Biological Aspects of Obesity

Introduction

Obesity is a complex health condition influenced by biological, genetic, and environmental factors. Understanding the mechanisms underlying obesity is essential for effective prevention and management.

White Adipose Tissue and Obesity

• White adipose tissue is the most abundant type of fat in the human body, distributed into subcutaneous (under the skin) and visceral (around organs) compartments.

• Adipose tissue remodeling involves hypertrophy (increase in cell size) more than hyperplasia (increase in cell number), which underlies the severity of obesity-related health risks.

• Adipocyte hypertrophy leads to changes in adipokine production, promoting inflammation, insulin resistance, and vascular changes.

• Ectopic fat deposits (fat stored in non-adipose tissues) can cause local cell dysfunction, impaired metabolism, and cell death, compounding obesity-related health risks.

• Accretion of adipose tissue often occurs alongside volume increases in other organs, diversifying obesity-related health risks.

Genetics of Obesity

Role of Genetics

Genetics plays a significant role in the development of obesity, with studies showing that body weight is highly heritable.

• Up to 70% of variation in body weight between individuals may be due to genetic makeup.

• Studies of twins show that the body weight of monozygotic twins (identical) is more concordant than that of dizygotic twins (fraternal), even when raised apart.

• This concordance persists over years and in response to controlled overfeeding.

Genes and Heredity

• A gene is the basic physical and functional unit of heredity, composed of DNA and acting as instructions to make proteins.

• Humans have over 20,000 genes, with each person inheriting two copies of each gene (one from each parent).

• Differences in less than 1% of our genes contribute to unique individual features.

• Small differences in the same gene are called alleles.

Obesity Susceptibility Genes

• Identification of genes that confer obesity susceptibility provides insight into mechanisms of weight regulation and fat distribution.

• These findings are leveraged to develop approaches for treatment and prevention.

Monogenic Forms of Obesity

Definition and Characteristics

Monogenic obesity results from a mutation in a single gene and is rare.

• Characterized by early-onset obesity, often beginning shortly after birth.

• Predominantly a hyperphagic condition, with constant hunger, lack of satiety, food-seeking behavior, and increased food consumption.

Examples of Monogenic Obesity

• Congenital leptin deficiency is associated with severe early-onset obesity in humans.

• Mutations in the LEPR (leptin receptor), POMC (pro-opiomelanocortin), and MC4R (melanocortin 4 receptor) genes can also cause monogenic obesity.

Table: Rare Monogenic Forms of Human Obesity

Polygenic Mechanism of Obesity

Definition and Mechanism

Common obesity is a polygenic trait, resulting from the interaction of many genes with environmental factors.

• Variants (alleles) with small effects on body weight are termed polygenic.

• Genome-wide association studies (GWAS) have identified specific sites (loci) associated with obesity.

• Single-nucleotide polymorphisms (SNPs) are substitutions of a single nucleotide at a specific position in the genome, which may impact obesity risk.

• Over 300 loci have been identified, but their individual contributions are small (less than 5% of individual risk).

• The FTO gene is one of the most significant, increasing the risk of obesity by 1.20-1.32-fold and BMI by 0.37 kg/m2.

Obesity Staging and Management

Obesity Staging Criteria

Staging of obesity provides a framework for managing individuals with obesity, considering medical comorbidities, mental health, and functional status.

• Stage 0: No cardiometabolic risk factors, no physical or psychological symptoms. Management: counseling and monitoring.

• Stage 1: Existing risk factors or mild complaints. Management: consideration of medical control for risk factors, counseling, and monitoring.

• Stage 2: Pronounced risk factors, moderate functional limitations, or psychological symptoms. Management: lifestyle intervention, medical care, and consideration of behavioral, drug, or surgical options.

• Stage 3: Significant end-organ damage, severe functional limitations, or grave psychological symptoms. Management: aggressive medical care, drug/surgical interventions, and palliative measures as needed.

Lifestyle Intervention

Lifestyle therapy is the first-line, low-risk approach for obesity treatment, aimed at promoting weight loss.

• Effective lifestyle therapy includes three components: dietary change, increased physical activity, and behavioral modification.

• Examples: calorie restriction, regular exercise, and cognitive-behavioral strategies.

Pharmacotherapy

Pharmacotherapy is indicated as an adjunct to diet, exercise, and behavioral therapy, with combination therapy having additive effects on weight loss and metabolic risk.

• Medications target various mechanisms to alter energy balance:

  • Interfere with nutrient absorption (e.g., Orlistat)

  • Delay gastric emptying (e.g., Liraglutide)

  • Act on neurotransmitter pathways in appetite circuits (e.g., Naltrexone-bupropion, Phentermine-topiramate)

• Additional medications may be indicated for comorbidities (e.g., insulin sensitizers for impaired glucose tolerance, statins for hyperlipidemia, antihypertensives, antidepressants, analgesics for osteoarthritis).

Bariatric Surgery

Bariatric surgery involves anatomical manipulations that restrict stomach size and/or interfere with nutrient absorption. It is highly effective for inducing weight loss and improving cardiometabolic comorbidities.

• Requires adaptation to profound changes in dietary intake and ongoing vitamin/mineral replacement.

• Potential complications: dumping syndrome, gastroesophageal reflux, hypoglycemia, and possible weight regain.

• Long-term data support reduction in obesity-related all-cause mortality.

Key Take Home Messages

• Genetic factors explain differences in body weight across individuals.

• Monogenic forms of obesity are rare and typically result from gene mutations affecting energy balance.

• Common obesity is polygenic, with over 300 loci identified as conferring susceptibility.

• Staging of obesity enables a framework for management, considering medical comorbidities, mental health, and functional status.

• Lifestyle interventions are considered first in treatment, followed by pharmacotherapy and surgery as needed.