## 3 Discussion  
In this study, we identified an integrated set of GM, metabolomic and body composition features associated with therapeutic response in patients with advanced melanoma (Figure 6). Interestingly, these signatures were independent of the immunotherapy or targeted therapy received, and they likely represented common prognostic biomarkers. In particular, all patients subsequently classified as responders were obese, non-sarcopenia, and enriched in certain fecal taxa and metabolites prior to initiation of therapy. They also had higher levels of adipose tissue compared to non-responders and, as expected, lower levels of NLR, which is an inflammatory biomarker of poor prognosis [17], as well as lower counts of neutrophils and MDSCs, which are immune cell subsets associated with melanoma progression [18; 19]. In particular, both MDSCs and tumor-infiltrating neutrophils are thought to exert immunosuppressive activity, as well as promote angiogenesis and tumor growth, thus contributing to immunotherapy resistance [20; 21]. Figure 5: Gut microbiota and metabolome differences at baseline in advanced melanoma patients in relation to the presence of sarcopenia. Boxplots showing the distribution of the relative abundance of taxa (**A**) and the relative concentration (\(\mu\)g/g of internal standard) of metabolites (**B**) differentially represented between sarcopenia and non-sarcopenia patients. Wilcoxon test, * for \(p\leq 0.05\), ** for \(p<0.01\). Although seemingly paradoxical, as discussed above, the relationship with obesity is not entirely surprising, as BMI has recently been found to be associated with better outcomes (namely, progression-free survival and overall survival) in patients with metastatic melanoma treated with targeted therapy or immunotherapy, which typically do not induce weight loss. The survival advantage in obese patients may be explained based on reverse causality, wherein patients with more aggressive disease have previous weight loss and BMI reduction, as well as enhanced \"metabolic reserve\" to withstand the wasting effects of cancer or its associated treatments [22]. Our study confirms, but also extends, this evidence by showing that therapeutic response is also associated with body composition parameters, such as TAT and VATSAT. Other authors have highlighted the potential link between SAT and sensitivity to PD-1/PD-L1 inhibition [23; 24]. Furthermore, it should be remembered that SAT is the compartment responsible for leptin production [25]. As discussed by [PERSON] et al. [26], obese patients would have an overexpression of PD-1, which is mainly expressed by T cells, due to the high leptin levels typical of obesity. Excess PD-1 would make T cells highly responsive to PD-1 inhibitors, paradoxically enhancing cancer responsiveness after anti-PD-1 therapy. However, it remains unclear why the association with obesity is not found in the chemotherapy setting, as well as how obesity-induced low-grade inflammation and immunosuppression could be an advantage [26; 27]. Even the data on sarcopenia, although recently debated [16], are partly expected, as loss of skeletal  
Figure 6: Summary of the study’s key findings. Thirty-one patients with unresectable IIIC-IV-stage cutaneous melanoma were characterized prior to initiation of targeted or first-line immunotherapy, anthropometry, body composition, nutritional status, physical activity, biochemical parameters, immunoprofiling, and fecal microbiome and metabolome. Patients subsequently classified as responders were found to be obese (with high body mass index (BMI) and high levels of total adipose tissue (TAT), visceral adipose tissue (VAT), subcutaneous adipose tissue (SAT), and intramuscular adipose tissue (IMAT)), non-sarcopenic, and enriched in certain fecal taxa (e.g., _Phascalorobacterium_) and metabolites (e.g., anethole), which were potentially endowed with immunostimulatory and oncoprotein activities. They also had a lower neutrophil-to-lymphocyte ratio, which is an inflammatory biomarker of poor prognosis.