Fermentation in the gut to prolong satiety : exploring mechanisms by which dietary fibres affect satiety in pigs

Obesity has become a major health problem in humans and companion animals. Although obesity is not common in farm animals, food restriction is often used to maintain low feeding costs and performance of, for instance, pregnant sows and fattening pigs. Food restriction may result in hunger and increased feeding motivation, which are associated with behavioural problems. Knowledge on the regulation of satiety is thus crucial to aid in the control of food intake in humans, and to improve welfare in food-restricted farm animals. Dietary fibres are believed to enhance satiety, but the effectiveness varies with the physicochemical properties of the fibre sources concerned. Therefore, the objective of this thesis was to identify whether and how dietary fibres with different physicochemical properties, such as bulkiness, viscosity, gelling and fermentability, affect satiety in the domestic pig, which was used both as a model for humans and as a target animal. In a study focusing on behavioural measures of satiety, pectin (viscous fibre) was the least satiating, whereas lignocellulose (bulking fibre) and resistant starch (fermentable fibre) were the most satiating fibres tested. In a subsequent study, increasing levels of guar gum, inulin, and resistant starch (all fermentable fibres), when replacing digestible starch, enhanced satiety throughout the day. Resistant starch was the most satiating fibre among all fibres tested, and used, in a subsequent study, to assess possible physiological and molecular mechanisms by which fermentation may affect satiety. Also in this study, resistant starch appeared to enhance satiety based on behavioural observations, i.e. reduced feeder-directed and drinking behaviours during 24 h. As expected, the satiating effects of resistant starch coincided with increased 24 h plasma short-chain fatty acids (SCFA) levels and decreased postprandial glucose and insulin plasma levels. Glucagon-like peptide-1 (GLP-1) plasma levels were lower in pigs fed resistant starch, whereas peptide tyrosine tyrosine (PYY) plasma levels were not affected by resistant starch, suggesting that these hormones do not play a role in the increased satiety induced by fermentation. Resistant starch consumption led to downregulation of genes involved in immune responses, and upregulation of genes involved in metabolic processes such as fatty acid and energy metabolism in the proximal colon. Moreover, correlation analysis inversely linked potential pathogenic microbial groups with plasma SCFA concentrations and with genes involved in fatty acid metabolism. These findings suggest that besides satiating effects, resistant starch has a beneficial effect on colonic health. In the last study, the long-term effects of a gelling fibre promoting satiation (alginate) and a fermentable fibre promoting satiety (resistant starch) on feeding patterns and growth performance were assessed. In the long-term, growing-finishing pigs compensated for a reduced dietary energy content by increasing voluntary food intake (alginate), or they became more efficient in the use of digestible energy (resistant starch). Moreover, dietary fibres increased the relative weight of the gastrointestinal tract and led to changes in body composition (less fat more muscle), which may be relevant for the maintenance of lean weight in humans. In conclusion, fermentable fibres are more satiating than viscous and bulking fibres. The satiating effects of fermentable fibres are likely mediated by an increased SCFA production, and a reduced and attenuated glucose supply. Under unrestricted feeding conditions, dietary fibres promoting satiation (alginate) and satiety (resistant starch) did not reduce long-term food intake and total body weight gain, yet, colon empty weight was increased and carcass growth was reduced. This implies that changes in body composition and intestinal weight or content, rather than body weight and body mass index (BMI) alone may be relevant to fully acknowledge the effects of fibres to aid in maintaining or promoting healthy body weight in humans.