Vitamin C se intenzivno istražuje kao dodatak prehrani zbog svoje antioksidativne aktivnosti te ostalih utjecaja na homeostazu i zaštitu od slobodnih radikala. Upravo zbog dokazanih blagotvornih učinaka mnogi ljudi konzumiraju količine vitamina C mnogo veće od RDA. Definirana je gornja granica unosa vitamina C od < 2000 mg za odrasle na dan (sigurna za većinu osoba). Dodaci prehrani koji sadrže vitamin C su jedni od najčešće korištenih, a dokazi o sigurnosti vitamina C u dozama manjim od 2000 mg/dan su konzistentni (53). Literaturni podaci ukazuju da se količina apsorbiranog željeza povećava povećanjem unosa vitamina C, osobito kod unosa s 25-50 mg/dan. Iznad toga, vitamin C ima vrlo mali učinak na količinu apsorbiranog željeza. Većina objavljenih studija snažno ukazuje da vitamin C u dozama do 2000 mg/dan ne povećava skladišta tjelesnog željeza u mjeri da bi to izazvalo bilo kakve klinički značajne neželjene učinke (53).

Neželjeni učinci uzimanja vitamina C:

• Nuspojave se obično javljaju nakon dugotrajnog uzimanja velikih doza vitamina C (doze veće od 1 g na dan mogu izazvati proljev, a doze veće og 600 mg mogu blago pojačati izlučivanje mokraće).
• Izrazito visoke doze vitamina C mogu biti "otimači" bakra iz organizma, a poznato je da je bakar esencijalni nutrijent. Stoga je preporuka svim osobama koje uzimaju izrazito visoke doze vitamina C da pritom osiguraju i adekvatan unos bakra.
• Budući da vitamin C povećava apsorpciju željeza, povišene doze vitamina C trebaju izbjegavati osobe koje imaju bolesti povezane s prekomjernim nakupljanjem željeza (primjerice: hemokromatoza, hemosideroza).
• Simptomi trovanja vitaminom C uključuju glavobolju, pojačano mokrenje, grčeve u donjem dijelu trbuha, proljev, povraćanje, vrtoglavicu, drhtavicu.
• Smatra se da osobe koje su sklone nastanku bubrežnih kamenaca (u formi kalcij - oksalata) trebaju izbjegavati dodatke vitamina C jer se ovaj vitamin u tijelu može pretvoriti u oksalat i povisiti razinu oksalata u urinu.

Interakcije s lijekovima

• Dugotrajna upotreba acetilsalicilne kiseline može uzrokovati nedostatak vitamina C. Stoga se osobama na dugotrajnoj terapiji acetilsalicilnom kiselinom preporučuje prehrana bogata voćem, povrćem, nemasnim mesom i morskim plodovima te cjelovitim žitaricama i mahunarkama (54). Ipak, mega doze vitamina C nisu dobrodošle uz acetilsalicilnu kiselinu, budući da visoka razina vitamina C zakiseljava urin, što otežava izlučivanje acetilsalicilne kiseline iz organizma te se tada lijek zadržava u organizmu dulje nego što je potrebno.
• Nedostatak vitamina C može se pojaviti kod uzimanja tetraciklinskih antibiotika, te je prilikom terapije potrebno osigurati prehranu bogatu vitaminom C.
• Oralni kontraceptivi koji sadrže estrogene mogu utjecati na sniženje razine vitamina C u plazmi i leukocitima, te je stoga uputno uzimati dodatni vitamin C tijekom uzimanja oralnih kontraceptiva.
• Velike doze vitamina C mogu blokirati djelovanje antikoagulansa varfarina, što zahtjeva povišenje doze da bi s održala učinkovitost. Osobe koje uzimaju antikoagulanse trebaju ograničiti unos vitamina C na 1 gram dnevno i liječnik treba pratiti njihovo protrombinsko vrijeme.
• Budući da visoke doze vitamina C interferiraju s interpretiranjem rezultata nekih laboratorijskih pretraga (glukoza u mokraći, etilestradiol, jetrene transaminaze, laktat dehidrogenaza u serumu, okultno krvarenje u stolici, bilirubin u serumu, željezo i feritin u plazmi, pH mokraće, mokraćna kiselina, oksalati) važno je obavijestiti liječnika o tome uzimate li dodatke vitamina C (55).

 

Važne činjenice o vitaminu C

• Vitamin C je vitamin topljiv u vodi
• Ljudski organizam ga ne može sintetizirati te se mora unositi hranom ili, u slučaju povećanih potreba, dodacima prehrani.
• U organizmu, vitamin C igra vrlo važne uloge: neophodan je u sintezi kolagena, pomaže funkciju imunološkog sustava, pomaže apsorpciju željeza, pretvara folnu kiselinu u njezin aktivni oblik, itd.
• Blagotvorni učinci vitamina C dokazani su u smislu skraćenja trajanja prehlade i ublažavanja simptoma.
• Objavljeni podaci pokazuju značajne zaštitne učinke vitamina C na karcinom dojke, želuca, jednjaka, gušterače, pluća, cerviksa i rektuma.
• Pojava kardiovaskularnih bolesti obrnuto je proporcionalna s unosom vitamina C i plazmatskim koncentracijama vitamina C
• Vitamin C je jedan od najsnažnijih antioksidansa
• Doze od 10 mg/dan vitamina C dovoljne su da preveniraju skorbut
• I u dozama do 2000 mg/dan vitamin C je siguran za većinu osoba

 

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