Residency · Residency · General Surgery
Obesity: Pathophysiology and Patient Selection for Bariatric Surgery
Introduction
Obesity is a chronic, relapsing, multifactorial disease affecting over 40% of adults in the United States. It is associated with over 200 comorbid conditions and is the second leading cause of preventable death after tobacco use. Despite advances in behavioral and pharmacologic therapies, metabolic and bariatric surgery (MBS) remains the most effective and durable treatment for severe obesity, producing sustained weight loss and resolution or improvement of obesity-related comorbidities. The general surgery resident must understand the pathophysiology of obesity, the evidence supporting bariatric surgery, and the principles of appropriate patient selection.
Definition and Classification
Body mass index, calculated as weight in kilograms divided by height in meters squared, is the standard classification system:
| BMI (kg/m²) | Classification |
|---|---|
| 25–29.9 | Overweight |
| 30–34.9 | Class I obesity |
| 35–39.9 | Class II obesity |
| ≥40 | Class III (severe) obesity |
| ≥50 | Super obesity |
| ≥60 | Super-super obesity |
Overweight is defined as a BMI of 25-29.9, Class I obesity as 30-34.9, Class II obesity as 35-39.9, Class III (severe) obesity as 40 or greater, super obesity as 50 or greater, and super-super obesity as 60 or greater. BMI has important limitations, as it does not distinguish fat mass from lean mass, does not account for fat distribution, and may underestimate metabolic risk in normal-weight individuals with central adiposity (the "metabolically obese, normal weight" phenotype). Waist circumference serves as a complementary measure of central and visceral adiposity, with elevated risk at greater than 102 cm (40 inches) in men and greater than 88 cm (35 inches) in women. The Edmonton Obesity Staging System classifies obesity by medical, functional, and psychological impairment (stages 0-4) rather than BMI alone.
Pathophysiology of Obesity
Energy Balance and Adipose Biology
Body weight is regulated by a complex interplay of hypothalamic circuits, gut hormones, adipokines, and autonomic nervous system signaling, and the body actively defends against weight loss through compensatory mechanisms. Adipose tissue functions as an endocrine organ, secreting adipokines including leptin, adiponectin, resistin, TNF-alpha, and IL-6, which regulate appetite, insulin sensitivity, inflammation, and metabolism. Leptin is produced by adipocytes in proportion to fat mass and signals satiety to the hypothalamus, but obese individuals have elevated leptin levels and demonstrate leptin resistance, blunting the anorexigenic signal. The set point theory holds that the body defends a biologically determined weight set point, and after weight loss, metabolic rate decreases, appetite increases, and hormonal changes promote weight regain.
Gut Hormones
Ghrelin is an orexigenic (appetite-stimulating) hormone produced primarily by the gastric fundus whose levels rise before meals and fall after eating; ghrelin levels increase with diet-induced weight loss, promoting weight regain. GLP-1 (glucagon-like peptide-1) is an incretin hormone from L-cells of the distal ileum that promotes satiety, slows gastric emptying, and enhances insulin secretion, and its levels increase after bariatric surgery, especially RYGB and sleeve gastrectomy. PYY (peptide YY) is an anorexigenic hormone from L-cells that reduces appetite; its levels are reduced in obesity and increase after bariatric surgery. Cholecystokinin is released from the duodenum in response to fat and protein and promotes satiety while stimulating pancreatic enzyme secretion and gallbladder contraction.
Metabolic Adaptations to Weight Loss
Metabolic adaptation (adaptive thermogenesis) causes resting metabolic rate to decrease beyond what is expected from the loss of lean mass alone and can persist for years after weight loss, as demonstrated in the Biggest Loser study. After diet-induced weight loss, hormonal changes include increased ghrelin, decreased leptin, decreased GLP-1, and decreased PYY, all of which promote weight regain. These adaptations explain why lifestyle interventions alone produce only 5-10% sustained weight loss in most patients and why bariatric surgery, which fundamentally alters gut hormone signaling, produces more durable results.
<image>Illustration of the gut-brain axis in appetite regulation showing the hypothalamus receiving signals from circulating hormones including leptin from adipose tissue, ghrelin from the stomach, GLP-1 and PYY from the distal ileum, and CCK from the duodenum, with arrows indicating orexigenic and anorexigenic effects and how bariatric surgery alters these hormonal signals</image>
Obesity-Related Comorbidities
Metabolic
Type 2 diabetes mellitus is the most significant metabolic comorbidity, with obesity being the strongest modifiable risk factor through insulin resistance from visceral adiposity; bariatric surgery achieves remission in 40-80% of patients. Dyslipidemia manifests as elevated triglycerides, low HDL, and elevated small dense LDL. Metabolic syndrome encompasses central obesity, hyperglycemia, hypertension, dyslipidemia, and a pro-inflammatory/pro-thrombotic state. Non-alcoholic fatty liver disease and NASH are present in 60-90% of patients with severe obesity, and bariatric surgery improves or resolves NASH in the majority.
Cardiovascular
Hypertension is present in 40-60% of obese patients and resolves or improves in 60-80% after bariatric surgery. Obesity cardiomyopathy results from volume overload and direct lipotoxic effects leading to heart failure. Coronary artery disease and stroke risk increase with BMI and visceral adiposity.
Respiratory
Obstructive sleep apnea is present in 40-90% of bariatric surgery candidates and should be screened with the STOP-BANG questionnaire, diagnosed with polysomnography, and treated with CPAP perioperatively. Obesity hypoventilation syndrome, defined by obesity (BMI 30 or greater), daytime hypercapnia (PaCO2 above 45 mmHg), and sleep-disordered breathing, carries high perioperative risk.
Musculoskeletal
Osteoarthritis of weight-bearing joints (knees and hips) is common, and weight loss significantly reduces symptoms. Chronic pain and disability limit mobility and physical activity, perpetuating the obesity cycle.
Psychosocial
Depression and anxiety have a bidirectional relationship with obesity. Binge eating disorder is present in 5-30% of bariatric surgery candidates and requires preoperative identification and treatment. Weight stigma involves pervasive societal discrimination that affects mental health, healthcare access, and outcomes.
Cancer Risk
Obesity increases the risk for at least 13 cancer types: endometrial, breast (postmenopausal), colorectal, esophageal adenocarcinoma, gastric cardia, hepatocellular, kidney, ovarian, pancreatic, gallbladder, thyroid, meningioma, and multiple myeloma. Bariatric surgery reduces cancer incidence by 30-50% in observational studies.
Non-Surgical Treatment of Obesity
Lifestyle Interventions
Comprehensive lifestyle programs incorporating dietary modification, increased physical activity, and behavioral therapy typically produce 5-10% weight loss at 1 year, with significant attrition and weight regain. Dietary approaches include caloric restriction (500-1000 kcal/day deficit), very low calorie diets (less than 800 kcal/day, medically supervised), Mediterranean, low-carbohydrate, or intermittent fasting approaches, and no single diet has been shown to be superior long-term.
Pharmacotherapy
GLP-1 receptor agonists represent a major advance, with semaglutide (Wegovy) producing 15-17% weight loss at 68 weeks in the STEP trials and tirzepatide (a dual GIP/GLP-1 agonist, Zepbound) achieving 20-22% weight loss. Other agents include phentermine-topiramate (Qsymia), naltrexone-bupropion (Contrave), and orlistat (Xenical). Limitations include weight regain after medication discontinuation in most patients, still-accumulating long-term safety data for newer agents, and cost and insurance coverage barriers. Anti-obesity medications may be used as an adjunct before or after bariatric surgery, and some patients achieve sufficient weight loss with pharmacotherapy alone.
Patient Selection for Bariatric Surgery
Current Indications (Updated ASMBS/IFSO 2022 Guidelines)
Surgery is indicated for BMI of 35 or greater regardless of comorbidities, and for BMI of 30-34.9 with metabolic disease (type 2 diabetes, hypertension, dyslipidemia, OSA, NAFLD/NASH, GERD, or other obesity-related conditions) that is inadequately controlled with non-surgical interventions. The requirement to demonstrate failure of non-surgical weight loss attempts has been removed in the updated guidelines, though some insurers still require it. Bariatric surgery is appropriate for adolescents with Class II obesity (BMI 35 or greater or 120% of the 95th percentile) with comorbidities, or Class III obesity (BMI 40 or greater or 140% of the 95th percentile).
<image>Flowchart illustrating the patient selection algorithm for bariatric surgery starting with BMI classification, assessment of obesity-related comorbidities, multidisciplinary evaluation including psychological assessment and nutritional counseling, and contraindication screening, leading to the choice of surgical procedure</image>
Contraindications
Absolute contraindications include active substance abuse, uncontrolled psychiatric illness that would impair compliance, inability to understand the procedure and comply with postoperative requirements, and physiologic unfitness for surgery. Relative contraindications include current smoking (cessation required for at least 6 weeks preoperatively), active inflammatory bowel disease (for some procedures), portal hypertension with esophageal varices, and severe coagulopathy. There is no upper age limit per se, but a careful risk-benefit analysis should be performed in elderly patients, and the lower age limit varies by institutional policy and guidelines.
Preoperative Evaluation
The multidisciplinary team includes a surgeon, dietitian, psychologist or psychiatrist, and primary care physician, and this team approach is the standard of care. Psychological evaluation assesses for binge eating disorder, depression, anxiety, substance use, eating behaviors, social support, and realistic expectations and functions as an opportunity for optimization rather than a gatekeeping tool. Nutritional counseling covers postoperative dietary progression, lifelong vitamin and mineral supplementation, and behavioral modification. Medical optimization includes glycemic control (HbA1c target below 8-9%), blood pressure management, OSA screening with CPAP initiation, smoking cessation, and VTE risk assessment. Some programs require 5-10% preoperative weight loss, which reduces liver volume, improves operative conditions, and demonstrates compliance, though evidence for improved outcomes is mixed. Endoscopy is recommended for all patients undergoing RYGB to assess for H. pylori, ulcers, and Barrett's esophagus, and is variably recommended for sleeve gastrectomy.
Evidence for Bariatric Surgery
The Swedish Obese Subjects Study is a landmark prospective controlled trial showing that bariatric surgery reduces long-term mortality by 29%, cardiovascular events, cancer incidence, and type 2 diabetes incidence compared to conventional treatment over more than 20 years. The STAMPEDE trial demonstrated that bariatric surgery (RYGB and sleeve gastrectomy) is superior to intensive medical therapy for diabetes control at 5 years, with 29% of surgical patients achieving HbA1c below 6% versus 5% with medical therapy alone. Bariatric surgery achieves 20-35% total weight loss maintained at 10-20 years, vastly superior to any non-surgical intervention. Observational studies consistently show a 30-50% reduction in all-cause mortality after bariatric surgery.
Key Clinical Pearls
Obesity is a chronic, progressive disease driven by complex neuroendocrine mechanisms and is not simply a failure of willpower. Bariatric surgery produces durable weight loss and comorbidity resolution because it fundamentally alters gut hormone signaling, unlike diet-induced weight loss. The 2022 ASMBS/IFSO guidelines expanded indications to include BMI of 30 or greater with metabolic disease, reflecting the strong evidence for metabolic benefits. Preoperative multidisciplinary evaluation including psychological assessment, nutritional counseling, and medical optimization is the standard of care. GLP-1 receptor agonists are transforming the obesity treatment landscape, but weight regain after discontinuation and the need for long-term use remain considerations compared to surgery.
References
- Eisenberg D, Shikora SA, Aarts E, et al. 2022 American Society for Metabolic and Bariatric Surgery (ASMBS) and International Federation for the Surgery of Obesity and Metabolic Disorders (IFSO) indications for metabolic and bariatric surgery. Surg Obes Relat Dis. 2022;18(12):1345-1356.
- Sjostrom L, Narbro K, Sjostrom CD, et al. Effects of bariatric surgery on mortality in Swedish obese subjects. N Engl J Med. 2007;357(8):741-752.
- Schauer PR, Bhatt DL, Kirwan JP, et al. Bariatric surgery versus intensive medical therapy for diabetes: 5-year outcomes (STAMPEDE). N Engl J Med. 2017;376(7):641-651.
- Wilding JPH, Batterham RL, Calanna S, et al. Once-weekly semaglutide in adults with overweight or obesity (STEP 1). N Engl J Med. 2021;384(11):989-1002.
