## 1 Introduction  
Global climate change influences people, ecosystems, and livelihoods across the world. However, the philosophy of sustainability also espouses broader principles that support the just treatment of farm workers and food pricing to provide the farmer with a liveable income [1]. To ensure these principles, the effective valorisation of food industry by-products has become very important. Our previous studies have shown that dairy industry by-product milk permeate (MP) could be effectively valorised to antimicrobial functional properties possessing ingredients for humans and animal nutrition [2]. MP is usually obtained after removing milk protein and fat by an ultrafiltration process [3]. This by-product contains unmodified lactose, minerals, and serum proteins. However, fermentation with selected lactic acid bacteria (LAB) strains can improve the physicochemical, antimicrobial, functional, and sensory characteristics of MP [4]. During the fermentation process, lactose in MP is converted to functional compounds, including galactooligosaccharides (GOSs) and high concentrations of viable LAB. This process results in animal feed with GOS-enriched antimicrobial properties [2,5]. For this reason, we hypothesised that fermented MP is a prospective feed supplement for newborn calves. Probiotics are widely used in animal feed supplements [6] for a variety of purposes, one of which is their ability to protect against infections caused by pathogenic bacteria [7,8]. Because probiotics promote the establishment of a beneficial gut flora and inhibit the growth of pathogenic bacteria in the intestine, they are expected to improve livestock health [9,10]. Probiotics for livestock have been studied using many different LAB strains individually and in combination [11, 12, 13]. In this study, we focused on LAB strains with antimicrobial properties used for MP, with a view to probiotics potentially reducing or replacing the use of antibiotics in animal husbandry [14]. Diarrhoea is one of the most common health problems in young dairy calves [15], and the use of probiotics has been proposed as an alternative for the prevention and alleviation of intestinal disorders, as well as immunomodulators [16]. Another promising approach is the use of prebiotics in the production of healthier feeds [17]. Prebiotics are nondigestible compounds that have a positive effect on the host by promoting the growth and activity of one or more bacteria in the colon [18]. Nondigestible oligosaccharides (NDOs) are the most promising prebiotics; this group include GOSs, which could be used in animal production as a potential alternative to antimicrobial growth promoters [19]. They participate in the formation of a healthier microbiota in which lactobacilli predominate and can exert health-promoting effects at the expense of more harmful organisms [20]. Finally, probiotic and prebiotic supplements have the greatest impact in the first weeks or months of a calf's life [16,21]. This may lead to the instability of their microbial populations, while the microbiota later in life is more stable and difficult to affect [22]. Because the gut microbiome is a key factor affecting gut health [23], the use of microbial products early in life could become a preferential treatment for improving calf health [24]. For this study, we conducted a 14-day experiment with newborn calves. An experimental group was given fermented MP containing both prebiotics (GOSs) and an LAB strain with viable antimicrobial properties. Although at the present time there is much information available on the digestion and absorption processes of the major macro- and micronutrients [25], there is still a wide gap in the literature when it comes to biomarkers for indicating the health status of young animals. Biomarkers could show how well the gastrointestinal tract performs its essential digestive and absorptive functions [26].