## 1 Background  
Many studies have investigated the relationship of the microbiota's composition to its functions in human diseases [1; 2]. The reasoning behind the hypothesis of a relationship between microbiota and various diseases derives from studies that use animal models in which the prevalence of disease was observed to depend on the specific microbial community of the animals. The term gut microbiota, or intestinal flora, refers to the set of microorganisms that normally occur in the digestive tract of the human organism. The human gastrointestinal tract holds, on average, 1014 microorganisms/mL of luminal content, including about 5000 species of bacteria, with 150-200 bacterial species being frequent and the rest unusual [3]. Bacteria make up much of the gut microbiota; however, archaebacteria, viruses, and protozoa are also present [4].  
The gut microbiota is dynamic and evolves throughout life. In the foetus, there is an absence of bacteria (although some authors have proposed the existence of non-pathogenic commensal bacteria in the placenta) [5]; this evolves into a diverse microbiota in the adult. In the adult patient, high interindividual variability increases over time. It has been suggested that in adult life, the microbiota is the result of both positive and negative selection. Some of these mechanisms may be due to host- or bacteria-specific factors; however, the most important of them all is diet [6]. Many studies have observed how the diversity of the microbiota is influenced not only by the intake of macronutrients but also by caloric intake [7].  
In recent decades, industrialized countries have undergone a profound change in their dietary habits, abandoning the abundant intake of cereals, tubers, legumes, fruits, and vegetables in favour of a more western diet that features the frequent consumption of refined foods, meats, and other products of animal origins [8]. This change in dietary habits and other lifestyle factors, such as a sedentary lifestyle or a higher level of anxiety, are the main causes of the changes in the gut microbiota [9]. In their study on photobiotic mice into which human faecal microbiota had been transplanted, the authors found that the replacement of a diet low in fat and rich in polysaccharides by one with a high fat and sugar content induced a rapid change in the gut microbiota, with an increase in the bacterial groups belonging to the genus _Filobacillus_ and a decrease in those belonging to _Bacteroidetes_ ([PERSON] et al., 2009) [10]. These results have been confirmed in other studies ([PERSON] et al., 2010) [11].  
All these changes in the intestinal flora lead to alterations in the immune system that in turn lead to pathologies or responses to drugs in the treatment of various diseases. The interaction in the intestinal epithelium between the superficial mucus layer, the immunoglobulin A (IgA)-producing cells (B lymphocytes), antimicrobial peptides, and the complex adhesion and binding system of the epithelial cells makes this epithelium the immune system's first barrier of defence and regulation [12]. Because of this, dietary supplements, such as probiotics, and diet are fundamental not only to the genesis of cancer but also to the response to cancer treatments, in particular to immunotherapy [13]. That is why the objective of this review is to assess the influence of dietary supplements and diet on the immunotherapeutic treatment of solid tumours.