Introduction: Diet may influence various aspects of human health. In fact, it is well known that diet can favour or not the development of various human pathologies, like diabetes, hypertension, and hypercholesterolaemia. Interestingly, diet has an influence in cancer development too (e.g., this relation has been studied for pancreatic, colonic, gastric, and breast cancers). Between the mechanisms that could explain this relation, there is epigenetic. In fact, thanks to epigenetic reprogramming, certain substances introduced with diet could affect gene expression, especially of those genes involved in cells’ proliferation and growth. In recent years, some studies have been published about the role that diet could have on chemotherapy outcome. Especially, various studies have analysed the effects of fasting and ketogenic diet (KD) during chemotherapy. The aim of this study is to summarize scientific evidences about diet and its effects on chemotherapy on humans and to better understand if these approaches deserve to be further investigated and might be suitable and beneficial during cancer treatment. Materials and Methods: We performed an electronic literature search of the PubMed database, using the combination of following terms: “fasting” or “ketogenic” with “chemotherapy,” “cancer treatment.” We included studies on humans about fasting and KD during chemotherapy, excluding reviews, case series including <10 patients, studies conducted on animals or limited to radiotherapy treatment, and studies that were mostly about molecular mechanisms. Results/Discussion In our analysis we included 4 studies (1 randomized controlled trial, 1 retrospective study, and 2 prospective pilot studies) about KD and 4 studies (1 prospective cohort study, 1 case series report, and 2 randomized trials) about fasting during oncological treatments. Authors suggested an improvement of quality of life (QoL) and fatigue in patients under chemotherapy, especially in the 8 days after chemotherapy treatment. We found that both fasting and KD demonstrated to be tolerable and feasible during oncological treatments. Conversely, data about survival outcomes are still controversial, but it should be underlined that it was not the outcome of these preliminary studies. Conclusions: All comparatives studies have demonstrated that even fasting then KD results in a reduction of collateral effects of adjuvant chemotherapy (due to reduction of drugs toxicity) and a better QoL than in patients that follow no diet. Unfortunately, despite the fact that various laboratory and animal studies confirm advantages from KD and fasting, few data are today disposable on humans: further studies are needed to confirm data exposed in this review.