Epidemiological and clinical evidence highlight the benefit of dietary fibre consumption on body weight. This benefit is partly attributed to the interaction of dietary fibre with the gut microbiota. Dietary fibre possesses a complex food structure which resists digestion in the upper gut and therefore reaches the distal gut where it becomes available for bacterial fermentation. This process yields SCFA which stimulate the release of appetite-suppressing hormones glucagon-like peptide-1 and peptide YY. Food structures can further enhance the delivery of fermentable substrates to the distal gut by protecting the intracellular nutrients during upper gastrointestinal digestion. Domestic and industrial processing can disturb these food structures that act like barriers towards digestive enzymes. This leads to more digestible products that are better absorbed in the upper gut. As a result, less resistant material (fibre) and intracellular nutrients may reach the distal gut, thus reducing substrates for bacterial fermentation and its subsequent benefits on the host metabolism including appetite suppression. Understanding this link is essential for the design of diets and food products that can promote appetite suppression and act as a successful strategy towards obesity management. This article reviews the current evidence in the interplay between food structure, bacterial fermentation and appetite control.

Occurrence of protozoa in the digestive tract of herbivores is dependent upon the occurrence of an environmentally compatible section of the tract and a retention time for gut contents in that section which exceeds the protozoan generation time. In general, herbivores can be classified as pre-gastric (foregut) or post-gastric (hindgut) fermentors. Fermentation in the foregut has evolved through enlargement of the stomach in some way to slow down ingesta passage rate and provide physical separation of the ingesta from the acid-secreting regions, as well as an adequate production of buffered saliva. Hindgut fermentations occur in the caecum-proximal colon area. Most of the protozoa in the digestive tract of herbivorous mammals belong to the class Kinetofragminophorea in the orders Prostomatida, Trichostomatida and Entodiniomorphida. They are anaerobic, live in conjunction with a large bacterial population and can ferment the structural polysaccharides of plants. End products of protozoal fermentation are similar to those of the bacterial population, i.e. volatile fatty acids, lactic acid, carbon dioxide and hydrogen. Additional products of the fermentation are vitamins and microbial protein which are subsequently utilized by foregut fermentors, whereas these products are essentially lost in hindgut fermentors unless they practice coprophagy. Only limited information is available on digestive and metabolic pathways of the gut protozoa, primarily because of our inability to grow them in axenic culture. Specific faunas appear to be associated with site of fermentation, animal species and diet. Diet in turn can affect pH and contents turnover time, both of which are very important in the establishment and growth of a protozoan population in the digestive tract.