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Advances in Physiology Education Jun 2010Nutrient digestion and absorption is necessary for the survival of living organisms and has evolved into the complex and specific task of the gastrointestinal (GI)...
Nutrient digestion and absorption is necessary for the survival of living organisms and has evolved into the complex and specific task of the gastrointestinal (GI) system. While most people simply assume that their GI tract will work properly to use nutrients, provide energy, and release wastes, few nonscientists know the details about how various nutrients are digested and how the breakdown products traverse the cells lining the small intestine to reach the blood stream and to be used by the other cells of the body. There have been several recent discoveries of new transporters that likely contribute to the absorption of oligopeptides and fatty acids. In addition, details are being clarified about how transporters work and in what forms nutrients can be absorbed. The enzymes that digest basic carbohydrates, proteins, and fats have been identified in various segments of the GI tract, and details are becoming clearer about what types of bonds they hydrolyze. Usually, detailed information about the digestion of basic nutrients is presented and learned in biochemistry courses and detailed information about absorption via transepithelial transport of the breakdown products of digestion is studied in physiology courses. The goal of this Staying Current article is to combine the details of the biochemistry of digestion with the updated information about the physiology of nutrient absorption into one source for teachers of physiology. Insights are included about some of the diseases and conditions that can bring about malabsorption of food in the GI tract and their consequences.
Topics: Dietary Carbohydrates; Digestion; Fatty Acids; Gastrointestinal Tract; Humans; Intestinal Absorption; Membrane Transport Proteins; Proteins
PubMed: 20522896
DOI: 10.1152/advan.00094.2009 -
Cell and Tissue Research Sep 2019Living cells depend on a constant supply of energy-rich organic molecules from the environment. Small molecules pass into the interior of the cell via simple diffusion...
Living cells depend on a constant supply of energy-rich organic molecules from the environment. Small molecules pass into the interior of the cell via simple diffusion or active transport carried out by membrane bound transporters; macromolecules, or entire cells, are taken up by endocytosis/phagocytosis, and are degraded intracellularly in specialized membrane bound compartments (lysosomes). Whereas all cells are capable of transporting molecules through the membrane, the efficient procurement, digestion and uptake of nutrients have become the function of specialized cell types and organs, forming the digestive system in multicellular animals. In mammals, for example, the digestive system is comprised of glandular organs with classes of cells specialized in the secretion of enzymes for the extracellular digestion of food particles (e.g., exocrine cells of the salivary gland, pancreas), as well as other organs with absorptive function (e.g., small intestine). Numerous other cell types, such as smooth muscle cells, neurons and enteroendocrine cells, are associated with glandular cells and intestinal cells to promote the digestive process.
Topics: Animals; Biological Evolution; Digestion; Digestive System; Immune System; Phagocytosis
PubMed: 31478136
DOI: 10.1007/s00441-019-03102-x -
Nutrients Apr 2020Food ingestion induces a metered response of the digestive system. Initially, the upper digestive system reacts to process and extract meal substrates. Later, meal... (Review)
Review
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.
Topics: Affect; Digestion; Eating; Feeding Behavior; Gastrointestinal Microbiome; Humans; Postprandial Period; Satiation; Satiety Response
PubMed: 32252402
DOI: 10.3390/nu12040986 -
Critical Reviews in Food Science and... Oct 2017Proteins are not equally digestible-their proteolytic susceptibility varies by their source and processing method. Incomplete digestion increases colonic microbial... (Review)
Review
Proteins are not equally digestible-their proteolytic susceptibility varies by their source and processing method. Incomplete digestion increases colonic microbial protein fermentation (putrefaction), which produces toxic metabolites that can induce inflammation in vitro and have been associated with inflammation in vivo. Individual humans differ in protein digestive capacity based on phenotypes, particularly disease states. To avoid putrefaction-induced intestinal inflammation, protein sources, and processing methods must be tailored to the consumer's digestive capacity. This review explores how food processing techniques alter protein digestibility and examines how physiological conditions alter digestive capacity. Possible solutions to improving digestive function or matching low digestive capacity with more digestible protein sources are explored. Beyond the ileal digestibility measurements of protein digestibility, less invasive, quicker and cheaper techniques for monitoring the extent of protein digestion and fermentation are needed to personalize protein nourishment. Biomarkers of protein digestive capacity and efficiency can be identified with the toolsets of peptidomics, metabolomics, microbial sequencing and multiplexed protein analysis of fecal and urine samples. By monitoring individual protein digestive function, the protein component of diets can be tailored via protein source and processing selection to match individual needs to minimize colonic putrefaction and, thus, optimize gut health.
Topics: Animal Feed; Diet; Digestion; Feces; Fermentation; Food Handling; Humans; Proteins
PubMed: 26713355
DOI: 10.1080/10408398.2015.1117412 -
The American Journal of Clinical... May 2014Yogurt is traditionally consumed throughout the world among populations who are seemingly unable to digest lactose. This review provides a historical overview of the... (Review)
Review
Yogurt is traditionally consumed throughout the world among populations who are seemingly unable to digest lactose. This review provides a historical overview of the studies that show lactose digestion and tolerance from yogurt by lactose-intolerant people. The lactose in yogurt is digested more efficiently than other dairy sources of lactose because the bacteria inherent in yogurt assist with its digestion. The bacterial lactase survives the acidic conditions of the stomach, apparently being physically protected within the bacterial cells and facilitated by the buffering capacity of yogurt. The increasing pH as the yogurt enters the small intestine and a slower gastrointestinal transit time allow the bacterial lactase to be active, digesting lactose from yogurt sufficiently to prevent symptoms in lactose-intolerant people. There is little difference in the lactase capability of different commercial yogurts, because they apparently contain Lactobacillus bulgaricus and Streptococcus thermophilus in sufficient quantities (10(8) bacteria/mL). However, Lactobacillus acidophilus appears to require cell membrane disruption to physically release the lactase. Compared with unflavored yogurts, flavored yogurts appear to exhibit somewhat reduced lactase activity but are still well tolerated.
Topics: Digestion; Gastrointestinal Tract; Humans; Hydrolysis; Lactase; Lactobacillus; Lactose; Lactose Intolerance; Streptococcus thermophilus; Taste; Yogurt
PubMed: 24695892
DOI: 10.3945/ajcn.113.073023 -
Comprehensive Physiology Apr 2013In vertebrates and invertebrates, morphological and functional features of gastrointestinal (GI) tracts generally reflect food chemistry, such as content of... (Review)
Review
In vertebrates and invertebrates, morphological and functional features of gastrointestinal (GI) tracts generally reflect food chemistry, such as content of carbohydrates, proteins, fats, and material(s) refractory to rapid digestion (e.g., cellulose). The expression of digestive enzymes and nutrient transporters approximately matches the dietary load of their respective substrates, with relatively modest excess capacity. Mechanisms explaining differences in hydrolase activity between populations and species include gene copy number variations and single-nucleotide polymorphisms. Transcriptional and posttranscriptional adjustments mediate phenotypic changes in the expression of hydrolases and transporters in response to dietary signals. Many species respond to higher food intake by flexibly increasing digestive compartment size. Fermentative processes by symbiotic microorganisms are important for cellulose degradation but are relatively slow, so animals that rely on those processes typically possess special enlarged compartment(s) to maintain a microbiota and other GI structures that slow digesta flow. The taxon richness of the gut microbiota, usually identified by 16S rRNA gene sequencing, is typically an order of magnitude greater in vertebrates than invertebrates, and the interspecific variation in microbial composition is strongly influenced by diet. Many of the nutrient transporters are orthologous across different animal phyla, though functional details may vary (e.g., glucose and amino acid transport with K+ rather than Na+ as a counter ion). Paracellular absorption is important in many birds. Natural toxins are ubiquitous in foods and may influence key features such as digesta transit, enzymatic breakdown, microbial fermentation, and absorption.
Topics: Animals; Digestion; Eating; Gastrointestinal Tract; Humans; Intestinal Absorption; Metagenome
PubMed: 23720328
DOI: 10.1002/cphy.c110054 -
Nutrients Mar 2021Food ingestion induces homeostatic sensations (satiety, fullness) with a hedonic dimension (satisfaction, changes in mood) that characterize the postprandial experience.... (Review)
Review
Food ingestion induces homeostatic sensations (satiety, fullness) with a hedonic dimension (satisfaction, changes in mood) that characterize the postprandial experience. Both types of sensation are secondary to intraluminal stimuli produced by the food itself, as well as to the activity of the digestive tract. Postprandial sensations also depend on the nutrient composition of the meal and on colonic fermentation of non-absorbed residues. Gastrointestinal function and the sensitivity of the digestive tract, i.e., perception of gut stimuli, are determined by inherent individual factors, e.g., sex, and can be modulated by different conditioning mechanisms. This narrative review examines the factors that determine perception of digestive stimuli and the postprandial experience.
Topics: Brain; Conditioning, Psychological; Digestion; Eating; Female; Gastrointestinal Microbiome; Gastrointestinal Tract; Homeostasis; Humans; Intestinal Absorption; Intestinal Diseases; Male; Meals; Postprandial Period; Satiation; Sensation; Sex Characteristics
PubMed: 33801924
DOI: 10.3390/nu13030893 -
Gut Jan 1994The normal gut is adapted to intermittent feeding with complex macromolecular substrates of low sodium content. The high permeability of the upper small intestine to... (Review)
Review
The normal gut is adapted to intermittent feeding with complex macromolecular substrates of low sodium content. The high permeability of the upper small intestine to sodium, together with sodium rich saliva and pancreaticobiliary secretions results in large sodium fluxes into the lumen. These substantial sodium influxes are matched by equally large effluxes from the ileum and proximal colon, which are comparatively impermeable to sodium and capable of active sodium absorption. Resection of these distal, sodium absorbing regions of the intestine, lead to problems with sodium depletion. Controlled transit of chyme is essential to permit time for optimum digestion and absorption and a range of feedback control mechanisms exist. Partially digested nutrients, both in the duodenum and ileum, exert inhibitory feedback to delay delivery of further nutrients and here again surgery may compromise these reflexes. Brush border hydrolase values are strongly influenced by luminal nutrient concentrations, being impaired by malnutrition and total parenteral nutrition, but restored by enteral feeding. Viscous fibre slows absorption and may delay transit through mechanisms that are as yet uncertain. Whether and how novel substrates activate normal control mechanisms will be important factors determining their effectiveness and patient acceptability.
Topics: Digestion; Digestive System Physiological Phenomena; Gastrointestinal Motility; Humans; Intestinal Absorption; Intestinal Mucosa; Sodium
PubMed: 8125391
DOI: 10.1136/gut.35.1_suppl.s5 -
Gastroenterology May 2019G-protein-coupled receptors (GPCRs) are the largest family of transmembrane signaling proteins. In the gastrointestinal tract, GPCRs expressed by epithelial cells sense... (Review)
Review
G-protein-coupled receptors (GPCRs) are the largest family of transmembrane signaling proteins. In the gastrointestinal tract, GPCRs expressed by epithelial cells sense contents of the lumen, and GPCRs expressed by epithelial cells, myocytes, neurons, and immune cells participate in communication among cells. GPCRs control digestion, mediate digestive diseases, and coordinate repair and growth. GPCRs are the target of more than one third of therapeutic drugs, including many drugs used to treat digestive diseases. Recent advances in structural, chemical, and cell biology research have shown that GPCRs are not static binary switches that operate from the plasma membrane to control a defined set of intracellular signals. Rather, GPCRs are dynamic signaling proteins that adopt distinct conformations and subcellular distributions when associated with different ligands and intracellular effectors. An understanding of the dynamic nature of GPCRs has provided insights into the mechanism of activation and signaling of GPCRs and has shown opportunities for drug discovery. We review the allosteric modulation, biased agonism, oligomerization, and compartmentalized signaling of GPCRs that control digestion and digestive diseases. We highlight the implications of these concepts for the development of selective and effective drugs to treat diseases of the gastrointestinal tract.
Topics: Allosteric Regulation; Digestion; Digestive System Diseases; Dimerization; Drug Discovery; Endosomes; Humans; Receptors, G-Protein-Coupled; Signal Transduction
PubMed: 30771352
DOI: 10.1053/j.gastro.2019.01.266 -
Food Research International (Ottawa,... Nov 2020Mushrooms are significant sources of amino acids and bioactive amines; however, their bioaccessibility can be affected by processing and during in vitro digestion. Fresh...
Mushrooms are significant sources of amino acids and bioactive amines; however, their bioaccessibility can be affected by processing and during in vitro digestion. Fresh Agaricus bisporus mushroom was submitted to cooking and canning and samples were submitted to in vitro gastric and gastric-intestinal digestions. An UHPLC method was used for the simultaneous determination of 18 free amino acids, 10 biogenic amines and ammonia in the samples. Fresh mushroom contained 14 free amino acids, with predominance of alanine and glutamic acid; spermidine was the only amine detected; and ammonia was also detected. Spermidine levels were not affected by cooking, but there was a significant loss (14%) after canning. Spermidine levels were not affected by the in vitro gastric and intestinal digestion, suggesting full bioaccessibility. There was a significant decrease on total amino acids levels after cooking and canning, with higher losses for aspartic and glutamic acids in cooked and for aspartic acid and valine in canned mushrooms. After the in vitro gastric and intestinal digestions, the total levels of amino acids increased and two additional amino acids (arginine and methionine) were detected. During in vitro digestion many of the amino acids were released mainly in the intestinal phase. After in vitro digestion, amino acids per gram of protein of mushrooms are adequate for most FAO amino acid pattern for adults. Multivariate analysis confirmed that protein hydrolysis in processed mushrooms is higher in intestinal phase. Bioaccessibility data for spermidine in A. bisporus is a novelty and increase the value of this food.
Topics: Agaricus; Amines; Amino Acids; Digestion; Spermidine
PubMed: 33233206
DOI: 10.1016/j.foodres.2020.109616