nutrients Article Food, Eating, and the Gastrointestinal Tract Dan M Livovsky 1 , Teorora Pribic 2,3,4 and Fernando Azpiroz 2,3,4,* 1 Digestive Diseases Institute, Shaare Zedek Medical Center, Hebrew University of Jerusalem, 9103102 Jerusalem, Israel; [email protected] 2 Digestive System Research Unit, University Hospital Vall d’Hebron, Passeig de la Vall d’Hebron 119, 08035 Barcelona, Spain; [email protected] 3 Centro de Investigación Biomédica en Red de Enfermedades Hepáticas y Digestivas (Ciberehd), Av. Monforte de Lemos 3-5, 28029 Madrid, Spain 4 Departament de Medicina, Universitat Autònoma de Barcelona, 08193 Cerdanyola del Vallès, Spain * Correspondence: [email protected]; Tel.: +34-93-274-6222; Fax: +34-93-489-4456 Received: 2 March 2020; Accepted: 30 March 2020; Published: 2 April 2020 Abstract: Food ingestion induces a metered response of the digestive system. Initially,the upper digestive system reacts to process and extract meal substrates. Later, meal residues not absorbed in the small bowel, pass into the colon and activate the metabolism of resident microbiota. Food consumption also induces sensations that arise before ingestion (e.g., anticipatory reward), during ingestion (e.g., gustation), and most importantly, after the meal (i.e., the postprandial experience). The postprandial experience involves homeostatic sensations (satiety, fullness) with a hedonic dimension (digestive well-being, mood). The factors that determine the postprandial experience are poorly understood, despite their potential role in personalized diets and healthy eating habits. Current data suggest that the characteristics of the meal (amount, palatability, composition), the activity of the digestive system (suited processing), and the receptivity of the eater (influenced by multiple conditioning factors) may be important in this context. Keywords: food ingestion; digestion; satiety; digestive well-being 1. Introduction The importance of a healthy diet is well recognized, but to become acceptable, a diet needs to be attractive and gratifying. In this regard, it is crucial to understand the factors that determine the responses to ingested food and in particular the lasting effects following ingestion, i.e., the postprandial experience. In this context, the Nutrients Special Issue entitled Food and Diet for Gut Function and Dysfunction focuses on the role of the responses of the digestive system to food ingestion in normal conditions and in the disease state. The aim of the present paper is to provide an introductory overview to the Special Issue, outlining the effects of food ingestion on the brain-gut axis, i.e., the relations between digestive responses and the sensory experience. Purposely, this review is sketchy and descriptive; most of the experimental factual/information is provided in the different articles that compose the Special Issue. A comprehensive review on this subject, analyzing the relations between gastronomy and neurogastroenterology, has been previously published [1]. 2. Physiological Responses to Meal Ingestion During fasting, the gastrointestinal tract exerts a cyclic activity, alternating between periods of quiescence and periods of intense motor and secretory activity. The function of this stereotyped pattern, known as the migrating motor complex (MMC), appears to be the propulsion of residues from the lumen of the small intestine into the colon; thus, it is considered to be the intestinal housekeeper [2]. With the initiation of a meal, the digestive system is stimulated leading to the suppression of the interdigestive motor pattern and the activation of the digestive process. The digestive process Nutrients 2020, 12, 986; doi:10.3390/nu12040986 www.mdpi.com/journal/nutrients Nutrients 2018, 10, x FOR PEER REVIEW 2 of 15 involves three semi‐sequential phases that overlap over time: cephalic, luminal, and post‐absorptive. Nutrients 2020, 12, 986 2 of 14 The cephalic phase refers to events before and during the ingestion period. Indeed, even before ingestion, the digestive system starts with a series of preparatory procedures, associated in normal involvesconditions three with semi-sequential an anticipatory phases reward that sensation, overlap over e.g., time: anticipation cephalic, luminal,of a desired and post-absorptive.meal stimulates Thesalivary cephalic and phase gastric refers secretion. to events Food before ingestion and during and theswallowing ingestion period.activate Indeed,oropharyngeal even before and ingestion,oesophagogastric the digestive responses system (salivation, starts with oesophageal a series of preparatory peristalsis, procedures,and gastric associatedreceptive relaxation). in normal conditionsThe walls withof the an stomach anticipatory are contracted reward sensation, and virtually e.g., anticipation collapsed during of a desired fasting meal and stimulates meal arrival salivary into andthe gastricstomach secretion. induces an Food active ingestion relaxation and swallowing(gastric accommodation). activate oropharyngeal Solid particles and oesophagogastricactivate the antral responsespump with (salivation, peristaltic oesophagealactivity, which peristalsis, starts a grinding and gastric process receptive that transforms relaxation). the meal The walls into a of liquid the stomachchime. A are gradual contracted re‐contraction and virtually of the collapsedstomach during during the fasting postprandial and meal phase arrival pushes into the the chime stomach into inducesthe small an bowel active [3]. relaxation The activity (gastric of accommodation).the stomach and small Solid bowel particles adapts activate to the the requirements antral pump of with the peristalticdigestive activity,process, whichso that, starts food a is grinding digested process and subsequently that transforms absorbed the meal by intoa sequence a liquid of chime. complex A gradualphysical re-contraction and chemical ofprocesses the stomach that begin during in the the postprandialmouth and extend phase to pushes the terminal the chime ileum. into Ultimately, the small bowelnon‐absorbed [3]. The activityresidues of reach the stomach the colon and [4]; small these bowel dietary adapts residues to the requirements serve as substrates of the digestive for gut process,microbiota, so that, and food in return, is digested the andgut microbiota subsequently can absorbed affect host by a physiology sequence of and complex digestive physical function and chemical(Figure 1). processes that begin in the mouth and extend to the terminal ileum. Ultimately, non-absorbed residues reach the colon [4]; these dietary residues serve as substrates for gut microbiota, and in return, the gut microbiota can affect host physiology and digestive function (Figure1). Figure 1. Digestive response to ingestion. The upper digestive system extracts meal substrates by a process of digestion and absorption. Non-absorbed meal residues pass into the colon and feed the microbiota. Figure 1. Digestive response to ingestion. The upper digestive system extracts meal substrates by a Indeed,process theof digestion large majority and absorption. of human Non microbiota‐absorbed inhabit meal theresidues colon, pass which into provides the colon a and dedicated feed the niche for thismicrobiota. population of symbiotic organisms. Microbiota fermentation of meal residues releases a series of metabolites that, in turn, serve as substrates for other subsets of microbiota. Hence, the colon contains a biomassIndeed, formed the large by microbiota, majority of mealhuman residues, microbiota and inhabit secondary the colon, metabolic which products provides in a dedicated dynamic chainniche of for metabolic this population reactions. of symbiotic The total volumeorganisms. of colonic Microbiota biomass fermentation consists of of approximately meal residues 500–800 releases mLa series with of a 30%metabolites daily turnover that, in (100–200 turn, serve mL as daily substrates fecal output) for other [5]. subsets Microbiota of microbiota. metabolism Hence, of some the mealcolon residues contains releases a biomass gas; formed hence, by colonic microbiota, gas production meal residues, reflects and microbiota secondary metabolic metabolic activityproducts [6 in]. Approximatelya dynamic chain 100 minof metabolic after ingestion, reactions. unabsorbed The total residues volume start of arriving colonic into biomass the colon, consists and gas of productionapproximately increases. 500–800 The mL plateau with a lasts 30% approximately daily turnover 4–6(100 h‐200 depending mL daily on fecal the mealoutput) composition [5]. Microbiota and thenmetabolism gradually of declines some meal [7], butresidues the effect releases persists gas; as hence, long as colonic substrates gas remainproduction available, reflects such microbiota that the residuemetabolic loads activity of consecutive [6]. Approximately meals contribute 100 min to gasafter production ingestion, [unabsorbed8]. The volume residues of gas start produced arriving within into 6the h after colon, a meal and rangesgas production from 200 mLincreases. with a standardThe plateau breakfast lasts approximately to 600 mL with 4–6 a flatulogenic h depending meal on [ 7the]. mealThe composition activity of and the then muscles gradually of thedeclines abdominal [7], but andthe effect thoracic persists walls as dependslong as substrates on its content. remain Specifically,available, such an increase that the in residue intraabdominal loads of volumeconsecutive induces meals an adaptivecontribute relaxation