Awareness of the need to improve the sustainability of livestock by reducing the loss of natural resources has increased significantly. This study investigated the effects of two categories of food industry leftovers, also referred to as former foodstuff products (FFPs), on pig gut microbiota and intestinal volatile fatty acid (VFA) production. Thirty-six female postweaning piglets (28 days old, Large White
Landrace, 6.5 ± 1.1 kg) were separated into three groups and fed a conventional diet (CTR), and diets in which cereals were partially replaced (30% w/w) by sugary confectionery products (FFPs-C) or salty bakery products (FFPs-B), respectively. After 42 days of dietary treatments, faeces were collected from the rectal ampulla, snap-frozen, and used for next-generation sequencing to analyse the composition and the alpha and beta diversity indexes of the microbial population. The concentration of VFAs in the intestinal content collected at the slaughterhouse was also analysed. The study demonstrated that balanced diets can be obtained by the inclusion of both FFPs-C and FFPs-B, with a similar chemical composition compared to traditional diets. Neither the FFPs-C nor FFPs-B diets affected the abundance and biodiversity indexes of the microbial community. Only a few taxa, normally attributed to a healthy gut, increased with FFPs-C and FFPs-B compared to the CTR. The experimental diets had no impact on the production of the VFAs in the faeces. Lastly, the inclusion at 30% (w/w) of both categories of FFP diets slightly affected the faecal microbiota. FFPs could thus be used as a promising alternative to traditional ingredients in pig diets; however, additional analyses are needed to further investigate the presence of potentially pathogenic bacteria. The effects of such ingredients on other markers of gut health, and on product quality when used in the fattening period also need to be investigated.