The role of dietary nucleotides in single-stomached animals

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Abbreviations: CMP, cytosine 5'-monophosphate; IMP, inosine 5'-monophosphate; UMP, uridine 5'-monophosphate

Introduction

Until 2006, in-feed antibiotics were used to overcome weaning-associated problems in pig production. Nowadays, an interest in antibiotic-free animal production has been established worldwide due to the risk of antibiotic resistance in humans⁽¹⁾. As weaning is accompanied by morphological, histological and microbial changes in the gastrointestinal tract of young mammals^(2,3), nutritional strategies are being developed to aid immature animals to overcome weaning-associated problems and withstand infectious diseases. Among others, dietary nucleotides are one group of bioactive agents which may have the potential to diminish challenges related to weaning⁽⁴⁾. Nucleotides are naturally present in all foods of animal and vegetable origin⁽⁵⁾ but in lower concentrations than in mammalian milk⁽⁶⁾. Soluble nucleotides have been determined in milk from various mammals, contributing to as much as 20 % of its non-protein fraction⁽⁷⁾. The nucleotide pattern in milk is species-specific, and concentrations vary with stage of lactation. Nucleotides and their metabolic products play key roles in many biological processes, for example, as monomeric units of nucleic acids, in transferring chemical energy, in biosynthetic pathways, as biological regulators and as coenzyme components⁽⁸⁾. Although nucleotides are synthesised endogenously, there is also evidence that dietary supplementation with nucleotides may exert beneficial effects both in humans and animals (for reviews, see Carver & Walker⁽⁹⁾ and Taylor-Pickard⁽¹⁰⁾). In animals, including pigs and rodents, positive effects on small-intestinal growth and development⁽¹¹⁾, on intestinal hyperaemia⁽¹²⁾, on stimulation of systemic immunity⁽¹³⁾ and on hepatic composition⁽¹⁴⁾ have been described. In addition, a protective role in the prevention of diarrhoea in pigs has been reported by Martinez-Puig et al.⁽¹⁵⁾. Accordingly, Gil et al.⁽¹⁶⁾ described in human subjects beneficial effects of supplemental soluble nucleotides on the composition of the faecal microbiota, as measured by an increase in bifidobacteria and lactobacilli. Inconsistent results, however, have been found for small-intestinal disaccharidase activity as influenced by nucleotide supplementation⁽