Ultra-processed foods (UPF) are known for their unbalanced nutrient profile and for being associated with unfavorable health outcomes in human. However, UPF are currently dominating global food systems, generating high amounts of waste along the food chain. Recycling UPF as feedstuff represents a way for reducing UPF waste and for replacing human-edible grains to reduce feed-food competition, making livestock production more sustainable. Monogastric such as pigs are the best candidates for consuming UPF waste given their human-like digestive tract. In a previous trial, 36 post-weaning piglets (28 day old, 6.5 ± 1.1kg) were assigned to three isoenergetic and isonitrogenous diets for 42 days: i) CTR, control (common grains), ii) salty (SA), iii) sugary (SU). For SA and SU diets, the 30% of grains was replaced with former food products (FFP), which are food leftovers falling in the UPF category. The same experiment was conducted on 36 growing pigs (22.4 ± 1.7 kg) for 110 days until slaughter (110 ± 3 kg). After observing that FFP inclusion did not cause any detrimental effect in both cases, we conducted an in-depth evaluation of the metabolic status of FFP-fed animals using omics-based approaches, focusing on plasma metabolome in piglets, and liver proteome and plasma peptidome in pigs. Mass spectrometry-based untargeted approaches were performed on plasma samples collected from piglets at trial onset and end and on liver tissue samples collected from pigs after slaughter. The piglets’ plasma metabolome was more affected by time than SA and SU diets (36 vs 2 metabolites modulated, respectively), with only caffeine and theobromine significantly increased in SA and SU groups compared to CTR. Caffeine and theobromine were not correlated with any other metabolic perturbation and since they are present in cocoa and coffee, it is possible that FFP could contain cocoa and coffee-containing ingredients, thus explaining their higher plasma level in SA and SU groups. Considering pigs, the SA and SU diets significantly modulated few liver proteins (125 out of 2881 total identified) mostly related to lipid metabolism and cellular structure but with no metabolic interaction among them, indicating a limited impact of FFP on liver proteome. Plasma peptidomes were not significantly different among groups, although three peptides related to vascular homeostasis and anti-hypertension were exclusively identified in the SA group, suggesting a potential self-regulating response to face the higher Na content of the SA diet. To conclude, the 30% dietary inclusion of FFPs in piglet and pig diets did not lead to significant unintended effects on metabolism assessed at multiple levels, thus preserving the wellbeing of FFP-fed animals. Therefore, a wider application of FFP as feed ingredients is advisable to reduce the environmental impact of pig production, although further research is needed to strengthen all these outcomes.