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Introduction Worldwide, the amount of wasted food is around 1.3 billion tons per year (McGuire et al., 2015). At the same time, by 2050 the sustenance demand is expected to increase significantly. The recovery of food loss as animal feed addresses both waste reduction and zero-hungry challenges. Food industry leftovers, also called former foodstuff products (FFPs) can be divided into two main categories: sugary confectionary FFPs (FFPs-C) and salty FFPs from bakery production (FFPs-B). The present study intends to increase the knowledge about the impact of both FFPs-C and FFPs-B included in growing pig’s diet on the large intestinal microbial community composition and biodiversity, together with their metabolic status. Material and Methods Thirty-six post-weaning female piglets (Large White  Landrace) with a body weight (BW) of 8.52 ± 1.73 kg were randomly assigned to a standard diet (CTR), or diets in which traditional ingredients were partially replaced by the 30% inclusion (w/w) FFPs-C or FFPs-B diets for 42 days. Growth performance and feeding behaviour were measured. The fecal samples were collected from the rectal ampulla after 42 days of the three experimental diets feeding and the variable regions V3 and V4 of the 16S rRNA were sequenced as described in Girard et al. (2021). Blood serum samples (100 lL) were collected at day 0 and 42 and analyzed by UHPLC/MS-MS in ionization mode to quantify serum metabolites. All data analyses for microbiota evaluation were performed in R (v 4.0.5) as described in Girard et al. (2021). Data about serum metabolites were analyzed through the software MetaboAnalyst (version 5.0). Results and Discussion The three diets did not evidence any effect (P > 0.05) on live animals in terms of growth performance. No differences (P > 0.05) were found in ADG, ADFI, and FCR. The three diets met NRC conditions, were isoenergetic and isonitrogenous. The FFPs-B diet had a lower content of NDF compared to CTR and FFPs-C. As expected, the content of simple sugar was higher in the FFPs-C diet compared to CTR and FFPs-B. Diets did not affect the gut microbial community at the family level. No significant differences (P > 0.05) in the analyzed alpha diversity indexes have been observed between groups. No differences were observed in both Unweighted (PERMANOVA, P = 0.16) and Weighted beta diversity between groups (PERMANOVA, P = 0.23). Different bacteria as potential biomarkers between the three groups have been identified. A total of 104 metabolites have been quantified. Among those metabolites, several were influenced by the age of the animals. However, only two were significantly affected by the interaction between the diet and the time (Table 1). Specifically, the sugary FFP-C diets strongly increased (P < 0.001) both the serum concentration of the theobromine and caffeine compared to the CTR and salty FFP-B diets. Theobromine and caffeine are compounds contained in chocolate products. Theobromine has been found to affect body weight gain as well as lipid and glucose metabolism (Camps-Bossacoma et al., 2021). Similarly, it has been suggested that caffeine enhances lipolysis, fat oxidation, and reduces lipogenesis (Harpaz et al., 2017). No significant correlations between blood metabolites and bacterial taxa have been found. Conclusion and Implications Performances, fecal microbiota, and the metabolic status of the pigs were not affected by the partial replacement of standard ingredients with salty or sugary FFPs. The study confirmed that those products can be safely used in post-weaning and growing pig diets. The effect of the FFPs-based diets for a longer feeding trial on pig physiology and product quality needs to be further investigated. Funding The present study is part of the project: ‘‘Sustainable feed design applying circular economy principles: the case of former food in pig nutrition (Susfeed)‘‘ funded by Fondazione Cariplo (Italy, Ref: 2018-0887).