Although the chemoprevention and anti-cancer activities of dietary polyphenols have been evidenced through both in vitro and in vivo studies, most of the human clinical trials were unsuccessful or even harmful. Debates on the beneficial roles of dietary polyphenols in cancer therapy are increasing. Many dietary polyphenols studies are conducted by in vitro experiments, but the nature of these studies does not consider the complexity of metabolic processes that are present in vivo. These can often cause instability in the dietary polyphenols, thereby leading to unsuccessful extrapolation into animal or human studies. Dietary polyphenols often have low bioavailability, which is mainly due to poor bioaccessibility and significant metabolism mediated by both host enzymes and colon microbiota. Some metabolites or catabolites are more potent and absorb better than the parent component. It is recognised that the oral bioavailability of dietary polyphenols is underestimated when the bioactive metabolites or catabolites are not considered. Notably, dietary polyphenols and their metabolites undergo further cellular metabolism within the cancer cells, which confers “cellular bioavailability” as an additional step to influence the actions of dietary polyphenols. Moreover, there are growing controversies in using dietary polyphenols for both chemopreventive and anti-cancer applications. A clear therapeutic window for dietary polyphenols as specific chemopreventive or chemotherapeutic agents is required. This review, thus, aims to identify key issues that were ignored by most of the studies, or are critical for future investigation.

尽管饮食多酚的化学预防和抗癌活性已通过体外和体内研究得到证实，但大多数人类临床试验均未成功甚至有害。关于饮食中多酚在癌症治疗中的有益作用的争论正在增加。许多饮食多酚研究是由进行体外实验，但这些研究的性质，不考虑所存在的代谢过程的复杂性在体内。这些通常会导致饮食中的多酚不稳定，从而导致无法在动物或人体研究中进行推断。饮食中的多酚通常具有较低的生物利用度，这主要是由于生物利用度差以及宿主酶和结肠微生物群介导的新陈代谢显着。一些代谢物或分解代谢物比母体成分更有效并且吸收更好。公认的是，当不考虑生物活性代谢物或分解代谢物时，饮食中多酚的口服生物利用度会被低估。值得注意的是，饮食中的多酚及其代谢产物会在癌细胞内进一步发生细胞代谢，从而赋予“细胞生物利用度”作为影响饮食中多酚作用的附加步骤。此外，在饮食中使用多酚进行化学预防和抗癌治疗的争议越来越大。需要饮食中多酚作为特定的化学预防剂或化学治疗剂的明确治疗窗口。因此，本次审查旨在确定大多数研究忽略的关键问题，或者这些问题对于未来的研究至关重要。