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Medical Textbook Library - Complete Reference

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Obesity Coverage in the Library

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Obesity - Comprehensive Medical Reference

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1. DEFINITION AND CLASSIFICATION

BMI Classification Table (WHO)

• For very muscular individuals, BMI may overestimate body fat
• For any given BMI, women have a higher percentage of body fat than men
• Asian populations have worse health outcomes at lower BMI thresholds - lower cutoffs are proposed for the Asia-Pacific region
• BMI-based definitions were established largely in predominantly white, Western populations

Fat Distribution: Waist-to-Hip Ratio

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2. EPIDEMIOLOGY

• United States (2017-2018): 42.4% of U.S. adults aged ≥20 had obesity - nearly a 3-fold increase since 1976-1980 (when prevalence was 14.5%)
• Racial disparities (US): Non-Hispanic Black people 49.6% > Hispanic 44.8% > non-Hispanic White 42.2% > non-Hispanic Asian 17.4%
• Sex: Prevalence slightly higher in women overall; Black women have the highest prevalence at 56.9%
• Global (WHO, 2016): >1.9 billion adults overweight; >650 million obese; 39% of adults overweight, 13% obese. Obesity has nearly tripled worldwide since 1975
• Children: In 2016, 340 million children and adolescents aged 5-19 were overweight or obese. 38 million children under age 5 were overweight or obese in 2019
• Most of the world's population now lives in countries where overweight and obesity kills more people than underweight

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3. PATHOBIOLOGY AND ETIOLOGY

3.1 Energy Balance

3.2 Genetic Factors

• Leptin/leptin receptor (LEP/LEPR) mutations: Rare, cause severe early-onset obesity with hyperphagia; leptin-deficient patients respond dramatically to leptin replacement
• MC4R (melanocortin 4 receptor) mutations: ~5% of patients with severe, early-onset obesity have heterozygous loss-of-function MC4R mutations; features include hyperphagia, hyperinsulinemia, and increased linear growth
• POMC (proopiomelanocortin) mutations: Homozygous/compound heterozygous mutations cause hyperphagia, early-onset obesity, isolated ACTH deficiency, and hypopigmentation
• Prohormone convertase 1 (PC1) deficiency: Severe early-onset obesity and ACTH deficiency due to impaired POMC processing
• Common variants (GWAS): The genetic predisposition to obesity is largely mediated through the brain's control of food intake - individuals carrying obesity-predisposing genetic variants tend to eat more and be less readily satiated

3.3 Why Doesn't Leptin Prevent Obesity?

3.4 Environmental Factors (Obesogenic Environment)

3.5 Constitutional and Epigenetic Influences

• Undernutrition in first two trimesters of pregnancy → increased probability of adult obesity in offspring
• Undernutrition in last trimester / early postnatal period → decreased risk of adult obesity, but increased risk of hypertension, abnormal glucose tolerance, and cardiovascular disease
• Infants of diabetic mothers tend to be fatter; their children have greater obesity prevalence aged 5-19
• Intrauterine exposure to diabetic environment → increased risk of diabetes and obesity in offspring

3.6 Secondary Causes of Obesity

• Polycystic ovarian syndrome (PCOS)
• Hypothyroidism
• Cushing's syndrome
• Hypothalamic disease

• Diabetes medications: insulin, sulfonylureas, thiazolidinediones
• Steroid hormones
• Antipsychotics: clozapine, olanzapine, risperidone
• Mood stabilizers: lithium
• Antidepressants: TCAs, MAOIs, paroxetine, mirtazapine
• Antiepileptics: valproate, gabapentin, carbamazepine

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4. PHYSIOLOGIC REGULATION OF ENERGY BALANCE

Central Regulation

• Arcuate nucleus (ARC): Contains two opposing populations - AgRP/NPY neurons (appetite-stimulating) and POMC/CART neurons (appetite-suppressing)
• Leptin (from adipocytes): acts on ARC to suppress AgRP/NPY and stimulate POMC/CART → decreases appetite
• Insulin (from pancreas): also acts centrally to suppress food intake
• Ghrelin (from stomach): rises before meals, stimulates appetite; falls postprandially
• GLP-1, PYY, CCK (gut hormones): released postprandially, signal satiety

Peripheral Regulation

• Resting energy expenditure increases (not decreases) with obesity
• When obese patients lose weight by dieting, they tend to be more "energy efficient" than persons who were never obese - particularly in how many calories they burn during defined activity bouts

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5. ADVERSE CONSEQUENCES OF OBESITY

5.1 Metabolic Complications

• Obesity is the single most important modifiable risk factor for T2DM
• Visceral adiposity drives insulin resistance through multiple mechanisms: elevated free fatty acids, adipokine dysregulation (↑ TNF-α, IL-6, resistin; ↓ adiponectin), ectopic fat deposition in liver and muscle
• ADA recommends screening individuals at an earlier age if they are overweight with one additional risk factor

• Upper body/visceral obesity is associated with: ↑ triglycerides, ↓ HDL cholesterol, ↑ proportion of small dense LDL particles
• Mechanism: increased hepatic VLDL secretion driven by increased delivery of free fatty acids from visceral fat; reduced HDL and increased small dense LDL are indirect consequences of elevated triglyceride-rich VLDL
• Genetic polymorphisms in apolipoprotein E, lipoprotein lipase, apo B-100, and apo A-II modulate expression of these lipid abnormalities

• Cluster of: central obesity + hypertriglyceridemia + low HDL + hypertension + elevated fasting glucose

5.2 Cardiovascular Complications

• Obesity is an independent risk factor for coronary artery disease, heart failure, and stroke
• Hypertension: obesity increases plasma volume and cardiac output, activates the renin-angiotensin-aldosterone system, and increases sympathetic nervous system activity
• Obesity cardiomyopathy: eccentric left ventricular hypertrophy from increased cardiac output demands
• Obstructive sleep apnea (see below) - a major driver of cardiovascular risk in obesity

5.3 Pulmonary Complications

• Defined as BMI ≥30 + chronic daytime hypercapnia (PaCO₂ >45 mmHg) in the absence of other causes
• Mechanism: reduced respiratory system compliance, decreased functional residual capacity, increased work of breathing
• Treatment: weight loss, positive airway pressure (CPAP or BiPAP), supplemental oxygen

• Caused by enlargement of upper airway soft tissue → collapse of upper airways during inspiration during sleep
• More prevalent in men and women with upper body/visceral obesity
• Obstruction leads to: apneas, hypoxemia, hypercarbia, elevated catecholamines and endothelins
• Frequent arousals → poor sleep quality
• Associated with increased risk of hypertension; severe OSA can lead to right-sided heart failure and sudden death
• Goldman-Cecil notes: pulmonary function testing in obese patients shows reduced lung volumes (restrictive pattern)

5.4 Gastrointestinal Complications

• NAFLD/NASH (now MASLD/MASH): Strongly associated with obesity and metabolic syndrome; can progress to cirrhosis
• Gastroesophageal reflux disease (GERD): Increased intraabdominal pressure drives reflux
• Gallstones: Obesity promotes cholelithiasis through supersaturation of bile with cholesterol
• Colorectal polyps and cancer: Insulin resistance and hyperinsulinemia promote colonic epithelial proliferation

5.5 Endocrine Complications

• PCOS: Insulin resistance associated with obesity may trigger development of PCOS in susceptible women; characterized by hirsutism, irregular menses, anovulation
• Male hypogonadism: Obese men may suffer mild to severe hypothalamic hypogonadism (improving with weight loss)
• Elevated conversion of androgens to estrogens in adipose tissue

5.6 Musculoskeletal and Other Systemic Complications

• Osteoarthritis: Excess body weight increases prevalence of lower extremity degenerative joint disease
• Venous stasis: Occasionally aggravated by right-sided heart failure in severely obese patients
• Pseudotumor cerebri (idiopathic intracranial hypertension): Associated with obesity, especially in young women
• Gout: Hyperuricemia driven by insulin resistance and renal urate retention

5.7 Cancer Risk

• Breast, esophageal, liver, gallbladder, stomach, pancreatic, colorectal, ovarian, uterine/endometrial, renal cell, thyroid, meningioma, and multiple myeloma
• Likely mechanisms: higher insulin-like growth factor (IGF-1) levels, elevated estrogens from peripheral aromatization, chronic inflammation

5.8 Psychosocial Consequences

• Serious social, economic, and medical discrimination
• Depression and anxiety
• Impaired quality of life
• People with obesity may become socially and economically disadvantaged

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6. EVALUATION AND DIAGNOSIS

Clinical Assessment

1. Weight
2. Height (to calculate BMI)
3. Waist circumference

• Men: >102 cm (>40 in) - high risk
• Women: >88 cm (>35 in) - high risk

History

• Age of onset and course of weight gain
• Dietary habits and physical activity patterns
• Review of medications (secondary causes)
• Sleep history (regularity, duration, efficiency, satisfaction)
• Psychosocial stressors - sleep deprivation is associated with metabolic alterations in appetite regulation, sympathetic overactivity, reduced insulin sensitivity, and circadian rhythm changes
• Stress contributes to obesity in part via adrenal cortical axis activation and elevated cortisol
• Family history (genetic predisposition)
• Patient's motivation for weight management

Physical Examination

• Blood pressure
• Signs of secondary causes (e.g., cushingoid features, thyroid abnormalities)
• Acanthosis nigricans (insulin resistance marker)
• Signs of complications (edema, hepatomegaly)

Laboratory Evaluation

• Fasting glucose and HbA1c
• Fasting lipid panel
• Liver function tests / hepatic imaging for NAFLD
• TSH (exclude hypothyroidism)
• If Cushing's suspected: overnight dexamethasone suppression test or 24-hr urinary free cortisol

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7. TREATMENT

1. Sustained weight reduction
2. Improvement or prevention of comorbidities
3. Improved quality of life

7.1 Lifestyle Modification (Foundation of All Treatment)

• Caloric restriction (typically 500-1000 kcal/day deficit) targeting 0.5-1 kg/week weight loss
• Any dietary pattern achieving caloric deficit can be effective (low-fat, low-carbohydrate, Mediterranean, etc.)
• Reducing energy-dense foods, sugar-sweetened beverages, and large portion sizes

• Physical fitness is an important predictor of all-cause mortality independent of BMI
• Aerobic exercise, resistance training, or both
• Sedentary behavior is independently associated with adverse metabolic outcomes
• Exercise preserves lean mass during weight loss

• Cognitive behavioral therapy (CBT)
• Motivational interviewing
• Self-monitoring (food diaries, step counters)
• Regular follow-up and support groups

7.2 Pharmacological Treatment

7.3 Surgical Treatment (Bariatric Surgery)

• BMI ≥40 kg/m² without comorbidities, OR
• BMI ≥35 kg/m² with significant obesity-related comorbidities (T2DM, hypertension, OSA, etc.)
• Failure of prior nonsurgical attempts

• 28-33% average total body weight loss at 12-18 months (RYGB/sleeve)
• Significant improvement in T2DM, hypertension, dyslipidemia, OSA, and quality of life
• Meta-analysis: surgery associated with reduced mortality (OR = 0.55 cardiovascular death, OR = 0.70 all-cause mortality) vs. no surgery
• Swedish Obese Subjects Study (15-year data): 78% reduction in incidence of T2DM development in surgically treated patients
• 68.2% of patients with T2DM experienced complete remission within 5 years; however, one-third redeveloped T2DM within 5 years
• Mortality rate from bariatric surgery: generally <1%, varies with procedure, patient age, comorbidities, and surgical team experience

• RYGB: Stromal stenosis or marginal ulcers (5-15%); treated with endoscopic balloon dilation and acid suppression
• Adjustable banding: Minimal absorptive abnormalities; micronutrient deficiencies uncommon if diet balanced
• Restrictive-malabsorptive procedures: Risk of micronutrient deficiencies - vitamin B₁₂, iron, folate, calcium, vitamin D - require lifelong supplementation

• Not yet requiring insulin
• Shorter duration of disease
• Lower HbA1c at baseline
• Earlier-stage T2DM

7.4 Non-Surgical Endoscopic Techniques

1. Intraluminal gastric balloons - space-occupying devices limiting gastric volume, decreasing hunger. Must be removed after 6 months. Weight loss: 6-12% at device removal. Adverse effects: nausea, vomiting, abdominal pain.
2. Endoscopic sleeve gastroplasty (ESG) - suturing reduces gastric volume, induces early satiety. More durable than balloons.
3. Aspiration therapy - endoscopic tube placed in stomach allowing aspiration of portion of meal after eating.

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8. SPECIAL POPULATIONS AND CONSIDERATIONS

Sleep and Obesity

• Sleep deprivation is associated with metabolic alterations in appetite regulation (↑ ghrelin, ↓ leptin), sympathetic nervous system overactivity, reduced insulin sensitivity, and circadian rhythm disturbances
• OSA must be identified and treated - CPAP therapy can improve metabolic parameters

Obesity in Pregnancy

• Maternal obesity increases risks of gestational diabetes, pre-eclampsia, stillbirth, macrosomia, and cesarean delivery
• Intrauterine exposure to diabetic/obese environment increases offspring's risk of obesity and T2DM

Childhood Obesity

• Using age- and sex-specific BMI cutoffs: overweight ≥91st percentile; obesity ≥99th percentile
• Childhood obesity tracks strongly to adult obesity and metabolic disease

Medications to Prefer/Avoid in Obese Patients

• Antihypertensives: beta-blockers (especially older agents), alpha-blockers
• Alternatives: ACE inhibitors, ARBs, calcium channel blockers, diuretics

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9. PROGNOSIS

• Even modest weight loss (5-10% of body weight) produces clinically meaningful improvements in blood pressure, glycemia, lipids, and sleep apnea
• Weight regain is common after non-surgical interventions; obesity is a chronic, relapsing disease requiring long-term management
• Bariatric surgery produces the most durable weight loss outcomes
• Physical fitness independently predicts mortality regardless of BMI - improving cardiorespiratory fitness even without weight loss has substantial health benefits

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Summary

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