VII. 1 Terminology
A plain-language explanation of the technical terms used throughout the book, so each concept becomes clear in its own context.
Terminology
16S rRNA sequencing — A genetic method for identifying gut bacteria by reading the sequence of a specific region of ribosomal RNA. Fast and inexpensive, but it only reveals the presence of bacteria — not whether they are active or what compounds they produce.
Acetate — The most abundant short-chain fatty acid (SCFA) produced by bacterial fermentation in the gut, accounting for roughly 60% of total SCFAs. Generated from fermentable fiber, it enters the circulation and acts as a metabolic substrate for peripheral tissues and as a lipogenesis precursor in hepatocytes. It modulates appetite through the brain's energy-sensing system and stimulates IgA production by B lymphocytes. Its level reflects overall microbial fermentation activity.
ACTH – Adrenocorticotropic hormone — A hormone produced by the pituitary gland that stimulates the adrenal cortex to release cortisol. It is a key step of the HPA axis stress response: hypothalamus → CRH → pituitary → ACTH → adrenal gland → cortisol.
ADH – Antidiuretic hormone — A hormone produced in the hypothalamus that reduces renal water excretion. It rises during dehydration and, together with aldosterone, triggers a sympathetic stress response that impairs insulin sensitivity.
Akkermansia muciniphila — A bacterial species that inhabits the intestinal mucus layer. Reduced abundance is associated with weakened mucus barrier function and increased LPS translocation. A fiber-rich diet and polyphenols promote its growth.
Aldosterone — A hormone produced by the adrenal cortex that regulates renal sodium reabsorption. It rises during dehydration — which explains why fluid deficit can produce symptoms resembling a stress response.
Bacteroidetes — One of the major phyla of the gut microbiota, primarily involved in carbohydrate fermentation. The shift in the Firmicutes/Bacteroidetes ratio documented in obesity is associated with increased energy harvest from food.
Beta-glucan — A partially water-soluble fiber found mainly in oats and barley. It forms a gel-like matrix in the gut, slows glucose absorption, and stimulates Lactobacillus strains.
Bifidobacterium — A beneficial bacterial genus in the colon. It produces lactic acid and acetate and stimulates IgA secretion. Reduced abundance is associated with impaired SCFA production and weakened gut barrier integrity.
BMAL1 / CLOCK — Key transcription factors regulating the molecular circadian clock. They form a 24-hour feedback loop — not only in the brain, but also in peripheral clocks of the liver, gut, and adipose tissue.
Bristol Stool Scale — Categorizes stool consistency into seven types (1 = hard, lumpy → 7 = liquid). Types 3–4 are considered normal. The scale serves as an indirect proxy for gut transit time and hydration status.
Butyrate — The most important short-chain fatty acid (SCFA), accounting for roughly 15–20% of total SCFAs. It is the primary energy source for colonic epithelial cells (colonocytes) and, as an HDAC inhibitor, reinforces tight junction proteins and activates anti-inflammatory genes, thereby supporting intestinal barrier integrity. Its deficiency increases gut permeability. Restoring butyrate production is a key goal and a marker of successful FMT-supported recovery.
CCK – Cholecystokinin — A satiety hormone secreted by I-cells of the small intestine in response to protein and fat. It signals fullness via the vagus nerve and slows gastric emptying.
CGM – Continuous Glucose Monitor — A small subcutaneous sensor that measures interstitial fluid glucose every minute. It is not a diagnostic device but a personal lifestyle-feedback tool; clinical decisions require medical interpretation.
Chrononutrition — The scientific study of meal timing in the context of circadian rhythm. Insulin sensitivity and GLP-1 secretion vary throughout the day — nutrient processing is more favorable in the morning.
Circadian rhythm — The body's internal clock that regulates sleep, metabolism, and hormonal cycles. It also influences microbial activity and composition over a 24-hour cycle. Disruption of circadian rhythm can negatively affect microbiota stability. The body's internal ~24-hour biological clock, which regulates sleep, hormone secretion, metabolism, and the immune system. Irregular schedules, late-night eating, and artificial light can disrupt it.
CKD – Chronic Kidney Disease — A persistent decline in kidney function, staged 1–5 based on GFR. At CKD stage 3 or higher, certain supplements and drugs (SGLT2 inhibitors, high protein intake) become risky.
Cortisol — The "stress hormone" produced by the adrenal cortex. It is naturally elevated in the morning (the cortisol awakening response) and declines throughout the day. Chronically elevated levels increase gut permeability, impair insulin sensitivity, and promote visceral fat accumulation.
CRH – Corticotropin-releasing hormone — A hormone produced by the hypothalamus and the first step of the stress response. It triggers ACTH release, which in turn drives cortisol secretion. Chronic stress leads to CRH overproduction, which also affects gut motility and permeability.
CRP – C-reactive protein — One of the most sensitive blood markers of inflammation. Elevated values can indicate low-grade systemic inflammation, which is associated with insulin resistance and gut health.
Cytokine — Small signaling proteins secreted by immune cells that regulate inflammatory and immune responses through cell-to-cell communication.
Diet break — A short, 1–2 week stabilization phase during a weight-loss diet in which caloric intake is moderately increased. Its purpose is to attenuate metabolic adaptation and improve adherence. It is not "cheating" but a planned biological tool.
Dopamine — A key neurotransmitter of the brain's reward system. The sugar-fat-salt combination in ultra-processed foods activates dopaminergic neurons of the nucleus accumbens, triggering craving.
Dysbiosis — An imbalance in the composition, diversity, or functional capacity of the gut microbiota, associated with a range of clinical conditions. Dysbiosis may involve loss of beneficial species, overgrowth of pathogenic or pro-inflammatory taxa, reduced diversity, or disrupted metabolic output. It is not a disease in itself but an ecological imbalance. FMT is used to shift the system toward functional stability.
Endocannabinoid system — The body's own network of cannabinoid-like signaling molecules. Low-intensity exercise (Zone 2) activates it — one of the mechanisms by which a post-meal walk reduces acute cravings.
Energy density — The caloric content of a food per gram (kcal/g). Low (0.5–1.5 kcal/g): vegetables; high (4–6 kcal/g): chips, biscuits. Low-energy-density foods mechanically distend the stomach — producing satiety with fewer calories.
Enteric nervous system — An autonomous nervous system within the gut wall, consisting of about 500 million neurons; independently regulates peristalsis, secretion, and blood flow.
Epigenetics — The regulation of gene activity without altering the DNA sequence. Butyrate, for example, acts as an HDAC inhibitor and modulates the expression of tight junction proteins.
Estrogen — A sex hormone that protects insulin sensitivity and lean mass. Its decline during menopause leads to increased visceral fat accumulation and a higher risk of insulin resistance.
Faecalibacterium prausnitzii — A commensal bacterium of the Firmicutes phylum, consistently associated with gut health in human studies. It produces butyrate and has anti-inflammatory properties. Its abundance is frequently reduced in dysbiotic states and may be partially restored after FMT.
Fecal microbiota transplantation (FMT) — The transfer of a processed stool preparation from a healthy, screened donor into the gastrointestinal tract of a recipient. It delivers a complex microbial community with the aim of restoring disrupted ecological functions. It is the central therapeutic intervention described in this guide.
Fermented foods — Foods produced by microbial or enzymatic transformation (e.g., yogurt, kefir, sauerkraut, kimchi, miso); many contain live microorganisms.
Firmicutes — One of the major phyla of the gut microbiota, whose members include many butyrate-producing bacteria (Faecalibacterium, Roseburia). The Firmicutes/Bacteroidetes ratio may shift in obesity.
FODMAP — Fermentable Oligosaccharides, Disaccharides, Monosaccharides And Polyols. In large amounts they ferment rapidly, causing bloating in sensitive intestines. A low-FODMAP diet is the most evidence-based dietary intervention for IBS — but it is only recommended for 4–8 weeks.
GABA – Gamma-aminobutyric acid — The main inhibitory neurotransmitter of the brain. Certain microbiota members (e.g., Lactobacillus) influence the GABA/glutamate balance — one of the mechanisms by which gut microbial status affects mood.
Gastrin — A hormone produced by gastric G-cells in response to a meal. It stimulates gastric acid secretion and, as part of the gastrocolic reflex, triggers colonic contraction.
Gastrocolic reflex — Gastric distension during a meal triggers colonic contraction through neural (vagus) and hormonal (gastrin, CCK) signals. It is strongest in the morning and activates 15–45 minutes after eating. This is the basis for recommending a morning toilet routine.
GFR – Glomerular filtration rate — A measure of renal filtration capacity (mL/min/1.73 m²). Normal: >90. CKD is staged 1 to 5 with progressively decreasing GFR.
Ghrelin — The "hunger hormone" produced by the stomach. It rises before meals and stimulates appetite. After weight loss it can remain chronically elevated — one of the reasons the body tends to drift back toward its previous weight.
GLP-1 receptor agonists — Drugs that activate the glucagon-like peptide-1 receptor (e.g., semaglutide, liraglutide), lowering blood glucose, reducing appetite, and supporting weight loss.
GLP-1 – Glucagon-like peptide-1 — An incretin hormone secreted by intestinal L-cells in response to a meal. It slows gastric emptying, enhances insulin secretion, and signals satiety to the brain. Naturally stimulated by protein and fiber intake.
Glucagon — A hormone produced by pancreatic alpha cells that raises blood glucose by breaking down hepatic glycogen. GLP-1 suppresses glucagon secretion — one of the mechanisms underlying blood glucose stabilization.
Gluconeogenesis — The hepatic process of synthesizing glucose from non-carbohydrate sources (amino acids, lactate). It is stimulated by fasting and low-carbohydrate intake and is upregulated in insulin resistance.
GLUT4 – Glucose transporter 4 — The muscle-cell glucose transporter that translocates to the cell surface in response to insulin and, independently of insulin, in response to muscle contraction. A post-meal walk reduces the postprandial glucose excursion via GLUT4 activation.
Gluten-free diet — A diet that fully avoids gluten-containing proteins in wheat, barley, rye, and their derivatives; essential treatment for celiac disease and severe gluten sensitivity.
Glycemic index (GI) — A measure of how rapidly a food raises blood glucose (on a 0–100 scale). On its own it can be misleading, as it does not account for portion size — the glycemic load (GL) is a more accurate indicator.
Glycemic load (GL) — The product of the glycemic index (GI) and the amount of carbohydrate per serving. It reflects the real glycemic burden of a meal and is clinically more relevant than the GI alone.
Glycogen — The short-term storage form of carbohydrate — primarily in the liver (~100 g) and skeletal muscle (~400 g). It depletes early in low-carbohydrate intake and releases bound water — accounting for the rapid initial "weight loss".
Gut motility — The rate at which intestinal contents are propelled by peristaltic contractions. Slow motility leads to constipation and a shift toward protein fermentation. It can be modulated through exercise, fiber intake, and hydration.
Gut permeability — The permeability of the intestinal epithelial layer. Normally, tight junctions prevent bacteria and toxins from entering the circulation. When they weaken, the basis for LPS translocation is established.
Gut-brain axis (GBA) — A bidirectional communication system between the gut and the nervous system, in which the gut microbiota also participates.
HbA1c – Glycated hemoglobin — A marker reflecting average blood glucose over the previous 2–3 months. Normal: <5.7%; prediabetes: 5.7–6.4%; diabetes: ≥6.5%.
HDAC – Histone deacetylase — An enzyme that removes acetyl groups from histone proteins. Butyrate acts as an HDAC inhibitor — it enhances expression of tight junction proteins and activates anti-inflammatory genes.
Helicobacter pylori — A Gram-negative, spiral-shaped bacterium that colonizes the gastric mucosa; a risk factor for peptic ulcer, chronic gastritis, and gastric cancer.
HOMA-IR – Homeostatic Model Assessment of Insulin Resistance — A fasting-based index that estimates insulin resistance from glucose and insulin levels. Indicative range: values around 2.5 suggest clinical insulin resistance. Cut-offs vary by laboratory and population — interpretation always requires a clinician.
HPA axis — The hypothalamic–pituitary–adrenal axis: the primary neuroendocrine stress-response system. Chronic activation of the HPA axis elevates cortisol, which alters gut motility, mucosal barrier integrity, and microbial composition. Dysbiosis itself can dysregulate HPA activity, creating a bidirectional stress-microbiota loop.
Hyperinsulinemia — Chronically elevated insulin levels, typically as a compensatory response to insulin resistance. It promotes fat storage, drives leptin resistance, and increases the risk of type 2 diabetes.
Hypothalamus — A small region of the brain and the central regulator of metabolism, hormones, and the autonomic nervous system. It houses appetite regulation (arcuate nucleus), the circadian rhythm pacemaker (suprachiasmatic nucleus), and the stress response (CRH).
IBD – Inflammatory Bowel Disease — Immune-mediated intestinal disease with tissue inflammation (Crohn's disease, ulcerative colitis). Distinct from IBS. During an active flare, restrictive diets may worsen symptoms.
IBS – Irritable Bowel Syndrome — A functional bowel disorder without tissue inflammation — symptoms arise from gut-brain axis and microbiota dysregulation. The low-FODMAP diet is its strongest evidence-based dietary intervention.
IgA – Immunoglobulin A — The primary defensive immunoglobulin of the intestinal mucosa. It binds bacteria within the mucus layer, preventing their adhesion to the epithelium. Acetate (SCFA) stimulates IgA production.
IL-6 – Interleukin-6 — A pro-inflammatory cytokine produced by immune cells and adipose tissue. Chronically elevated levels indicate low-grade systemic inflammation and are associated with insulin resistance.
Inflammasome (NLRP3) — A protein complex in immune cells — the body's "danger sensor". It is activated by bacterial toxins and metabolic stress. Butyrate and beta-hydroxybutyrate (ketone body) inhibit it — one of their anti-inflammatory mechanisms.
Insulin — Hormone produced by pancreatic beta cells in response to elevated blood glucose. It transports glucose into cells and promotes glycogen and fat storage. Chronically elevated levels drive fat storage and insulin resistance.
Insulin resistance — A reduced sensitivity of cells to insulin. The pancreas must secrete progressively more insulin to compensate. Contributors include visceral adipose inflammation, sleep deprivation, and dysbiosis. It can be estimated by HOMA-IR and reversed through lifestyle change.
Insulin sensitivity — A measure of how effectively cells respond to insulin. With high insulin sensitivity, small amounts of insulin are sufficient. It can be improved through exercise, fiber intake, sleep, and stress reduction.
Intermittent fasting — A dietary strategy alternating eating windows and fasting periods (e.g., 16:8 time window, 5:2 day pattern, alternate-day fasting).
Inulin — A fermentable prebiotic fiber found mainly in onion, garlic, and artichoke. It preferentially stimulates Bifidobacterium strains. Large doses can cause bloating — gradual introduction is recommended.
Jet lag — A circadian desynchrony caused by crossing time zones: sleep disturbance, appetite loss, mental fatigue, and digestive issues.
Kefir — A milk- or water-based fermented beverage made with kefir grains (a bacteria-yeast symbiosis); contains dozens of microorganisms.
Ketoacidosis — A life-threatening metabolic state in which accumulating ketone bodies acidify the blood; most often occurs with insulin deficiency.
Lactate — A byproduct of anaerobic muscle metabolism — and, in fact, an important energy substrate. The lactate threshold used in Zone-2 training is the marker of the aerobic/anaerobic transition.
Lactobacillus — A group of lactic acid bacteria in the gut. They produce lactic acid and acetate and stimulate IgA secretion. Found in many commercial probiotic products.
Lactose intolerance — Bloating, diarrhea, and abdominal discomfort caused by reduced lactase enzyme activity that normally digests lactose (milk sugar).
Leaky gut — A popular term for increased gut permeability. The mechanism itself is documented, but "leaky gut syndrome" is not a validated standalone clinical diagnosis — no standardized diagnostic test exists.
Leptin — A hormone produced by adipose tissue that signals energy-store fullness to the brain and suppresses appetite. It drops after weight loss — the body interprets this as a "starvation state". In obesity, leptin resistance may develop.
Leptin resistance — A blunting of leptin (the satiety hormone) signaling in the brain. The brain falsely perceives hunger even with replete energy stores. It is associated with chronic inflammation and insulin resistance.
Levothyroxine — A synthetic form of the thyroid T4 hormone used to treat hypothyroidism. It should be taken on an empty stomach; calcium, magnesium, iron, and coffee reduce its absorption.
Lipolysis — The breakdown of adipose triglycerides into free fatty acids and glycerol. It is stimulated by low insulin and high glucagon levels and inhibited by elevated insulin.
Liraglutide — A GLP-1 receptor agonist (daily injection) used to treat diabetes and obesity. The predecessor of semaglutide — similar in mechanism but with a shorter duration of action.
LPS – Lipopolysaccharide — A component of the Gram-negative bacterial cell wall ("endotoxin"). With increased gut permeability it translocates into the circulation, where it drives metabolic endotoxemia.
Melatonin — The "sleep hormone" produced by the pineal gland in darkness. Through the MTNR1B receptor it suppresses pancreatic insulin secretion — which is why late-evening carbohydrate intake produces higher blood glucose.
Metabolic endotoxemia — A persistent, low-level presence of bacterial LPS in the circulation, enabled by increased gut permeability. It drives low-grade systemic inflammation and impairs insulin sensitivity.
Metabolic syndrome — The co-occurrence of abdominal obesity, hypertension, dyslipidemia, and insulin resistance; a major risk for cardiovascular disease and type 2 diabetes.
Metformin — The first-line drug for type 2 diabetes. It suppresses hepatic glucose output. Long-term use impairs vitamin B12 absorption — periodic monitoring is recommended.
Microbiota — The collection of microorganisms (bacteria, archaea, fungi, viruses) inhabiting the gut and other mucosal surfaces, which influences digestion, immunity, and metabolism. Distinct from the microbiome: that is a broader concept encompassing the microorganisms together with their collective genes and genetic activity.
Mitochondria — The cellular "power plant" — the site of ATP production. Zone-2 exercise stimulates mitochondrial biogenesis, which improves fat-oxidation capacity.
MTNR1B – Melatonin receptor 1B — The melatonin receptor on pancreatic beta cells. Its activation suppresses insulin secretion. About 30% of the population carries a risk variant, in whom the effect of evening eating is more pronounced.
Mucin — Heavily glycosylated proteins that form the mucus protective layer of the gut mucosa; key components of the gut barrier.
Mucus (intestinal mucus) — The mucus layer covering the intestinal epithelium. It acts as a physical barrier between bacteria and the epithelium. On a low-fiber diet, bacteria begin to digest the mucus, thinning the protective layer.
MUFA – Monounsaturated fatty acids — Fatty acids with a single double bond (e.g., oleic acid); major sources include olive oil, avocado, and nuts.
Neuroinflammation — An inflammatory process within nervous-system tissue. LPS translocation, via cytokines crossing the blood-brain barrier, triggers neuroinflammation — implicated in the biology of depression and anxiety.
NF-κB — Nuclear factor kappa B — a central transcription factor of the immune response and low-grade systemic inflammation. In its inactive form it resides in the cytoplasm; upon inflammatory stimuli (LPS, cytokines, oxidative stress) it translocates to the nucleus and activates pro-inflammatory genes (TNF-α, IL-6, COX-2). Gut-derived LPS and chronic hyperinsulinemia can also activate it — one of the mechanisms linking dysbiosis to insulin resistance.
NOVA classification — A food processing classification system (Monteiro et al.): 1 = unprocessed / minimally processed, 2 = culinary ingredients, 3 = processed foods, 4 = ultra-processed foods (UPF).
NREM sleep — Non-Rapid Eye Movement sleep — the deep, restorative stage of sleep. Growth hormone release and immune-system regeneration occur predominantly during NREM. High evening blood glucose impairs its quality.
NTS – Nucleus tractus solitarii — A brainstem nucleus that serves as the first processing station for gut signals (satiety, hunger) arriving via the vagus nerve. Vagal fibers activated by GLP-1, CCK, and PYY project to the NTS.
Nucleus accumbens — A forebrain region and a key structure of the reward system. Its dopaminergic activation triggers craving. Ultra-processed foods preferentially engage this area.
Omega-3 fatty acids — Essential polyunsaturated fatty acids (EPA, DHA, ALA) that are key components of cell membranes and anti-inflammatory mediators.
Pectin — A water-soluble, gel-forming fiber found mainly in apple peel and berries. It slows glucose absorption and feeds fermentative gut bacteria.
PEM – Post-Exertional Malaise — A hallmark symptom of chronic fatigue syndrome (ME/CFS) and Long COVID: worsening of symptoms appearing 24–48 hours after physical exertion.
Peristalsis — The wave-like contractions of intestinal smooth muscle that propel contents toward the rectum. Stress inhibits it; the vagus nerve and the gastrocolic reflex stimulate it. Physical activity also augments it mechanically.
Polyphenols — A family of plant-derived phenolic compounds (e.g., flavonoids, anthocyanins) with antioxidant effects that serve as important substrates for gut microbiota.
Postbiotic — A preparation of inactivated microorganisms or their components that confers a health benefit on the host.
Postprandial — "After a meal". Most often refers to the postprandial glucose peak and the subsequent glucose "dip" — emerging 30–90 minutes after a meal and capable of triggering strong hunger and cravings.
Prebiotic — A substrate selectively utilized by beneficial gut microbes that confers a health benefit on the host.
Probiotic — The deliberate intake of live microorganisms for a health benefit. Effects are strain- and condition-specific. Butyrate-producing strains are absent from commercial products due to their oxygen sensitivity.
Progesterone — A sex hormone produced during the luteal phase of the menstrual cycle. It causes 1–2 kg of transient fluid retention in the second half of the cycle — this is not fat gain.
Propionate — A short-chain fatty acid (SCFA) produced by gut microbes from fermentable fiber, accounting for roughly 20–25% of total SCFAs. It is absorbed into the bloodstream and processed mainly in the liver, where it serves as a major gluconeogenesis substrate contributing to glucose regulation; it is antilipogenic and activates AMPK. Through GPR41/GPR43 receptors it suppresses pro-inflammatory cytokine production, promotes Treg cell differentiation, and acts as a central satiety signal. Shifts in its level are a sensitive marker of microbial activity after FMT.
Psyllium — A fiber powder made from the husk of Plantago seeds. It swells strongly in water. A mixed soluble/insoluble fiber that slows glucose absorption and increases stool bulk. Typical dose: 5–10 g daily with plenty of water.
PYY – Peptide YY — A satiety hormone secreted by intestinal L-cells in response to protein and fiber. It slows gastric emptying and suppresses appetite. Together with GLP-1 and CCK it forms the natural satiety-signaling system.
REM sleep — Rapid Eye Movement sleep — the dreaming stage. Critical for memory consolidation and emotional processing. It is the first to be lost under sleep restriction — and the next day can bring increased cravings.
Resistant starch — The fraction of starch that escapes absorption in the small intestine. Among the best sources are cooked-and-cooled rice and potatoes, and unripe bananas. A strong driver of butyrate production.
Reverse dieting — A gradual restoration of caloric intake — typically by 50–100 kcal per week — until metabolism normalizes. Applied after a weight-loss diet to prevent weight regain.
Seasonal affective disorder (SAD) — A recurrent depressive episode appearing from autumn to spring, triggered by reduced sunlight. Metabolically relevant: it increases caloric intake and reduces physical activity. Treatments include light therapy, morning light exposure, and exercise.
Semaglutide — A GLP-1 receptor agonist (once-weekly injection) used for diabetes (Ozempic) and obesity (Wegovy). It slows gastric emptying and reduces appetite. After discontinuation, body weight typically rebounds.
Serotonin — A neurotransmitter and tissue hormone. 90–95% is produced in the gut, where it regulates motility — but it does not cross the blood-brain barrier. Brain serotonin's role in mood regulation depends indirectly on microbiota-modulated tryptophan metabolism.
SGLT2 inhibitor — A class of glucose-lowering drugs that promote renal glucose excretion. Combined with a ketogenic diet they carry a risk of ketoacidosis — medical supervision is required.
Shift work — A work schedule outside the normal daytime pattern (night, rotating, or early shifts); causes chronic circadian desynchrony.
Short-chain fatty acids (SCFA) — Short (C1–C6) chain fatty acids produced by the colonic microbiota through fermentation of soluble dietary fibers. The three principal SCFAs — acetate, propionate, and butyrate — together account for roughly 95% of the total. They fuel colonocytes, reinforce the gut barrier, exert anti-inflammatory and immunomodulatory effects, and influence metabolism, appetite, and brain activity via the gut–brain axis.
Shotgun metagenomics — Sequencing of the entire genetic material of the gut microbiota — revealing not only which bacteria are present but also their functional capacity. More accurate than 16S rRNA sequencing, but more expensive.
SIBO – Small Intestinal Bacterial Overgrowth — Excessive bacterial proliferation in the small intestine — causing bloating, diarrhea, and nutrient malabsorption.
SSRI – Selective Serotonin Reuptake Inhibitor — The most commonly prescribed class of antidepressants. They raise brain serotonin levels. Side effects can include appetite changes and weight fluctuations.
Suprachiasmatic nucleus — A nucleus in the hypothalamus — the body's "master clock". It receives light signals from the eyes and synchronizes the peripheral clocks of internal organs.
Synbiotic — A combined preparation of probiotic and prebiotic that work synergistically or complementarily on host health.
Th17 cells — A CD4+ helper T-cell subset that produces IL-17 and IL-22; central to antibacterial and antifungal defense and to autoimmune inflammation.
Tight junctions — Protein complexes (claudin-1, occludin, ZO-1) between adjacent intestinal epithelial cells that seal the paracellular spaces. Their integrity is essential for preventing paracellular passage of bacteria and toxins; when they weaken, gut permeability increases. Dysbiosis-associated disruption of tight junctions contributes to epithelial permeability. Butyrate enhances their expression through its HDAC-inhibitor action.
Time-restricted eating — The daily form of intermittent fasting: eating is restricted to an 8-12 hour window (e.g., 16:8 pattern); only water or non-caloric fluids the rest of the time.
Tirzepatide — A dual incretin-mimetic — a combined GLP-1 and GIP receptor agonist (Mounjaro, Zepbound). It has produced greater average weight loss than semaglutide in head-to-head trials.
TNF-α – Tumor necrosis factor alpha — A central pro-inflammatory cytokine produced mainly by macrophages; key in inflammation, autoimmune disease, and metabolic disorders.
Treg – Regulatory T cell — The "brakes" of the immune system — T-lymphocytes that suppress excessive inflammatory responses. Propionate and butyrate (SCFAs) promote their differentiation in the colon.
UPF – Ultra-Processed Foods — NOVA category 4: industrially manufactured products containing non-food-derived additives (emulsifiers, flavor enhancers, artificial sweeteners, hydrogenated fats). They are characterized by high energy density, low fiber content, rapid absorption, and enhanced activation of the brain's reward system. Higher consumption is associated with overeating, blood glucose volatility, reduced microbiota diversity, and elevated metabolic disease risk.
Vagus nerve — The tenth cranial nerve, running from the brainstem to the abdominal organs. The principal pathway of gut-brain communication: about 80% of signals from the gut reach the brain via the vagus.
VO2max — The body's maximum oxygen uptake capacity — the strongest indicator of aerobic fitness. Improvable with Zone-2 training.
Zone-2 exercise — A low-intensity, sustainable aerobic training zone (~60–70% of maximum heart rate). It burns predominantly fat, improves mitochondrial function and insulin sensitivity, and stays below the lactate threshold.