<P> Although certain levels of antioxidant vitamins in the diet are required for good health, there is considerable debate on whether antioxidant - rich foods or supplements have anti-disease activity . Moreover, if they are actually beneficial, it is unknown which antioxidant (s) are needed from the diet and in what amounts beyond typical dietary intake . Some authors dispute the hypothesis that antioxidant vitamins could prevent chronic diseases, while others maintain such a possibility is unproved and misguided from the beginning . </P> <P> Polyphenols, which often have antioxidant properties in vitro, are not necessarily antioxidants in vivo due to extensive metabolism . In many polyphenols, the catechol group acts as electron acceptor and is therefore responsible for the antioxidant activity . However, this catechol group undergoes extensive metabolism upon uptake in the human body, for example by catechol - O - methyl transferase, and is therefore no longer able to act as electron acceptor . Many polyphenols may have non-antioxidant roles in minute concentrations that affect cell - to - cell signaling, receptor sensitivity, inflammatory enzyme activity or gene regulation . </P> <P> Although dietary antioxidants have been investigated for potential effects on neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis, these studies have been inconclusive . </P> <P> Tirilazad is an antioxidant steroid derivative that inhibits the lipid peroxidation that is believed to play a key role in neuronal death in stroke and head injury . It demonstrated activity in animal models of stroke, but human trials demonstrated no effect on mortality or other outcomes in subarachnoid haemorrhage and worsened results in ischemic stroke . </P>

Which of the following is not part of the group of vitamin supplements called antioxidants