A
Abdominal Distension
A measurable increase in abdominal girth, often caused by gas, fluid, or stool buildup in the gastrointestinal tract. Unlike bloating, which is a subjective sensation, distension can be objectively observed and measured.
Absorption
The process by which nutrients from digested food pass through the intestinal wall into the bloodstream. Most absorption occurs in the small intestine, where the lining is covered in tiny finger-like projections called villi that maximize surface area.
Acid Reflux
A condition where stomach acid flows back up into the esophagus, causing a burning sensation commonly known as heartburn. Chronic acid reflux occurring more than twice a week may indicate gastroesophageal reflux disease (GERD).
Anus
The opening at the end of the digestive tract through which stool exits the body. It contains two muscular rings (sphincters) that control bowel movements -- one voluntary and one involuntary.
Autoimmune Protocol (AIP)
An elimination diet designed to reduce inflammation and identify food triggers in people with autoimmune conditions. It removes grains, legumes, dairy, eggs, nuts, seeds, nightshades, and certain additives before gradually reintroducing them.
B
Bacteroidetes
One of the two dominant phyla of bacteria in the human gut, along with Firmicutes. Bacteroidetes are especially good at breaking down complex carbohydrates and plant fibers, and shifts in their balance relative to Firmicutes have been linked to obesity and metabolic health.
Bifidobacterium
A genus of beneficial bacteria commonly found in the human gut, especially in the large intestine. Bifidobacteria help ferment dietary fiber, produce B vitamins, and support the intestinal barrier. They are among the first microbes to colonize an infant's gut.
Bile
A yellow-green digestive fluid produced by the liver and stored in the gallbladder. Bile is released into the small intestine to help break down and absorb dietary fats and fat-soluble vitamins.
Bile Acid Malabsorption
A condition where bile acids are not properly reabsorbed in the small intestine and instead pass into the colon, causing chronic watery diarrhea. It is an underdiagnosed cause of IBS-like symptoms and can be identified with a SeHCAT scan.
Biofilm
A structured community of microorganisms that adhere to surfaces and are enclosed in a self-produced protective matrix. In the gut, biofilms can be formed by both beneficial and harmful bacteria and may play a role in chronic infections and antibiotic resistance.
Bloating
A subjective feeling of increased pressure, fullness, or tightness in the abdomen, commonly reported by people with IBS. Bloating can occur with or without visible abdominal distension and is often triggered by certain foods, stress, or changes in gut motility.
Bowel Habit
The typical pattern of an individual's bowel movements, including frequency, consistency, and ease of passage. Changes in bowel habit -- such as new onset of constipation or diarrhea -- are a key factor in diagnosing functional gut disorders like IBS.
Breath Test
A non-invasive diagnostic test that measures hydrogen and methane gases in exhaled breath after consuming a sugar solution (such as lactulose or glucose). Breath tests are commonly used to detect small intestinal bacterial overgrowth (SIBO) and carbohydrate malabsorption.
Bristol Stool Scale
A medical chart that classifies stool into seven types based on shape and consistency, ranging from Type 1 (hard lumps) to Type 7 (entirely liquid). It is widely used by clinicians and patients to monitor bowel health and track changes over time.
Butyrate
A short-chain fatty acid produced when gut bacteria ferment dietary fiber. Butyrate is the primary energy source for the cells lining the colon and plays a vital role in maintaining the intestinal barrier, reducing inflammation, and supporting overall gut health.
C
Candida
A genus of yeast that naturally lives in small amounts in the gut, mouth, and skin. When the balance of gut microorganisms is disrupted, Candida can overgrow and potentially cause symptoms such as bloating, fatigue, and oral or genital thrush.
Celiac Disease
An autoimmune disorder in which ingestion of gluten triggers an immune response that damages the lining of the small intestine. This damage impairs nutrient absorption and can cause symptoms ranging from diarrhea and bloating to fatigue and anemia.
CFU (Colony Forming Units)
A unit of measurement used to estimate the number of viable bacteria or yeast cells in a probiotic supplement. A higher CFU count does not necessarily mean a more effective product -- strain type, survival through stomach acid, and clinical evidence matter more.
Cholecystokinin (CCK)
A hormone released by cells in the upper small intestine in response to dietary fat and protein. CCK stimulates the gallbladder to release bile, prompts the pancreas to secrete digestive enzymes, and signals the brain to promote feelings of satiety.
Colitis
Inflammation of the inner lining of the colon, which can result from infections, autoimmune conditions (such as ulcerative colitis), reduced blood flow, or certain medications. Symptoms typically include diarrhea, abdominal pain, and sometimes rectal bleeding.
Colon
The largest part of the large intestine, extending from the cecum to the rectum. The colon's primary roles are absorbing water and electrolytes from indigestible food matter and housing trillions of bacteria that ferment remaining fiber into beneficial short-chain fatty acids.
Colonoscopy
A diagnostic procedure in which a long, flexible tube with a camera (colonoscope) is inserted through the rectum to examine the entire colon and the end of the small intestine. It is used to screen for colorectal cancer, investigate symptoms, and remove polyps.
Constipation
A condition characterized by infrequent bowel movements (typically fewer than three per week), hard or lumpy stools, straining, and a sensation of incomplete evacuation. It can result from low fiber intake, dehydration, inactivity, medications, or functional disorders like IBS-C.
Cortisol
A steroid hormone produced by the adrenal glands in response to stress. Chronic elevation of cortisol can negatively affect gut health by increasing intestinal permeability, altering the microbiome, reducing digestive enzyme output, and slowing gut motility.
Crohn's Disease
A chronic inflammatory bowel disease (IBD) that can affect any part of the gastrointestinal tract, from the mouth to the anus, though it most commonly involves the end of the small intestine. Symptoms include abdominal pain, persistent diarrhea, weight loss, and fatigue.
D
Dehydration
A state in which the body loses more fluids than it takes in, leading to insufficient water for normal functions. In the context of gut health, dehydration is a common consequence of prolonged diarrhea or vomiting and can worsen constipation.
Diarrhea
The passage of loose or watery stools, typically three or more times per day. Diarrhea can be acute (caused by infections or food poisoning) or chronic (lasting more than four weeks), and may result from malabsorption, inflammation, or functional disorders like IBS-D.
Dietary Fiber
Plant-based carbohydrates that resist digestion in the human small intestine and pass into the colon, where they may be fermented by gut bacteria. Fiber is classified as soluble (dissolves in water, forms a gel) or insoluble (adds bulk to stool), and both types support digestive health.
Digestive Enzymes
Proteins produced by the body (primarily in the mouth, stomach, pancreas, and small intestine) that break food down into absorbable nutrients. Key examples include amylase (starches), protease (proteins), and lipase (fats). Supplemental enzymes are sometimes used for conditions like lactose intolerance or pancreatic insufficiency.
Disaccharides
A class of sugars made up of two simple sugar molecules bonded together. In the FODMAP framework, the key disaccharide is lactose (glucose + galactose), found in dairy products. People who lack sufficient lactase enzyme cannot properly digest lactose.
Diverticulitis
Inflammation or infection of small pouches (diverticula) that can form in the walls of the colon, typically in the lower left side. Symptoms include severe abdominal pain, fever, nausea, and changes in bowel habits. A high-fiber diet may help prevent diverticula from forming.
Duodenum
The first and shortest section of the small intestine, connecting the stomach to the jejunum. The duodenum receives partially digested food from the stomach along with bile from the gallbladder and enzymes from the pancreas, making it a critical site for chemical digestion.
Dysbiosis
An imbalance in the composition or function of the gut microbial community, where potentially harmful organisms outgrow beneficial ones. Dysbiosis has been associated with conditions including IBS, inflammatory bowel disease, obesity, and mental health disorders.
E
Electrolytes
Minerals such as sodium, potassium, magnesium, and chloride that carry an electrical charge and are essential for nerve signaling, muscle contraction, and fluid balance. The colon plays an important role in absorbing electrolytes, and imbalances can occur with chronic diarrhea or vomiting.
Elimination Diet
A structured dietary approach where specific foods or food groups suspected of causing symptoms are removed for a period (usually 2-6 weeks) and then systematically reintroduced one at a time to identify triggers. The low-FODMAP diet is one well-known example.
Endoscopy
A medical procedure that uses a thin, flexible tube equipped with a light and camera to visually examine the interior of the digestive tract. Upper endoscopy (gastroscopy) examines the esophagus, stomach, and duodenum, while colonoscopy examines the colon.
Enteric Nervous System
A vast network of over 100 million neurons embedded in the walls of the gastrointestinal tract, sometimes called the 'second brain.' It independently regulates gut motility, secretion, and blood flow, and communicates bidirectionally with the central nervous system via the vagus nerve.
Esophagus
The muscular tube that connects the throat (pharynx) to the stomach. It moves food downward through coordinated muscle contractions called peristalsis. At its lower end, the lower esophageal sphincter prevents stomach acid from flowing back up.
F
Fermentation
A metabolic process in which gut bacteria break down undigested carbohydrates (such as fiber and FODMAPs) in the colon, producing gases (hydrogen, methane, carbon dioxide) and beneficial short-chain fatty acids. Excessive fermentation can contribute to bloating and gas in sensitive individuals.
Firmicutes
One of the two most abundant phyla of bacteria in the human gut microbiome, alongside Bacteroidetes. Firmicutes include many important genera such as Lactobacillus, Clostridium, and Faecalibacterium. An elevated Firmicutes-to-Bacteroidetes ratio has been studied in relation to obesity.
Flatulence
The expulsion of gas from the digestive tract through the anus. It is a normal physiological process -- most people pass gas 10-20 times per day. Excessive flatulence may result from high-fiber diets, FODMAP-rich foods, swallowed air, or bacterial overgrowth.
FODMAP
An acronym standing for Fermentable Oligosaccharides, Disaccharides, Monosaccharides, And Polyols -- a group of short-chain carbohydrates that are poorly absorbed in the small intestine. In susceptible individuals, FODMAPs draw water into the gut and are rapidly fermented by bacteria, causing bloating, gas, and altered bowel habits.
Food Intolerance
A non-immune-mediated adverse reaction to food, typically caused by an inability to properly digest or process a specific component (such as lactose or fructose). Unlike food allergies, intolerances do not involve the immune system and are generally not life-threatening, though they can significantly affect quality of life.
Food Sensitivity
A broad, informal term used to describe adverse reactions to food that do not fit neatly into the categories of food allergy or classical food intolerance. Symptoms may be delayed (hours to days) and can include digestive discomfort, headaches, fatigue, or skin issues.
FOS (Fructo-oligosaccharides)
A type of prebiotic fiber found naturally in foods like onions, garlic, bananas, and asparagus. FOS feeds beneficial gut bacteria such as Bifidobacterium but is also a FODMAP, meaning it can trigger symptoms in people with IBS when consumed in larger amounts.
Fructans
A type of oligosaccharide FODMAP consisting of chains of fructose molecules. Found in wheat, onions, garlic, and many other common foods, fructans are a frequent trigger for digestive symptoms in people with IBS because humans lack the enzyme needed to fully break them down.
Fructose
A simple sugar (monosaccharide) naturally found in fruits, honey, and some vegetables. In the FODMAP framework, excess fructose -- the amount of fructose that exceeds glucose in a food -- can be poorly absorbed and cause digestive symptoms in sensitive individuals.
Fructose Malabsorption
A digestive condition in which the small intestine has a reduced ability to absorb fructose, leading to unabsorbed fructose passing into the colon where it is fermented by bacteria. This can cause bloating, gas, pain, and diarrhea, and is identified using a hydrogen breath test.
Functional Gastrointestinal Disorder (FGID)
A group of disorders characterized by chronic or recurring digestive symptoms -- such as pain, bloating, or altered bowel habits -- that cannot be explained by structural or biochemical abnormalities on standard tests. IBS and functional dyspepsia are common examples.
G
Galactans (GOS)
A type of oligosaccharide FODMAP made up of chains of galactose sugars, found mainly in legumes (lentils, chickpeas, beans) and some nuts. Humans lack the enzyme alpha-galactosidase needed to break them down, so they pass to the colon and are fermented by bacteria.
Gallbladder
A small, pear-shaped organ located beneath the liver that stores and concentrates bile. When you eat a meal containing fat, the gallbladder contracts and releases bile into the duodenum to aid in fat digestion and absorption.
Gas
A natural byproduct of digestion, produced primarily when gut bacteria ferment undigested carbohydrates in the colon. Swallowed air also contributes to intestinal gas. While passing gas is normal, excessive gas can be a symptom of food intolerances, SIBO, or IBS.
Gastritis
Inflammation of the stomach lining, which can be caused by Helicobacter pylori infection, excessive use of NSAIDs, alcohol, or autoimmune conditions. Symptoms may include upper abdominal pain, nausea, and a feeling of fullness. Chronic gastritis can lead to ulcers if untreated.
Gastroenterologist
A physician who specializes in diagnosing and treating diseases of the digestive system, including the esophagus, stomach, intestines, liver, gallbladder, and pancreas. Gastroenterologists perform procedures like endoscopies and colonoscopies and manage conditions such as IBS, IBD, and celiac disease.
Gastroparesis
A condition in which the stomach takes too long to empty its contents into the small intestine, even without a physical blockage. Common symptoms include nausea, vomiting, early fullness, and bloating. It is often associated with diabetes but can also be idiopathic.
GERD (Gastroesophageal Reflux Disease)
A chronic digestive condition in which stomach acid frequently flows back into the esophagus, causing persistent heartburn, regurgitation, and potentially damaging the esophageal lining. Lifestyle changes, medications, and sometimes surgery are used to manage it.
Gluten
A group of proteins found in wheat, barley, and rye that gives dough its elasticity. Gluten triggers an autoimmune response in people with celiac disease and may cause symptoms in those with non-celiac gluten sensitivity. It is distinct from fructans, which are the FODMAP component of wheat.
Gut Barrier
The multi-layered defense system of the intestinal wall that selectively allows nutrients to be absorbed while blocking harmful substances like pathogens and toxins from entering the bloodstream. It consists of a mucus layer, epithelial cells joined by tight junctions, and immune cells.
Gut Hormones
Chemical messengers produced by specialized cells in the gastrointestinal tract that regulate digestion, appetite, and metabolism. Key examples include ghrelin (hunger), leptin (satiety), cholecystokinin (bile release), and GLP-1 (insulin secretion and appetite suppression).
Gut Microbiome
The collective community of trillions of microorganisms -- including bacteria, fungi, viruses, and archaea -- that reside in the gastrointestinal tract, primarily in the large intestine. This ecosystem plays essential roles in digestion, immune function, vitamin synthesis, and protection against pathogens.
Gut Motility
The coordinated muscular contractions (peristalsis) that move food and waste through the gastrointestinal tract. Abnormal motility -- either too fast or too slow -- is a core feature of many digestive conditions, including IBS-D (rapid transit) and IBS-C (slow transit).
Gut-Brain Axis
The bidirectional communication network linking the gastrointestinal tract and the brain through neural, hormonal, and immune pathways. This connection explains why stress can worsen digestive symptoms and why gut health can influence mood, cognition, and mental health.
Gut-Directed Hypnotherapy
A specialized form of clinical hypnosis designed to reduce IBS symptoms by targeting the gut-brain connection. It uses relaxation techniques, visualization, and positive suggestions focused on gut function. Multiple clinical trials have shown it to be effective for IBS management.
H
Helicobacter pylori (H. pylori)
A type of bacteria that infects the stomach lining and is the most common cause of stomach ulcers and a major risk factor for gastric cancer. About half the world's population carries H. pylori, though many people remain asymptomatic. It is treated with a combination of antibiotics and acid-suppressing medication.
Histamine Intolerance
A condition in which the body accumulates excess histamine due to reduced activity of the enzyme diamine oxidase (DAO), leading to symptoms that can mimic allergies, including headaches, flushing, hives, and digestive discomfort. Fermented and aged foods are common triggers.
Hydrogen (in breath testing)
A gas produced by gut bacteria during fermentation of undigested carbohydrates. Elevated hydrogen levels on a breath test can indicate carbohydrate malabsorption (such as lactose or fructose) or small intestinal bacterial overgrowth (SIBO).
I
IBS-C (IBS with Constipation)
A subtype of irritable bowel syndrome in which the predominant bowel habit is constipation -- infrequent stools that are often hard and difficult to pass (Bristol Stool Scale types 1-2). Abdominal pain and bloating typically accompany the constipation.
IBS-D (IBS with Diarrhea)
A subtype of irritable bowel syndrome in which the predominant bowel habit is diarrhea -- frequent, loose or watery stools (Bristol Stool Scale types 6-7). It is often accompanied by urgency, abdominal cramping, and anxiety about access to restrooms.
IBS-M (Mixed IBS)
A subtype of irritable bowel syndrome in which the individual alternates between constipation and diarrhea. At least 25% of bowel movements are hard/lumpy and at least 25% are loose/watery. This fluctuating pattern can make dietary and lifestyle management more challenging.
Ileocecal Valve
A sphincter muscle located at the junction of the small intestine (ileum) and the large intestine (cecum). It controls the flow of digested material from the small intestine into the colon and helps prevent bacteria from the colon from migrating back into the small intestine.
Inflammation
The body's immune response to injury, infection, or irritation, characterized by redness, swelling, heat, and pain. In the gut, chronic low-grade inflammation can damage the intestinal lining, disrupt the microbiome, and contribute to conditions like IBD and may play a role in IBS.
Inflammatory Bowel Disease (IBD)
A group of chronic conditions involving persistent inflammation of the gastrointestinal tract. The two main types are Crohn's disease and ulcerative colitis. IBD is distinct from IBS -- it involves visible, measurable inflammation and structural damage, whereas IBS is a functional disorder.
Insoluble Fiber
A type of dietary fiber that does not dissolve in water and adds bulk to stool, helping it pass more quickly through the intestines. Found in whole grains, nuts, seeds, and the skins of fruits and vegetables. It is particularly beneficial for preventing constipation.
Intestinal Permeability
The degree to which the intestinal lining allows substances to pass through into the bloodstream. Increased intestinal permeability (sometimes called 'leaky gut') means that the tight junctions between gut lining cells have loosened, potentially allowing toxins and bacteria to cross the barrier.
Irritable Bowel Syndrome (IBS)
A common functional gastrointestinal disorder affecting up to 10-15% of the global population, characterized by recurrent abdominal pain associated with changes in bowel habits (constipation, diarrhea, or both). It is diagnosed using the Rome criteria and managed through dietary, lifestyle, psychological, and pharmaceutical approaches.
L
Lactase
A digestive enzyme produced in the lining of the small intestine that breaks down lactose (the sugar in milk) into glucose and galactose for absorption. A decline in lactase production, which is genetically normal in most of the world's adult population, leads to lactose intolerance.
Lactobacillus
A genus of beneficial bacteria found throughout the gastrointestinal and urogenital tracts. Lactobacilli produce lactic acid, which helps maintain an acidic environment that inhibits harmful bacteria. Many strains are used in probiotic supplements and fermented foods like yogurt and sauerkraut.
Lactose
A disaccharide sugar found in milk and dairy products, composed of glucose and galactose. Lactose requires the enzyme lactase for digestion. In the FODMAP framework, lactose is the key disaccharide that can cause symptoms in people with insufficient lactase production.
Lactose Intolerance
A common condition in which the body produces insufficient lactase enzyme to fully digest the lactose in dairy products, leading to bloating, gas, cramping, and diarrhea after consuming milk, cheese, or other dairy foods. It affects an estimated 65-70% of the global adult population.
Large Intestine
The final major section of the gastrointestinal tract, consisting of the cecum, colon, rectum, and anal canal. Its primary functions are absorbing water and electrolytes, forming and storing stool, and hosting the majority of the gut's microbial population.
Laxatives
Medications or supplements used to promote bowel movements and relieve constipation. Types include osmotic laxatives (draw water into the colon), stimulant laxatives (trigger intestinal contractions), bulk-forming agents (add fiber), and stool softeners. Long-term use of certain types should be discussed with a healthcare provider.
Liver
The largest internal organ, located in the upper right abdomen. In digestion, the liver produces bile, processes absorbed nutrients from the intestines, detoxifies harmful substances, and stores glycogen for energy. It plays a critical role in metabolizing medications and filtering blood from the digestive tract.
Loperamide
An over-the-counter antidiarrheal medication that works by slowing gut motility and increasing water absorption in the intestines. It is commonly used for short-term relief of acute diarrhea and is sometimes prescribed as part of an IBS-D management plan.
Low-FODMAP Diet
A three-phase dietary approach developed by Monash University for managing IBS symptoms. Phase 1 eliminates high-FODMAP foods for 2-6 weeks; Phase 2 systematically reintroduces each FODMAP group to identify personal triggers; Phase 3 establishes a personalized long-term diet that avoids only specific triggers.
M
Malabsorption
A condition in which the small intestine cannot properly absorb nutrients from food into the bloodstream. Causes include celiac disease, Crohn's disease, pancreatic insufficiency, and certain infections. Symptoms may include weight loss, diarrhea, fatty stools, and nutritional deficiencies.
Mannitol
A sugar alcohol (polyol) found naturally in cauliflower, mushrooms, celery, and sweet potatoes. It is also used as an artificial sweetener. Mannitol is poorly absorbed in the small intestine and can trigger digestive symptoms in people sensitive to polyol FODMAPs.
Methane
A gas produced by specific microorganisms called archaea (methanogens) in the gut, not by typical bacteria. Elevated methane levels on a breath test are associated with constipation-predominant symptoms, as methane slows intestinal transit. The condition is sometimes called intestinal methanogen overgrowth (IMO).
Microbiome Diversity
A measure of the variety and abundance of different microbial species in the gut ecosystem. Higher diversity is generally associated with better health outcomes, while reduced diversity has been linked to conditions like IBD, obesity, and antibiotic use. A varied, fiber-rich diet promotes microbial diversity.
Migrating Motor Complex (MMC)
A cyclical pattern of electrical activity and muscular contractions that sweeps through the stomach and small intestine during fasting (between meals). The MMC acts as a 'housekeeper,' clearing residual food particles and bacteria from the small intestine. Disrupted MMC function is implicated in SIBO.
Monosaccharides
The simplest form of sugar, consisting of a single sugar molecule. In the FODMAP framework, the relevant monosaccharide is excess fructose -- fructose present in a food in a higher amount than glucose. Fruits like apples, pears, and mangoes are common high-fructose sources.
Mucosa
The innermost lining of the gastrointestinal tract, consisting of an epithelial layer, a layer of connective tissue (lamina propria), and a thin muscle layer. The mucosa produces mucus to protect itself, secretes digestive enzymes and hormones, and absorbs nutrients.
O
Oligosaccharides
Short chains of sugar molecules that form one of the FODMAP groups. The two main types relevant to IBS are fructans (found in wheat, onions, garlic) and galactans/GOS (found in legumes). Humans lack the enzymes to digest them, so they pass into the colon and are fermented by bacteria.
Osmotic Effect
The process by which poorly absorbed substances (like certain FODMAPs) draw water into the intestinal lumen through osmosis. This increases the volume of liquid in the gut, potentially causing loose stools, urgency, and diarrhea in sensitive individuals.
P
Pancreas
A glandular organ located behind the stomach that serves both digestive and hormonal functions. It produces digestive enzymes (amylase, lipase, protease) that are released into the duodenum to break down food, and hormones (insulin, glucagon) that regulate blood sugar levels.
Pathogen
A microorganism -- such as a bacterium, virus, parasite, or fungus -- capable of causing disease. In the gut, common pathogens include Salmonella, E. coli, Clostridioides difficile, and norovirus. A healthy gut microbiome and intact gut barrier help defend against pathogenic invasion.
Pelvic Floor
A group of muscles that form a sling across the base of the pelvis, supporting the bladder, uterus (in women), and rectum. Pelvic floor dysfunction -- either too tight or too weak muscles -- can contribute to constipation, fecal incontinence, and incomplete bowel evacuation.
Peppermint Oil
An herbal supplement with antispasmodic properties that relaxes the smooth muscle of the intestinal wall. Enteric-coated peppermint oil capsules have been shown in clinical trials to reduce abdominal pain, bloating, and gas in people with IBS.
Peristalsis
The wave-like, coordinated contractions of muscles in the walls of the gastrointestinal tract that push food and waste material forward through the digestive system. Peristalsis begins in the esophagus and continues through the stomach, small intestine, and colon.
Polyols
Sugar alcohols such as sorbitol, mannitol, xylitol, and maltitol that form one of the FODMAP groups. They are found naturally in certain fruits (stone fruits, apples) and vegetables, and are widely used as artificial sweeteners in sugar-free products. Polyols are poorly absorbed and can cause digestive symptoms.
Postbiotics
Bioactive compounds produced as byproducts of probiotic bacterial metabolism, including short-chain fatty acids, enzymes, peptides, and vitamins. Postbiotics can confer health benefits similar to probiotics and are being studied as a more stable and shelf-friendly alternative.
Prebiotic
A type of non-digestible dietary compound (often a fiber) that selectively feeds and promotes the growth of beneficial gut bacteria. Common prebiotics include inulin, FOS, GOS, and resistant starch. Found in foods like garlic, onions, leeks, asparagus, bananas, and oats.
Probiotics
Live microorganisms that, when consumed in adequate amounts, confer a health benefit on the host. Common probiotic genera include Lactobacillus, Bifidobacterium, and Saccharomyces. They are available as supplements and in fermented foods like yogurt, kefir, kimchi, and sauerkraut.
Prokinetics
Medications that enhance gastrointestinal motility by strengthening the coordinated muscular contractions that move food through the digestive tract. They are used to treat conditions like gastroparesis and may help with constipation-predominant IBS or impaired migrating motor complex function.
Proton Pump Inhibitors (PPIs)
A class of medications that reduce stomach acid production by blocking the enzyme system in the stomach lining responsible for acid secretion. PPIs are used to treat GERD, stomach ulcers, and gastritis. Long-term use may affect nutrient absorption and gut microbiome composition.
R
Rectum
The final segment of the large intestine, approximately 12-15 centimeters long, connecting the sigmoid colon to the anus. The rectum stores stool until it is ready to be expelled and sends signals to the brain indicating the need for a bowel movement.
Reintroduction Phase
The second phase of the low-FODMAP diet in which individual FODMAP groups are systematically tested one at a time over several days to identify which specific types and amounts trigger symptoms. This phase is essential for avoiding unnecessarily restrictive long-term eating.
Resistant Starch
A type of starch that resists digestion in the small intestine and reaches the colon intact, where it is fermented by gut bacteria to produce short-chain fatty acids, especially butyrate. Found in cooked and cooled potatoes, green bananas, legumes, and certain whole grains.
Rifaximin
A non-absorbable antibiotic that acts locally in the gut and is used to treat conditions like SIBO and IBS-D. Unlike systemic antibiotics, rifaximin has minimal impact on the overall gut microbiome outside the small intestine and has been shown to reduce bloating, diarrhea, and abdominal pain.
Rome Criteria
The internationally recognized diagnostic framework for functional gastrointestinal disorders, including IBS. The current version (Rome IV) requires recurrent abdominal pain at least one day per week in the last three months, associated with changes in stool frequency or form, with symptom onset at least six months prior.
S
Serotonin
A neurotransmitter and signaling molecule, approximately 90-95% of which is produced in the gut by enterochromaffin cells. In the digestive system, serotonin regulates motility, secretion, and pain perception. It also plays a well-known role in mood regulation, providing a direct biochemical link between gut and brain health.
Short-Chain Fatty Acids (SCFAs)
Metabolic byproducts -- primarily acetate, propionate, and butyrate -- produced when gut bacteria ferment dietary fiber in the colon. SCFAs nourish colon cells, strengthen the gut barrier, regulate inflammation, and influence immune function. They are considered a key marker of a healthy gut ecosystem.
Small Intestinal Bacterial Overgrowth (SIBO)
A condition in which an abnormally large number of bacteria colonize the small intestine, where bacterial populations should normally be low. SIBO causes symptoms like bloating, gas, abdominal pain, and diarrhea, and is diagnosed with a lactulose or glucose breath test.
Small Intestine
The longest part of the gastrointestinal tract (approximately 6 meters), consisting of the duodenum, jejunum, and ileum. The small intestine is the primary site of chemical digestion and nutrient absorption, with its inner surface amplified by folds, villi, and microvilli to maximize absorptive area.
Soluble Fiber
A type of dietary fiber that dissolves in water to form a gel-like substance. Soluble fiber slows digestion, helps regulate blood sugar levels, and can ease both diarrhea (by absorbing excess water) and constipation (by softening stool). Found in oats, psyllium, flaxseed, and many fruits.
Sorbitol
A sugar alcohol (polyol) found naturally in stone fruits like apples, pears, cherries, and peaches, and widely used as an artificial sweetener in sugar-free products. Sorbitol is poorly absorbed in the small intestine and can cause bloating, gas, and diarrhea in sensitive individuals.
Stomach
A muscular, J-shaped organ that receives food from the esophagus and mixes it with gastric acid and enzymes to begin protein digestion. The stomach's churning action converts food into a semi-liquid paste called chyme, which is gradually released into the duodenum.
Stool Test
A laboratory analysis of a fecal sample used to detect infections, inflammation, parasites, blood, or digestive issues. Common stool tests include fecal calprotectin (for inflammation), fecal occult blood, stool cultures, and comprehensive stool analysis panels that assess microbiome composition.
Stress Response
The body's physiological reaction to perceived threats, involving the release of cortisol and adrenaline. Chronic stress can significantly impact gut health by altering motility, increasing intestinal permeability, changing the microbiome, and amplifying visceral pain sensitivity -- a key mechanism in IBS symptom flares.
Synbiotics
Products that combine probiotics and prebiotics in a single formulation, designed so that the prebiotic component selectively supports the growth of the co-administered probiotic organisms. The goal is to enhance the survival and colonization of the beneficial bacteria.
T
Tight Junctions
Protein complexes that seal the spaces between adjacent epithelial cells in the intestinal lining, forming a selective barrier that controls what passes from the gut lumen into the bloodstream. Dysfunction of tight junctions is central to increased intestinal permeability.
Transit Time
The total time it takes for food to travel from ingestion through the entire gastrointestinal tract and be excreted as stool. Normal transit time typically ranges from 12 to 36 hours. Abnormally fast transit is associated with diarrhea, while slow transit contributes to constipation.
Tricyclic Antidepressants (TCAs)
A class of medications originally developed for depression that are now commonly prescribed at low doses for IBS, particularly IBS-D. At sub-antidepressant doses, TCAs can reduce visceral pain sensitivity, slow gut transit, and improve overall IBS symptom severity.
V
Vagus Nerve
The longest cranial nerve in the body, running from the brainstem to the abdomen. It serves as the primary communication highway of the gut-brain axis, carrying signals between the gut and the brain. About 80% of its fibers are afferent (gut-to-brain), relaying information about the state of the digestive system.
Villi
Tiny finger-like projections that line the inner surface of the small intestine, dramatically increasing its surface area for nutrient absorption. Each villus is covered in even smaller projections called microvilli, and together they create a surface area roughly the size of a tennis court.
Visceral Hypersensitivity
An increased sensitivity to pain and discomfort arising from internal organs, particularly the gut. It is considered one of the central mechanisms underlying IBS, causing normal physiological processes like gas production or intestinal stretching to be perceived as painful or uncomfortable.