Natural antioxidants such as fucoxanthine (FX) and phlorotannin (PhT) are known for their bioactivities and potential health benefits. However, both suffer from poor water solubility which limits their bioavailability.… In this study, nanoencapsulation with polyvinylpyrrolidone (PVP) was utilized as a nano-enabling technique to improve the bioaccessibility of both FX and PhT, which also bring the benefit of protecting the antioxidants against unwanted oxidation. Different PVP Loading ratios were investigated to evaluate the impact on bioavailability, and it was found that for both FX and PhT, 1:8 loading ratio (payload:PVP) was optimal in bring about the best performance in terms of encapsulation efficiency (EE) and loading capacity (LV). Dynamics release profile of PhT from PhT-PVP nanocomplexes in simulated gastrointestinal fluids was characterized, and it was shown that these nanocomplexes were non-toxic against HaCaT keratinocytes in vitro, while effectively reducing endogenous oxygen species (ROS) to reduce oxidative stress-induced cell death. Meanwhile, FX-PVP nanocomplexes were shown to effectively transport into Caco-2 colon cancer cells and deliver high dosage of FX, which, at high dosage, became pro-oxidant, and indeed accelerated H2O2-induced cell death in caco-2 cells. Apparently, by carefully designing and utilizing nano-enabled encapsulation techniques, natural antioxidants can be manipulated effectively to behave as either anti- or pro-oxidant to achieve desirable health-improving results (either reducing oxidative-stress induced cell death, or promote it for cancer cells). The technique has the potential to become an effective means for producing functional foods and therapeutic out of natural ingredients.