Pathobiology of Obesity¶
Chapter 413 | Part 12: Endocrinology and Metabolism · Part 12 – Endocrinology & Metabolism · Chapter 413
Key Clinical Points¶
- Obesity is defined as excess adipose tissue mass that adversely affects health; BMI (weight/height²) is the standard clinical proxy.
- WHO criteria: Overweight = 25–30 kg/m²; Obesity ≥ 30 kg/m².
- Waist-to-hip ratio >0.9 in women or >1.0 in men indicates central adiposity associated with increased risk for type 2 diabetes and dyslipidemia.
- Leptin resistance explains why elevated leptin levels in obesity do not prevent weight gain, though leptin still functions to defend against fat loss.
- Genetic factors are significant; up to 20% of children with severe obesity have rare chromosomal abnormalities or highly penetrant genetic mutations.
- Monogenic obesity (e.g., MC4R mutations) is the most common cause of severe early-onset obesity (~5–6% of cases).
- Setmelanotide (an MC4R agonist) is used for specific defects in POMC, PCSK1, and the leptin receptor.
- Insulin resistance is a hallmark of obesity, driven by adipose tissue inflammation and ectopic fat storage in non-adipose tissues (liver/muscle).
- Genetic diagnosis informs management (difficulty of weight loss via diet/exercise) and surgical risk assessment (bariatric surgery feasibility).
- Obesity complications include mechanical issues (osteoarthritis, sleep apnea) and metabolic ones (dyslipidemia, T2D, NAFLD).
DEFINITION & OVERVIEW¶
• Definition: Obesity is defined as a state of excess adipose tissue mass that adversely affects health. • Measurement Proxy: Body Mass Index (BMI) ◦ Calculation: weight/height² (kg/m²) • WHO Classifications: ◦ Overweight: 25–30 kg/m² ◦ Obesity: ≥ 30 kg/m² ◦ Note: BMI may overestimate fat in very muscular individuals. • Fat Distribution Metric: Waist-to-hip ratio ◦ Women >0.9 → associated with type 2 diabetes and dyslipidemia ◦ Men >1.0 → associated with type 2 diabetes and dyslipidemia
EPIDEMIOLOGY¶
• Prevalence Trends: ◦ Significant increase over time; e.g., 14.5% (1976–1980) to 42.4% of U.S. adults (2017–2018). ◦ Global trend: Obesity nearly tripled worldwide since 1975. • Demographic Variations: ◦ Non-Hispanic Black: 49.6% ◦ Hispanic: 44.8% ◦ Non-Hispanic White: 42.2% ◦ Non-Hispanic Asian: 17.4% ◦ Prevalence is generally higher in women than men. • Pediatric Impact: ◦ Overweight ≥ 91st percentile; Obesity ≥ 99th percentile (adjusted for age and sex). ◦ 2019: 38 million children <5 years old were overweight or obese. ◦ 2016: 340 million children/adolescents aged 5–19 were overweight or obese.
ETIOLOGY & PATHOPHYSIOLOGY¶
• Energy Balance: ◦ Accumulation of triglycerides in adipose tissue occurs when intake exceeds expenditure. ◦ Small excess (as little as 7 kcal/d) can lead to obesity over years or decades. • Genetic Predisposition: ◦ Significant role confirmed by twin studies; identical twins show similar adiposity regardless of environment. ◦ ~20% of children with severe obesity have rare chromosomal abnormalities or highly penetrant genetic mutations. • Leptin Dynamics: ◦ Produced by fat cells; levels rise as fat stores expand and fall during caloric restriction. ◦ Leptin Resistance: High leptin in obesity does not prevent weight gain but still defends against fat loss. ◦ Congenital Deficiency: Leads to severe hyperphagia and early-onset obesity. • Insulin Resistance (IR) Mechanisms (Figure 413-5): ◦ Pathway 1: Adipose Tissue Inflammation Expansion of adipose tissue → Recruitment of macrophages/immune cells → Release of inflammatory cytokines → Disturbance of insulin action in muscle and liver. ◦ Pathway 2: Lipotoxicity Expansion of adipose tissue → Limited fat cell capacity for continuing storage → Storage of lipid in non-adipose tissue (liver, muscle) → Defective glucose handling in liver and muscle. ◦ Outcome: Insulin resistance and compensatory hyperinsulinemia.
CLINICAL FEATURES¶
• Syndromic Disorders (Table 413-1): ◦ Prader-Willi: Autosomal dominant; features include hypotonia, failure to thrive in infancy, developmental delay, short stature, hypogonadotropic hypogonadism, sleep disturbance, and obsessive behavior. ◦ Bardet-Biedl: Autosomal recessive; features include syndactyly/brachydactyly/polydactyly, developmental delay, retinal dystrophy or pigmentary retinopathy, hypogonadism, and renal abnormalities. ◦ Carpenter’s: Autosomal recessive; features include acrocephaly, brachydactyly, developmental delay, congenital heart defects, growth retardation, and hypogonadism. ◦ Tubby: Autosomal recessive; features include progressive cone-rod dystrophy and hearing loss. • Monogenic & Genetic Mutations (Table 413-2): ◦ Leptin: Autosomal recessive; severe hyperphagia, frequent infections, hypogonadotropic hypogonadism, mild hypothyroidism. ◦ Proopiomelanocortin (POMC): Autosomal recessive; hyperphagia, cholestatic jaundice or adrenal crisis due to ACTH deficiency, pale skin and red hair. ◦ Carboxypeptidase E: Autosomal recessive / Autosomal dominant; severe insulin resistance. ◦ Single-minded 1 (SIM1): Autosomal dominant; hyperphagia, accelerated linear growth, speech and language delay, autistic traits. ◦ TrkB: Autosomal dominant; hyperphagia, speech and language delay, variable developmental delay, hyperactivity, behavioral problems including aggression. ◦ MC4R Mutations: Common in severe early-onset obesity (~5–6% of cases); associated with hyperphagia.
DIFFERENTIAL DIAGNOSIS¶
• Primary vs. Secondary Obesity: ◦ Primary: Genetic or endocrine conditions (e.g., Leptin deficiency, MC4R mutations). ◦ Secondary: Resulting from other systemic disorders. ◦ Note: Clinical assessment should screen for treatable endocrine and neurologic conditions to inform management and surgical risk.
INVESTIGATIONS & DIAGNOSIS¶
- Initial Assessment: ◦ Calculate BMI (weight/height²). ◦ Measure waist-to-hip ratio to assess risk for T2D and dyslipidemia.
- Genetic Screening: ◦ Recommended for severe cases to identify treatable endocrine/neurologic conditions. ◦ Identifies specific mutations (e.g., POMC, PCSK1, leptin receptor) to guide pharmacotherapy.
MANAGEMENT & TREATMENT¶
- Pharmacologic Therapy: ◦ Setmelanotide: MC4R agonist; indicated for children with genetic defects in POMC, PCSK1, and the leptin receptor. ◦ Targeted Therapy: Selection of treatment depends on specific genetic diagnosis (e.g., identifying mutations in the melanocortin pathway).
- Surgical Intervention: ◦ Bariatric Surgery: Decision-making guided by genetic profile; feasible in some patients but high risk in others.
PROGNOSIS & COMPLICATIONS¶
• Mechanical Complications: ◦ Osteoarthritis of knees. ◦ Reflux esophagitis. ◦ Obstructive sleep apnea. • Metabolic/Endocrine Complications: ◦ Dyslipidemia. ◦ Type 2 diabetes (T2D). ◦ Non-alcoholic fatty liver disease (NAFLD). ◦ Polycystic ovarian syndrome (PCOS).
KEY PEARLS & CLINICAL TRAPS¶
• Leptin Paradox: High leptin in obesity does not prevent weight gain due to resistance, but it still acts to defend against fat loss. ◦ Genetic Screening Importance: Essential for severe cases to identify treatable conditions and determine if the patient can manage weight via diet/exercise or requires specific drugs like setmelanotide. ◦ Early Intervention: Potential for early-life interventions (in utero or postnatal) to address 'epigenetic' programming of obesity.
Reference Tables¶
TABLE 413-1 Classical Genetic Obesity Syndromes SYNDROME Prader-Willi¶
Harrison's 22e, p.3184
| SYNDROME | INHERITANCE | ADDITIONAL CLINICAL FEATURES |
|---|---|---|
| Prader-Willi | Autosomal dominant |
Hypotonia, failure to thrive in infancy, developmental delay, short stature, hypogonadotropic hypogonadism, sleep disturbance, obsessive behavior |
| Autosomal dominant |
||
| Bardet-Biedl | Autosomal recessive |
Syndactyly/brachydactyly/ polydactyly, developmental delay, retinal dystrophy or pigmentary retinopathy, hypogonadism, renal abnormalities |
| Autosomal recessive |
||
| Carpenter’s | Autosomal recessive |
Acrocephaly, brachydactyly, developmental delay, congenital heart defects; growth retardation, hypogonadism |
| Autosomal recessive |
||
| Tubby | Autosomal recessive |
Progressive cone-rod dystrophy, hearing loss |
TABLE 413-2 Obesity Syndromes due to Mutations in Genes Controlling Energy Homeostasis Pathways¶
Harrison's 22e, p.3185
| GENE AFFECTED | INHERITANCE | ADDITIONAL CLINICAL FEATURES |
|---|---|---|
| Leptin | Autosomal recessive |
Severe hyperphagia, frequent infections, hypogonadotropic hypogonadism, mild hypothyroidism |
| Autosomal recessive |
||
| Proopiomelanocortin | Autosomal recessive |
Hyperphagia, cholestatic jaundice or adrenal crisis due to ACTH deficiency, pale skin and red hair |
| Autosomal recessive |
||
| Carboxypeptidase E | Autosomal recessive |
Severe insulin resistance |
| Autosomal dominant |
||
| Single-minded 1 | Autosomal dominant |
Hyperphagia, accelerated linear growth, speech and language delay, autistic traits |
| Autosomal dominant |
||
| TrkB | Autosomal dominant |
Hyperphagia, speech and language delay, variable developmental delay, hyperactivity, behavioral problems including aggression |
| Autosomal dominant |