Metabolic & Weight Management

GLP-1 RA appetite suppression involves central and peripheral mechanisms. Central: GLP-1Rs are expressed in hypothalamic appetite centres (arcuate nucleus, paraventricular nucleus), brainstem satiety centres (area postrema, NTS), and reward centres (nucleus accumbens, ventral tegmental area). Semaglutide crosses the BBB (demonstrated in animal studies) and directly activates these GLP-1Rs. Functional MRI studies show reduced activation of food-reward brain regions in semaglutide-treated subjects. Peripheral: vagal afferent GLP-1Rs transmit satiety signals from the gut to the brainstem. The relative contribution of central vs peripheral mechanisms is debated — some evidence suggests central effects dominate for long-acting GLP-1 RAs. Patient-reported outcomes in weight management trials confirm subjective appetite reduction: significantly fewer food cravings, less hunger, and smaller portion sizes.

Historical anti-obesity drugs: fenfluramine/phentermine (withdrawn 1997, valvular heart disease), sibutramine (withdrawn 2010, cardiovascular risk), rimonabant (withdrawn 2009, psychiatric adverse effects). Currently approved: orlistat (lipase inhibitor, ~3% weight loss), phentermine-topiramate ER (~10%), naltrexone-bupropion ER (~5%), and the new peptide class — semaglutide 2.4mg (~15%), tirzepatide (~22.5%). The GLP-1 RA/incretin class has transformed bariatric pharmacotherapy from modestly effective (~3-5% weight loss) to substantially effective (15-25%), creating a viable alternative to bariatric surgery for many patients. Emerging pipeline: triple agonists (retatrutide, ~24% in Phase II), oral GLP-1 RAs (higher-dose oral semaglutide, ~15% in Phase III), amylin analogues (cagrilintide + semaglutide combination, ~22% in Phase II), and bimagrumab (activin receptor antagonist, preserved lean mass).

BMR accounts for approximately 60-70% of total daily energy expenditure and is primarily determined by lean body mass (muscle, organ tissue). Weight loss reduces BMR through: loss of metabolically active lean mass (each kg of lean mass accounts for approximately 13-28 kcal/day), and metabolic adaptation (reduction in BMR beyond what is predicted from the change in body composition — the body becomes more energy-efficient, potentially through reduced sympathetic nervous system activity, lower thyroid hormone levels, and decreased leptin). This metabolic adaptation persists for years after weight loss and creates a thermodynamic environment favouring weight regain. GLP-1 RA discontinuation weight regain is partly driven by this persistent metabolic adaptation combined with restored appetite signalling. Understanding metabolic adaptation is important for patient counselling about realistic long-term outcomes and the rationale for ongoing treatment.

Beta cell function is assessed by: HOMA-B (fasting insulin and glucose — steady-state index), C-peptide measurement (co-secreted with insulin 1:1 but not extracted by liver — better reflection of insulin secretion than insulin levels), glucose-stimulated insulin secretion (GSIS — insulin response to IV or oral glucose challenge), disposition index (insulin secretion adjusted for insulin sensitivity — the most physiologically meaningful measure), and proinsulin:insulin ratio (elevated in beta cell dysfunction). In T2D, beta cell mass and function progressively decline — at diagnosis, approximately 50% of functional beta cell mass is already lost. GLP-1 RAs may preserve beta cell function through: reducing glucotoxicity and lipotoxicity (improving the metabolic environment), stimulating beta cell proliferation and inhibiting apoptosis (demonstrated in rodent models — human relevance debated), and promoting insulin gene expression and proinsulin processing.

Assessment methods: DEXA (dual-energy X-ray absorptiometry — gold standard for clinical trials, measures total and regional fat mass, lean mass, and bone mineral content), BIA (bioelectrical impedance analysis — less accurate but portable and inexpensive), air displacement plethysmography (Bod Pod), CT/MRI (most accurate for regional fat distribution including visceral fat quantification, but costly and involves radiation for CT), and D2O dilution (total body water, research use). Body composition during pharmacological weight loss is a critical concern: ideally, weight loss should be predominantly fat with minimal lean mass loss. GLP-1 RA-induced weight loss typically comprises 60-70% fat mass and 30-40% lean mass — the lean mass component is a concern because lean mass loss can reduce basal metabolic rate and functional capacity. Strategies to preserve lean mass during pharmacotherapy: resistance exercise and adequate protein intake (1.2-1.6 g/kg/day).

BMI limitations: does not distinguish fat from muscle mass (athletes with high muscle mass may be misclassified as overweight), does not account for fat distribution (visceral vs subcutaneous — waist circumference or waist-to-hip ratio are complementary measures), varies with age/sex/ethnicity (WHO recommends lower BMI thresholds for Asian populations: overweight ≥23, obese ≥27.5), and may underestimate obesity in elderly (due to age-related muscle loss). Despite limitations, BMI remains the standard because of simplicity and extensive epidemiological validation. In GLP-1 RA clinical trials, BMI-based inclusion criteria define the study population: weight management trials require BMI ≥30 (or ≥27 with comorbidity), while diabetes trials may enrol patients across a wide BMI range.

CGM systems: a subcutaneous sensor (electrochemical glucose oxidase or fluorescence-based) measures interstitial glucose every 1-5 minutes, transmitted wirelessly to a receiver or smartphone app. Sensor duration: 7-14 days depending on system. Accuracy: mean absolute relative difference (MARD) of 8-12% for current devices. CGM-derived metrics beyond TIR/TBR/TAR include: glucose management indicator (GMI — estimated HbA1c from CGM data), area under the curve above/below threshold, episodes of hypoglycaemia/hyperglycaemia (duration and nadir/peak), and ambulatory glucose profile (standardised visualisation). CGM is increasingly used in clinical trials: the FDA accepts TIR as a meaningful endpoint, and CGM data provide richer glycaemic information than HbA1c alone. For GLP-1 RA trials, CGM data demonstrate improvements in TIR and reductions in glycaemic variability alongside HbA1c reduction.

Clinical assessment: scintigraphic gastric emptying study (gold standard — patient eats radiolabelled meal, gamma camera tracks gastric retention at 1, 2, 3, 4 hours; >10% retention at 4 hours = delayed), wireless motility capsule (SmartPill — measures pH, pressure, temperature throughout GI transit), and 13C-breath test (non-radioactive, measures gastric emptying of 13C-labelled substrate). GLP-1 RA-associated gastric emptying concerns include: impact on oral medication absorption (drugs requiring rapid absorption may have delayed Tmax and reduced Cmax — relevant for time-sensitive medications like oral contraceptives, antibiotics), anaesthesia risk (aspiration of gastric contents — guidelines suggest considering gastric emptying delay for patients on GLP-1 RAs undergoing general anaesthesia, though the degree of risk and appropriate management remain debated), and the gastroparesis safety signal (rare cases of severe, persistent delayed emptying beyond the expected pharmacological effect).

DKA pathophysiology: absolute or relative insulin deficiency → unrestrained lipolysis → hepatic ketogenesis → metabolic acidosis (pH <7.3, bicarbonate <18 mmol/L) with hyperglycaemia (usually >250 mg/dL) and ketonuria/ketonaemia. Treatment: IV insulin infusion, aggressive fluid replacement, electrolyte monitoring (especially potassium), and identification/treatment of precipitating factors. Glucagon is generally contraindicated in DKA because it stimulates hepatic glucose output and ketogenesis, potentially worsening the metabolic crisis. GLP-1 RAs: there are case reports of euglycaemic DKA (DKA with near-normal glucose levels) in patients on SGLT2 inhibitors, which are sometimes used alongside GLP-1 RAs. Clinicians should be aware of this possibility in patients on combination metabolic therapy.

Dyslipidaemia patterns in metabolic syndrome/T2D: elevated triglycerides (>150 mg/dL, due to hepatic VLDL overproduction driven by insulin resistance), low HDL-C (<40 mg/dL men, <50 mg/dL women, due to increased HDL catabolism), and small dense LDL particles (more atherogenic than large buoyant LDL, even when total LDL-C is near normal). GLP-1 RA effects on lipids: triglyceride reduction 10-20% (likely mediated through reduced hepatic VLDL production secondary to weight loss and improved insulin sensitivity), modest LDL-C reduction (0-5%), and modest HDL-C increase (1-3%). These lipid effects are secondary endpoints in metabolic peptide drug trials and contribute to the overall cardiovascular risk reduction observed in CVOTs. Tirzepatide Phase III data showed somewhat greater lipid improvements than selective GLP-1 RAs.

Total daily energy expenditure (TDEE) = BMR (60-70%) + thermic effect of food (TEF, ~10% — energy cost of digesting, absorbing, and processing nutrients) + physical activity energy expenditure (PAEE, 20-30% — most variable component). GLP-1 RAs primarily reduce energy intake (appetite suppression) rather than increasing energy expenditure. Some evidence suggests GLP-1 RAs may slightly reduce PAEE (patients are less hungry, potentially less active) while having minimal direct effect on BMR beyond what is expected from weight loss. Triple agonists (GLP-1/GIP/glucagon receptor) are hypothesised to increase energy expenditure through the glucagon receptor component — glucagon stimulates hepatic thermogenesis, fatty acid oxidation, and potentially brown adipose tissue activation. This could provide a metabolic advantage over GLP-1-only approaches.

FPG represents hepatic glucose output in the fasting state — elevated FPG reflects inadequate suppression of hepatic gluconeogenesis and glycogenolysis due to insulin resistance and relative insulin deficiency. Diagnostic criteria: normal <100 mg/dL (5.6 mmol/L), impaired fasting glucose 100-125 mg/dL (5.6-6.9 mmol/L, indicating prediabetes), diabetes ≥126 mg/dL (7.0 mmol/L, confirmed on repeat testing). FPG is measured in venous plasma after ≥8-hour overnight fast. GLP-1 RAs reduce FPG primarily through: suppression of overnight glucagon secretion (reducing hepatic glucose output) and enhancement of basal insulin secretion. Semaglutide typically reduces FPG by 25-40 mg/dL (1.4-2.2 mmol/L) in type 2 diabetes trials.

Gastric emptying rate is regulated by: duodenal nutrient feedback (fat and acid slow emptying via CCK and secretin), hormonal signals (GLP-1, PYY, amylin slow emptying; ghrelin, motilin accelerate), vagal tone (parasympathetic stimulation promotes emptying), and intrinsic gastric motility (antral contractions, pyloric relaxation). GLP-1 RAs slow gastric emptying dose-dependently, primarily through inhibition of vagal efferent output to gastric smooth muscle. Tachyphylaxis: the gastric emptying delay attenuates with chronic GLP-1 RA use — short-acting agents (exenatide BID, lixisenatide) cause more pronounced meal-by-meal slowing (because they provide intermittent receptor activation), while long-acting agents (semaglutide, dulaglutide) cause less gastric emptying delay at steady state due to continuous receptor activation and subsequent adaptation. This tachyphylaxis explains why long-acting GLP-1 RAs have relatively greater effects on fasting glucose (via glucagon suppression) than postprandial glucose.

GDM affects approximately 2-10% of pregnancies and increases risks of macrosomia, neonatal hypoglycaemia, pre-eclampsia, and future T2D (50% of women with GDM develop T2D within 5-10 years). Diagnosis: oral glucose tolerance test (75g OGTT) at 24-28 weeks gestation using IADPSG criteria. Management: dietary modification, exercise, and insulin if glycaemic targets are not met (insulin does not cross the placenta). GLP-1 RAs and most peptide drugs are NOT approved for use in pregnancy. Animal reproductive toxicity data for GLP-1 RAs have shown embryo-foetal effects at high doses. Women of childbearing potential are advised to discontinue GLP-1 RAs at least 2 months before planned pregnancy due to the prolonged half-life of some agents.

Hepatic gluconeogenesis converts non-carbohydrate substrates (lactate, pyruvate, glycerol, amino acids — particularly alanine and glutamine) to glucose through a multistep enzymatic pathway (pyruvate carboxylase → phosphoenolpyruvate carboxykinase/PEPCK → fructose-1,6-bisphosphatase → glucose-6-phosphatase). Gluconeogenesis is the primary source of hepatic glucose output in the fasting state. Regulation: stimulated by glucagon (activates PEPCK and G6Pase gene transcription via cAMP/PKA/CREB pathway), cortisol, and catecholamines; inhibited by insulin (suppresses PEPCK/G6Pase expression via Akt/FoxO1 pathway). In T2D, hepatic gluconeogenesis is inappropriately elevated due to: hepatic insulin resistance (failure of insulin to suppress gluconeogenic gene expression), elevated glucagon (alpha cell dysfunction), and increased gluconeogenic substrate delivery. GLP-1 RAs reduce gluconeogenesis through: glucagon suppression, improved hepatic insulin sensitivity, and weight loss effects.

Glycaemic control targets: ADA/EASD recommend HbA1c <7.0% for most adults with diabetes (individualised: <6.5% for those at low hypoglycaemia risk, <8.0% for elderly/comorbid patients). Beyond HbA1c, contemporary glycaemic metrics include: time in range (TIR — percentage of time glucose is within 70-180 mg/dL target, measurable by CGM; target ≥70%), time below range (TBR — <70 mg/dL; target <4%), time above range (TAR — >180 mg/dL; target <25%), and glycaemic variability (coefficient of variation <36%). GLP-1 RAs improve glycaemic control through multiple mechanisms: potentiating glucose-stimulated insulin secretion (glucose-dependent — low hypoglycaemia risk), suppressing glucagon secretion, slowing gastric emptying (reducing postprandial glucose excursions), and promoting weight loss (improving insulin sensitivity).

Hepatic glycogenolysis: glycogen phosphorylase cleaves glucose-1-phosphate from glycogen → conversion to glucose-6-phosphate → glucose-6-phosphatase releases free glucose into blood. The liver stores approximately 100g of glycogen (sufficient for 12-18 hours of fasting). Regulation: glucagon activates glycogen phosphorylase via cAMP/PKA (stimulating glycogenolysis) and inactivates glycogen synthase (inhibiting glycogen synthesis) — these coordinated effects mobilise stored glucose. Insulin has the opposite effects. Glucagon emergency treatment for hypoglycaemia exploits glycogenolysis: 1mg glucagon IM/SC/IV mobilises hepatic glycogen stores, raising blood glucose within 10-15 minutes. Glucagon is less effective in: glycogen-depleted states (starvation, chronic alcohol use, advanced liver disease), adrenal insufficiency, and chronic hypoglycaemia (depleted glycogen reserves).

HbA1c is formed by non-enzymatic glycation — glucose binds irreversibly to the N-terminal valine of the haemoglobin beta chain. The rate of glycation is proportional to average glucose concentration over the preceding 2-3 months (reflecting the ~120-day red blood cell lifespan). HbA1c assays: standardised to DCCT/NGSP reference method (reported as percentage) or IFCC reference method (reported as mmol/mol; conversion: IFCC = [DCCT-2.15] × 10.929). Conditions affecting HbA1c accuracy: haemoglobin variants (HbS, HbC, HbE — can interfere with some assay methods), conditions altering RBC lifespan (haemolytic anaemia, recent transfusion — shorter lifespan underestimates average glucose; iron deficiency, B12 deficiency — longer lifespan overestimates), and pregnancy (haemodilution). In GLP-1 RA trials, HbA1c reductions of 1.0-2.0 percentage points are typical at therapeutic doses.

Hyperinsulinaemia represents compensatory hypersecretion of insulin by pancreatic beta cells attempting to overcome peripheral insulin resistance. Fasting hyperinsulinaemia (insulin >25 mU/L) indicates systemic insulin resistance. Postprandial hyperinsulinaemia (exaggerated insulin response to meals) contributes to postprandial hypoglycaemia (reactive hypoglycaemia) and may promote weight gain (insulin is an anabolic, lipogenic hormone). Hyperinsulinaemia has been independently associated with: hypertension (insulin stimulates renal sodium retention and sympathetic nervous system), dyslipidaemia (hepatic triglyceride synthesis), endothelial dysfunction, and possibly cancer progression (insulin promotes cell proliferation through IGF-1 receptor cross-activation). GLP-1 RAs can reduce hyperinsulinaemia by: improving insulin sensitivity (less compensatory secretion needed) and providing glucose-dependent insulin secretion (more efficient insulin use).

Hypoglycaemia classification (ADA/EASD): Level 1 (glucose <70 mg/dL/3.9 mmol/L — alert value requiring treatment), Level 2 (glucose <54 mg/dL/3.0 mmol/L — clinically significant, associated with cognitive impairment), Level 3 (severe — requiring assistance from another person regardless of glucose level). GLP-1 RA monotherapy carries very low hypoglycaemia risk because insulin secretion is glucose-dependent — the GLP-1R-mediated cAMP/PKA signalling in beta cells requires concurrent glucose-mediated depolarisation (closure of KATP channels by ATP generated from glucose metabolism) for insulin granule exocytosis. When combined with sulfonylureas or insulin, hypoglycaemia risk increases because those agents stimulate insulin release independently of glucose. Clinical guidelines recommend reducing sulfonylurea or insulin doses when adding a GLP-1 RA.

Insulin resistance pathophysiology: lipid accumulation in muscle and liver (ectopic fat deposition from positive energy balance) activates inflammatory pathways (NF-κB, JNK) and generates diacylglycerols and ceramides that interfere with insulin receptor substrate (IRS) phosphorylation, disrupting the PI3K/Akt signalling cascade required for GLUT4 translocation and glucose uptake. Measurement: the hyperinsulinaemic-euglycaemic clamp (gold standard — measuring glucose infusion rate required to maintain euglycaemia during insulin infusion) and HOMA-IR (homeostatic model assessment — calculated from fasting glucose × fasting insulin / 405, a practical clinical proxy). GLP-1 RAs improve insulin resistance through: direct weight loss (visceral fat reduction improves hepatic and muscle insulin sensitivity), reduced hepatic glucose output, improved beta cell function (reducing glucotoxicity), and possible direct anti-inflammatory effects in adipose tissue.

Insulin sensitivity is the inverse of insulin resistance and can be measured by: euglycaemic-hyperinsulinaemic clamp (M-value: glucose disposal rate at steady-state insulin — gold standard), frequently sampled IVGTT with minimal model analysis (Si index), HOMA-S (100/HOMA-IR), Matsuda index (from OGTT data), and quantitative insulin sensitivity check index (QUICKI). Factors improving insulin sensitivity: weight loss (5-10% body weight loss can improve insulin sensitivity by 25-50%), exercise (both acute and chronic effects through AMPK activation and GLUT4 upregulation), reduced visceral adiposity, reduced inflammatory cytokines, and improved sleep quality. GLP-1 RA-induced weight loss of 10-20% produces substantial insulin sensitivity improvement — this is partly why some patients with type 2 diabetes achieve diabetes remission (HbA1c <6.5% off diabetes medications) after significant weight loss with these agents.

Lean body mass (LBM = total body weight minus fat mass) includes muscle, bone, organs, and water. Sarcopenia (age-related muscle loss) and sarcopenic obesity (low muscle mass with excess fat) are conditions where further lean mass loss from weight management therapy is particularly concerning. In semaglutide STEP trials, lean mass loss represented approximately 35-40% of total weight lost. Tirzepatide SURMOUNT data showed similar proportions. For context, bariatric surgery typically causes 20-35% lean mass loss as proportion of total. Resistance exercise during GLP-1 RA therapy can attenuate lean mass loss — the STEP 3 trial (semaglutide + intensive behavioural therapy including exercise) showed preserved lean mass compared to drug alone. The development of myostatin inhibitors and activin receptor antagonists as adjuncts to GLP-1 RAs is an active research area aimed at specifically preserving muscle during pharmacological weight loss.

HIV-associated lipodystrophy subtypes: lipoatrophy (loss of peripheral subcutaneous fat — face, limbs, buttocks; caused primarily by older NRTIs like stavudine and zidovudine), lipohypertrophy (accumulation of visceral fat — central abdomen, dorsocervical fat pad/buffalo hump; multifactorial — HIV infection itself, protease inhibitors, NRTIs), and mixed pattern (both features). Tesamorelin mechanism: GHRH analogue → pituitary GH release → GH acts on visceral adipose tissue → stimulates lipolysis (via hormone-sensitive lipase activation) and inhibits lipogenesis → reduced visceral fat. TERAHEART and TERASTRA Phase III trials showed ~15-18% reduction in trunk fat vs placebo at 26 weeks. Effect is maintained with continued treatment but reverses upon discontinuation (visceral fat returns to baseline within months). Side effect profile includes: arthralgia, injection site reactions, and potential IGF-1 elevation requiring monitoring.

De novo lipogenesis (DNL): hepatic conversion of excess carbohydrate-derived acetyl-CoA to fatty acids via the enzymes ACC (acetyl-CoA carboxylase) and FAS (fatty acid synthase), regulated by transcription factors SREBP-1c (activated by insulin) and ChREBP (activated by glucose). DNL contributes to hepatic steatosis (fatty liver) in insulin resistance/T2D — the combination of peripheral insulin resistance with hepatic hyperinsulinaemia drives fat synthesis while simultaneously impairing fat oxidation. GLP-1 RAs reduce hepatic lipogenesis through: improved insulin sensitivity (reduced hyperinsulinaemia → reduced SREBP-1c activation), weight loss (reduced substrate availability), and possible direct hepatic effects (GLP-1Rs are expressed on hepatocytes, though the significance of direct hepatic GLP-1R signalling is debated). These anti-steatotic effects underlie the investigation of GLP-1 RAs for NASH treatment.

Lipolysis pathway: hormone-sensitive lipase (HSL) and adipose triglyceride lipase (ATGL) hydrolyse stored triglycerides in adipocytes → glycerol + free fatty acids (FFAs) released into circulation. Hormonal regulation: stimulated by catecholamines (beta-adrenergic receptor → cAMP → PKA → HSL phosphorylation), glucagon, growth hormone, and cortisol; inhibited by insulin (activates PDE3B, reducing cAMP). Tesamorelin promotes lipolysis specifically in visceral adipose tissue through GH-mediated HSL activation. GLP-1 RAs promote lipolysis indirectly: weight loss reduces insulin levels (less lipolysis inhibition), improved insulin sensitivity shifts the hormonal balance toward fat mobilisation, and reduced caloric intake creates energy deficit requiring fat oxidation. Excessive lipolysis can cause: elevated circulating FFAs (promoting insulin resistance), ketogenesis (risk in insulin-deficient states), and lipotoxicity in non-adipose tissues.

Diagnostic criteria (IDF/AHA/NHLBI harmonised 2009): any 3 of 5: elevated waist circumference (population- and country-specific thresholds), triglycerides ≥150 mg/dL (or on treatment), HDL-C <40 mg/dL men / <50 mg/dL women (or on treatment), blood pressure ≥130/85 mmHg (or on treatment), fasting glucose ≥100 mg/dL (or on treatment). Prevalence: approximately 20-25% of adults globally, increasing with age and obesity prevalence. Pathophysiology centres on insulin resistance driven by visceral adiposity and chronic low-grade inflammation. GLP-1 RAs address multiple MetS components simultaneously: weight loss reduces waist circumference and visceral fat, improved insulin sensitivity reduces fasting glucose, weight loss and potential direct vascular effects reduce blood pressure by 2-5 mmHg, and improved lipid metabolism reduces triglycerides by 10-20%. This multi-target effect is a major advantage over single-target metabolic therapies.

BMI ≥40 (or ≥35 with severe comorbidities) carries the highest medical risk: 2-3× increased all-cause mortality, dramatically elevated rates of T2D (OR 5-12), cardiovascular disease, sleep apnoea (prevalence >50%), osteoarthritis, cancer (multiple types), depression, and reduced life expectancy by 8-14 years. Bariatric surgery has been the only effective treatment achieving durable >20% weight loss. GLP-1 RAs and dual agonists now produce weight losses in this range: semaglutide 2.4mg ~15% (STEP 1); tirzepatide 15mg ~22.5% (SURMOUNT-1). In patients with BMI ≥40, even larger percentage losses may be achievable with higher doses or combination approaches. Subgroup analyses from Phase III trials consistently show that patients with higher baseline BMI achieve similar or greater percentage weight loss, though they remain at higher absolute BMI post-treatment.

Obesity pathophysiology: energy imbalance (caloric intake > expenditure) driven by: genetic predisposition (>900 genes associated with BMI variation; heritability 40-70%), environmental factors (food availability, portion sizes, ultra-processed food, sedentary behaviour), neuroendocrine regulation (leptin resistance, altered ghrelin/GLP-1/PYY signalling, hypothalamic inflammation), gut microbiome (altered bacterial composition affecting energy harvest and metabolic signalling), and psychological factors (stress eating, food addiction neurobiology). Pharmacotherapy positioning: guidelines recommend anti-obesity medications as adjunct to lifestyle intervention for BMI ≥30 or ≥27 with comorbidity. GLP-1 RAs (semaglutide 2.4mg/Wegovy) and dual agonists (tirzepatide/Zepbound) produce weight losses of 15-25% — approaching bariatric surgery outcomes (25-35%) for the first time with pharmacotherapy. The paradigm shift toward viewing obesity as a chronic disease requiring long-term medical management (analogous to hypertension or diabetes) is driving sustained treatment approaches.

Clinical significance thresholds: ≥5% weight loss produces meaningful improvements in metabolic risk factors (HbA1c, blood pressure, lipids, sleep apnoea); ≥10% produces significant cardiovascular risk reduction and potential diabetes remission; ≥15% approaches bariatric surgery-level metabolic benefits. GLP-1 RA efficacy: liraglutide 3.0mg ~8%, semaglutide 2.4mg ~15%, tirzepatide 15mg ~22.5% (all placebo-adjusted at 68-72 weeks). Responder analysis: percentage achieving ≥5%/≥10%/≥15%/≥20% thresholds provides clinically meaningful context beyond mean weight loss. For semaglutide 2.4mg: ~86% achieve ≥5%, ~70% achieve ≥10%, ~50% achieve ≥15%, ~32% achieve ≥20%. Individual variability is significant — genetic, behavioural, and microbiome factors likely contribute to differential response.

Postprandial glucose (PPG) is typically measured 1-2 hours after eating. Normal PPG: <140 mg/dL (7.8 mmol/L) at 2 hours. In type 2 diabetes, PPG elevations often precede fasting hyperglycaemia and contribute significantly to overall glycaemia (particularly when HbA1c is 7.0-8.0%). GLP-1 RAs are particularly effective at reducing PPG through three synergistic mechanisms: glucose-dependent insulin secretion (insulin release proportional to meal-induced glucose rise), glucagon suppression (preventing postprandial hepatic glucose output), and delayed gastric emptying (slowing nutrient absorption — this is the dominant short-term mechanism, reducing PPG peak by 30-50%). Short-acting GLP-1 RAs (exenatide BID, lixisenatide) have greater PPG-lowering effect relative to FPG-lowering than long-acting agents, because the gastric emptying delay is subject to tachyphylaxis with continuous GLP-1R exposure.

Prediabetes criteria: impaired fasting glucose (IFG: FPG 100-125 mg/dL), impaired glucose tolerance (IGT: 2-hour OGTT glucose 140-199 mg/dL), or HbA1c 5.7-6.4%. Approximately 70% of people with prediabetes eventually progress to T2D. Risk factors: overweight/obesity, physical inactivity, family history, age >45, GDM history, and ethnicity (higher risk in Black, Hispanic, Native American, Asian American populations). Prevention: lifestyle intervention (5-7% weight loss, 150 min/week moderate activity) reduces T2D risk by approximately 58% (DPP trial). Pharmacological prevention: metformin reduces risk by approximately 31%. GLP-1 RAs have not been specifically studied for diabetes prevention as a primary indication, but their weight loss effects and metabolic improvements may reduce progression risk in clinical practice.

Satiety is regulated by a complex neuroendocrine network: peripheral signals (GLP-1, PYY, CCK — released from gut after eating → activate vagal afferents and directly stimulate brainstem areas; ghrelin — suppressed after eating), brainstem integration (nucleus tractus solitarius/NTS receives vagal inputs and circulating hormone signals), and hypothalamic processing (arcuate nucleus POMC/CART anorexigenic neurons activated, NPY/AgRP orexigenic neurons suppressed; signals relayed to paraventricular nucleus → reduced food intake). GLP-1 RAs promote satiety through: direct activation of GLP-1Rs on NTS neurons, hypothalamic arcuate and paraventricular neurons, and vagal afferent neurons; delayed gastric emptying (prolonging gastric distension signals); and possible effects on food reward pathways (reduced preference for high-fat, high-sugar foods — demonstrated in functional MRI studies with semaglutide).

SAT is distributed beneath the skin throughout the body, with gender-specific patterns: gluteofemoral predominance in women (estrogen-mediated), abdominal predominance in men. SAT serves important physiological functions: energy storage, thermal insulation, mechanical protection, and endocrine function (producing leptin, adiponectin, and other adipokines). SAT is generally considered less metabolically harmful than VAT — the 'metabolically healthy obese' phenotype is characterised by SAT-predominant fat distribution with normal insulin sensitivity and inflammatory markers. However, SAT can become dysfunctional in obesity, with hypertrophic adipocytes, inflammatory infiltration, and fibrosis. For peptide drug administration, SAT is the tissue layer into which subcutaneous injections are deposited — its thickness varies by body region and individual, affecting needle length selection and injection technique.

T1D is an autoimmune disease where CD4+ and CD8+ T cells destroy pancreatic beta cells, leading to absolute insulin deficiency. Onset typically occurs in childhood/adolescence but can occur at any age. Management requires exogenous insulin replacement (basal-bolus regimen or insulin pump). Pramlintide (synthetic amylin analogue) is the only non-insulin peptide drug specifically indicated for T1D — it addresses the amylin deficiency that accompanies insulin deficiency (amylin is co-secreted with insulin from beta cells). Pramlintide reduces postprandial glucagon secretion, slows gastric emptying, and promotes satiety, improving postprandial glucose control when added to insulin. GLP-1 RAs are NOT approved for T1D (inadequate beta cell mass to respond to GLP-1R stimulation), though investigational studies have explored adjunctive use.

T2D pathophysiology (DeFronzo's ominous octet): insulin resistance in muscle, liver, and adipose tissue; beta cell failure (progressive decline in insulin secretion); increased hepatic glucose output; decreased incretin effect; increased glucagon secretion; enhanced renal glucose reabsorption; neurotransmitter dysfunction (appetite dysregulation); and systemic inflammation. Treatment algorithm (ADA/EASD 2022): first-line metformin + lifestyle; second-line selection based on comorbidities — GLP-1 RA preferred for patients with established cardiovascular disease (ASCVD) or at high cardiovascular risk (based on CVOT data), patients with heart failure, and patients where weight management is a priority. Semaglutide, liraglutide, dulaglutide, exenatide, lixisenatide, and tirzepatide are all approved for T2D. The GLP-1 RA class has advanced from fourth-line agents to preferred second-line therapy over the past decade based on accumulating cardiovascular outcome and weight management evidence.

VAT is measured by: CT scan at L4-L5 level (gold standard — quantitative measurement of VAT area in cm²), MRI (similar accuracy without radiation), DEXA (whole-body composition with regional fat estimates), and waist circumference (clinical proxy — threshold values: >102cm/40in men, >88cm/35in women for elevated risk). VAT is metabolically active: it secretes pro-inflammatory adipokines (TNF-α, IL-6, MCP-1), contributes to hepatic insulin resistance through portal delivery of free fatty acids, and produces less adiponectin (an insulin-sensitising adipokine) than subcutaneous fat. GLP-1 RAs produce preferential VAT reduction relative to lean mass — body composition studies (DEXA and MRI) in semaglutide and tirzepatide trials show that approximately 30-40% of weight lost is lean mass and 60-70% is fat mass, with disproportionate VAT reduction.

Measurement technique: measured at the midpoint between the lowest palpable rib and the iliac crest, at the end of normal expiration, using a non-elastic tape. Risk thresholds (ATP III/IDF): elevated risk >94cm men / >80cm women (European); substantially increased risk >102cm / >88cm. Ethnic-specific thresholds exist: South Asian/Chinese: >90cm men / >80cm women. Waist circumference correlates with VAT area (r ≈ 0.80) and is the simplest clinical proxy for visceral adiposity. GLP-1 RA trials typically report waist circumference as a secondary endpoint: semaglutide 2.4mg reduces waist circumference by approximately 13-14cm (STEP 1), reflecting preferential visceral fat loss. Waist-to-height ratio (<0.5 is generally healthy) is an alternative metric that may be more universally applicable across populations.

Weight regain after GLP-1 RA discontinuation reflects: restoration of pre-treatment appetite signalling (hypothalamic setpoint returns to baseline), metabolic adaptation (reduced basal metabolic rate from weight loss persists, creating positive energy balance when appetite returns), loss of pharmacological gastric emptying delay, and potentially, loss of central reward pathway modulation. STEP 4 data: participants who discontinued semaglutide 2.4mg after 20 weeks regained approximately 2/3 of lost weight over 48 weeks off treatment. STEP 1 extension: participants maintaining semaglutide 2.4mg for 2 years sustained ~15% weight loss; those discontinuing at 1 year regained substantially. These data support the chronic disease model — obesity, like hypertension, requires ongoing treatment for maintained benefit. The durability question has implications for healthcare economics, patient expectations, and long-term treatment planning.