sostanze ad attività variabili delle quali le più importanti dal punto di vista dietetico sono l’ ergocalciferolo (calciferolo, ergosterolo,vitamina D2) e il colecalciferolo (colecalciferolo o vitamina D3). L’ergocalciferolo è una vitamina D naturale di origine vegetale mentre il colecalciferolo è di origine animale. La parte più importante di vitamina D è fornita dalla sintesi endogena a livello della cute, sotto l’influenza della luce.
Funzioni nell’organismo
La principale funzione della vitamina D riguarda il mantenimento della concentrazione fisiologica ematica di calcio e di fosfato.
Intestino: la principale funzione biologica è quella di aumentare l’efficacia dell’ assorbimento del calcio a livello intestinale.
Osso: il calcitriolo analogamente al paratormone, aumenta l’attività degli osteoclasti, le cellule dell’osso capaci di solubilizzare l’idrossiapatite.
Rene: il calcitriolo stimola il riassorbimento del calcio a livello dei tubuli distali.
Carenza
Una deficienza di vitamina D nella dieta, associata ad insufficiente esposizione ai raggi ultravioletti, provoca rachitismo nei bambini ed osteomalacia negli adulti. Il rachitismo colpisce i bambini nei primi anni di vita. La lesione caratteristica è la deficiente calcificazione delle ossa; i ridotti livelli ematici di calcio, infatti, ostacolano la normale deposizione dei cristalli di idrossiapatite nelle zone di mineralizzazione. Tuttavia, anche in assenza di tale deposizione, gli osteociti e gli osteoblasti continuano a produrre una matrice organica approssimativamente normale, costituita da collageno e mucopolisaccaridi. Di conseguenza nel bambino si ha la formazione di strutture ossee cedevoli che, sottoposte a carico, tendono a deformarsi.
Nell’ osteomalacia non si ha deformità ossea, ma osteopenia con aumentato rischio di frattura e un diffuso dolore osseo.
L’osteomalacia non va confusa con l’osteoporosi. La prima, come il rachitismo, consiste in una diminuzione della sola componente minerale delle ossa; l’osteoporosi risulta invece in una globale diminuzione di osso che riguarda sia la componente minerale che la componente organica e cellulare.
Forme familiari di rachitismo, resistente alla vitamina D, sono dovuti all’incapacità di convertire la D3 in calcitriolo per alterazione congenita dell’1∝– idrossilasi renale.
Fonti alimentari e fabbisogno
Le principali fonti alimentari sono costituite da:
- Pesci di acqua salata
- Sardine
- Salmone
- Tonno
- Aringa
- Oli di pesce
La vitamina D è anche presente in:
- Uova
- Carne
- Burro
In caso di mancata esposizione alla luce solare o di aumentato fabbisogno (gravidanza e allattamento) l’assunzione giornaliera per gli adulti dovrebbe essere di 10-15μg.
Eccesso di vitamina D
Un eccesso di vitamina porta aumentati livelli sierici di calcio che nei casi più gravi porta a calcificazione anomala di tessuti molli (rene, cuore). Quantità tossiche non vengono raggiunte con una dieta normale ma solo conseguenza di un eccessivo uso di supplementi vitaminici.
Compromissione reni, vomito, diarrea, calo di peso.
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